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malconium 11-14-2004 09:39 PM

Permant floor at a reasonable price?
4 Attachment(s)
It is now time to publish progress and details on how my full floor replacement is coming along. I am going to post the write-up in several frames so that it is easier for me to keep the photos in order. I anticipate posting a bit more information as the job progresses.

My AS is a 1973 31' unit. I have already replaced the floor once but made the mistake of using the new variety of treated plywood before I found out that it is way too corrosive to aluminum and steel. So I went looking for something else to use and finally decided to use a product called Polyboard. Check out the manufacturers website for product information:

I was able to buy sheets locally from GE Polymer Shapes for $52.70 each. This is actually less than I paid for my treated plywood for the first round of replacement. It is also way less than what it would cost to use all other composite or plastic floor panels that I have investigated. My initial concern was that while the material is very tough it is also not as stiff as plywood. I eventually decided that I would try to come up with an approach that provided support more frequently than the typical 24" on center that an AS has. I decided to try 12" on center (based on something I read at the manufacturers site) and see what would happen. I came up with a technique for adding strips of aluminum u-channel that is 3/4" x 3/4" x 1/8" thick. I found a great local supplier that sold me 16' pieces for just under $19 each.

I decided to do a trial installation focusing on the front floor panel before I bought enough Polyboard for the whole job (it takes 7 sheets to do my 31' unit). The front floor panel is a bit unique in that it is curved, it is only 3' wide rather than 4' like the rest of the sheets and it sits over the spare tire well. This means that the whole middle of the panel actually spans almost 3' instead of the more normal 2'. I thought this would be a great worst case test of my technique. So I bought one sheet of Polyboard and collected some scrap 3' pieces of the aluminum channel that a friend of mine let me have. The technique I decided to try for stiffening the floor was to add 4 strips of aluminum channel lengthwise down the center of the trailer. This divided up the area between the main frame channels into about 12" segments. The area outside of the main channels is about a 15" span from the frame to the bottom of the body. In my case the body has the u-channel/c-channel aluminum at the bottom except at the curves near the ends. It seemed to me that the c-channel would help stiffen the edge of the floor. I thought that the 15" span might be OK but felt that I could add re-enforcement there if I needed to. So far it seems to be fine without any. I should also note that the front part of my AS still has the belly pan on. I do not have any compelling reason to take it off so I decided to work on an approach that could be done entirely from above, for that area at least. In particular that means I have had to give some thought to the types of fasteners to us. All of the belly pan is off from behind the under-floor freshwater tank to the back of the AS.

To mount the aluminum I decided to cut 1/8" deep dados (grooves) into the bottom of the floor panels using my router. I mount the channel to the bottom of the floor panels using a few 3/4" pan head screws. With the aluminum recessed 1/8" into the floor panel the remainder sticks down by 5/8". This is the correct amount so that the end of the aluminum can sit on top of the lowered cross-members. On my model every other cross-member where the plywood seams fell were made 5/8" lower so that a strip of plywood could be added to help re-enforce the joints. Where the aluminum channel crosses the normal height cross-members I notch the aluminum to clear. The top part of the aluminum channel is recessed into the floor panel so that it can clear the top of the cross-member. In this fashion each piece of aluminum is supported by the frame at each cross-member. I also needed some strips of material 5/8" thick to support the floor panels at the lowered cross-members. I was able to use the remaining 12" of the front floor panel to cut 4 3" wide strips that are enough for all the needed spacers. I ran the strips through my table saw to cut them down to 5/8" thick. I have been attaching the strips to the edge of a floor panel before I install it. I am using a polyurethane glue made by Elmer's Glue that I bought at Home Depot. I am also using a couple of decking screws to attach it to the bottom of the panel while the glue sets up.

The photos below show the following:

1.) The front floor panel cut to shape and with the aluminum channel attached to the bottom.

2.) A close up shot of the notched aluminum strip at the front edge of the panel.

3.) A close up shot of the recessed aluminum channel at the rear of the panel.

4.) A shot showing the spacers attached to the bottom side of the panel between the aluminum channel.


malconium 11-14-2004 09:49 PM

Installing the first panel...
3 Attachment(s)
The 3 photos on this page show the first panel in its installed location. The curved panel can be placed down on the floor on the diagonal and rotated into position. Notice that I am using elevator bolts to attach the rear edge of the panel. I can get to the bottom side of all these bolts without having to remove the belly pan. I also used some self-drilling/taping screws through the bottom shim strips to help locate the panel while I drilled and installed the elevator bolts. If you look closely you can see the elevator bolts and the self drilling screws. You can also see the ends of the aluminum channel as they sit on top of the lowered cross-member. I forgot to mention earlier that GE Polymer Shapes stocks Polyboard in white and a dark grey. I elected to go with the white because I thought construction lines would be show more easily and that the white would be better at reflecting heat from the bottom side in hot weather. Indeed it does show pencil marks better but it also shows every dirty foot print too. It has been a bit rainy around here lately and it is almost impossible to keep from getting the floor dirty. That would have been true for the grey too but the dirt would not have shown as much.

Even though I have not finished bolting down the front edge of the panel I find that it is stiff enough to be an acceptable solution. As a practical matter the front area will be under the front couch anyway but I can stand on the floor without any real problems with sag. I weigh in at 230 lbs so I thought that was a pretty good test on the worst case condition for my floor. That was enough evidence to convince me to continue with the rest of the floor.


malconium 11-14-2004 09:51 PM

Production line panel preparation...
5 Attachment(s)
I went back to GE Polymer Shapes to buy 6 more sheets of Polyboard the next day. Polyboard is reportedly about 20% heavier than plywood. I can attest to the truth of that. I decided to do most of the rest of the floor using 1/2 sheets with a splice along the middle line of the AS. That made it much easier for me to deal with the weight of each sheet by myself and the smaller size is much easier to get into position . With a helper it should be possible to install full sheets crossways on the floor like the original panels. The Polyboard is flexible enough that it should be possible to lift the panel enough in the middle to get it to slip under the edge of the body. This would eliminate the step of having to add a spline down the center line of the AS where the joints are.

I was delighted to find out that my saw horses with a sheet of plywood on top made a work table that was about even with the tail gate of my pickup. All I had to do was slide a sheet out onto the top of the table to be able to work on it. No heavy lifting needed on full-sheets.

I made a template out of strips of plywood that I was able to clamp to the Polyboard sheet as a guide for my router for cutting the dados and for my circular saw for cutting the panels to size. Something to keep in mind too is that the Polyboard sheets are actually about 1/4" wider than 4'. This means that the panels have to be cut to both length and width. Cutting the panels is interesting in that there are shavings that look a lot like shredded coconut rather than like saw dust. Even with the router the shavings tend to stay together somewhat - unlike with wood where there would be fine dust all over the place.

The photos below show the following:

1.) A panel on my work table ready for processing.

2.) The plywood template clamped to the Polyboard.

3.) A mark on the template for aligning the template with the center of the panel.

4.) The router just having cut a dado.

5.) My circular saw against the guide for cutting the panel to size.


malconium 11-14-2004 09:54 PM

Installing the 1/2 sheets in the AS...
3 Attachment(s)
Each of the 1/2 sheets of Polyboard get two aluminum channels installed on their bottom sides before installing them in the AS. As I did for the first panel I have also been gluing and screwing the shim strips from the bottom side before putting the panel in the AS. If you look closely at the first photo you can see that the aluminum channel is notched in the middle where it will clear the tall cross-member that will fall below the center of the panel. Note that I left about a 1/8 gap between the first panel and the second set. The second photo shows the 1/2 panel in place with the first of the elevator bolts installed. You can also see how I have attached the splice strips on the center line of the trailer with glue and screws from the top side of the panel after I put it down. If you look real closely you might be able to see the row of screws along the front edge of the panel where it attaches to the shim strip at the back edge of the previous panel. I have not yet made up my mind if any additional fasteners are needed along this edge to supplement the glue and screws. Also notice that I put an elevator bolt down to the middle cross-member.

The third photo shows the location for the next 1/2 sheet. You can see the belly pan below the frame. This area is right next to the entry door so the frame has some variations from the norm in this area. Notice the shims on the back edge of the first panel to the left and the splice strips on the 1/2 panel at the bottom. So far I have installed a total of 3 of the 1/2 sheet pieces.


malconium 11-14-2004 09:55 PM

Some observations and conclusions...
Going through this process I have discovered a few things about the approach I have decided to take.

1.) Polyboard does seem to be strong enough and stiff enough when supported at a nominal 12" on center. There is a tiny amount of deflection when I stand in the middle of a panel but I do not think this to be a problem for my installation. It is possible that the amount of deflection might cause a problem for 12" square floor tile glued to the floor. I intend to use some sort of floating floor which should not be a problem at all. I am still leaning in favor of installing radiant heat tubing on top of the Polyboard too. If I do end up doing that I will have additional strips of some material (possibly Polyboard) that will add some additional stiffening.

2.) The use of the aluminum strips for re-enforcement the way that I am using them is a somewhat time consuming process what with routing the Polyboard, cutting the channels, drilling the mounting holes in the channel and attaching the channels to the Polyboard. It would have been easier to cut 1-1/2" strips of Polyboard and attached them with glue and screws to the bottom side of the panel. I do not know for sure, however, if this would have been stiff enough. Since I still have the rest of the floor to do I might perform an experiment with this technique on one of the 1/2 panels for the benefit of science. It might also be easier to add some additional steel strips of some sort to the frame before installing the Polyboard.

3.) Polyboard does not hold screws quite as tightly as plywood does. I found that I have to be careful not to strip the threads out when I set them with my screw gun. I did have to watch that a bit with plywood too but the Polyboard is more touchy in this regard. I don't think this is too much of an issue since a lot of the screws are temporarily holding the Polyboard while the glue sets. Decking screws seem to have enough tooth if they are used carefully. I have been wondering though if it would be a good idea to attach my aluminum channel to the bottom of the floor panels with bolts through from the top. In general though I have felt that these screws are mostly for holding the channel in place until the floor panel is down. The channel is supported at both ends of each section by the cross-members.

4.) Elevator bolts do not pull completely flush with the top surface of the Polyboard like I suspect they would with plywood. I get at most about 1/16" of projection about the surface. I do not think this will be a problem for me but others might want to set them down in more flush by cutting a little recess first - maybe with a plunge cut router and bit.

Well, I think this documents most of my findings so far so I think I will stop here for now. I look forward to discussion on the topic though. Feel free to send questions my way if you want to know more about the process.


Stefrobrts 11-14-2004 10:55 PM

I've been watching Malcolm's posts with interest, as he is not far from me. Saturday we had a chance to drop by and check out his project in person. The work he is doing looks really top notch. The floor looks good, and there was not as much flex in it as I was expecting. It was really quite solid, and I'm no lightweight. I think it's a really good idea for putting in a floor with no future possibility of rot.

Thanks for the tour, Malcolm!

markdoane 11-15-2004 06:14 AM


Great idea and nifty execution! You've done a great job, not only in design, but in writing it up so it's easy to understand each step, why and how you did it.

I would like to come out and look at it in person. Will you be available in a couple of weeks? I'll send you a PM when my schedule is more certain.

malconium 11-15-2004 06:06 PM

I am open for tours...

I should be in and around here in a couple of weeks except for a few days around Thanksgiving. You or anyone else is welcome to come by and have a look. Just let me know when you want to come.


malconium 11-15-2004 06:19 PM

One other thing about Polyboard...
I realized that I have not mentioned my only significant concern so far relative to the Polyboard installation. That has to do with the thermal expansion issue. The manufacturer's site suggests that a panel could expand as much as 1/4" in the 8' direction. The suggestion is that they be installed with a 1/4" gap between sheets. I decided to install mine with about 1/8" side to side gap since each panel is only 4' wide. I figure that if the panels expand in their lengthwise direction by as much as 1/4" that this will effectively be 1/8" on each side of the AS - which does not sound too bad to me. My actual preference for installing the Polyboard would be to install it in the hotest possible weather and let it shrink to a tight fit in cooler climates. As it is I have been doing my installation at about 50 degrees. I do not know what this will mean relative to the behaviour of the floor at higher temperatures. The factory site also suggests installing the material with slotted holes so that things can move a little. That does not seem at all practical to me. How am I going to drill slotted holes anyway? Besides part of the idea of using sheet goods on the floor is to help provide diagonal bracing for the frame. I don't think I want slots for that reason. I can only hope that the total movement of the panels over the temperature range that they might be subjected to will not cause any problems of any significance. I am not at all sure how to test that out without actually going ahead and doing the install. I have toyed with the idea of maybe heating the panels up some with a heat lamp or hot air gun but that would be hard to control and to do accurately.

It occurs to me that if panels are fully constrained horizontally by bolts, screws and direct abutment with other panels that they should just expand vertically shouldn't they? Does anyone have any thoughts about this?


malconium 11-24-2004 02:52 PM

Progress report and weight calculation...
I finally have all of the treated plywood removed and about 60% of the new Polyboard pieces in place. I will have to go back and finish fastening them down completely when they are all in place. The process is going fine. As I noted earlier it is a bit time consuming to cut and fit all the aluminum channel on the bottom side of the Polyboard sheets but it does seem to result in a stiff enough floor.

I got to wondering just how much I was adding to the weight with the Polyboard instead of plywood. The Polyboard seemed to me to be more than 20% heavier (as I noted earlier) than the plywood I was removing. I got out the bathroom scales and weighed a plywood half sheet and a Polyboard half sheet that was the same size as the plywood and with aluminum channel attached. I found that the Polyboard assembly was 66% heavier than the plywood! I did the math and summed up the total impact for my 31' floor. I estimate the following:

Polyboard with aluminum channel equals about 3.41lbs per square foot of floor

Polyboard floor (with aluminum channel) 694lbs
Plywood floor 416lbs
Total increase 278lbs

That is more than I had originally anticipated but still seems OK to me overall. There are a number of things that I am planning to do to the rest of my AS that will result in some weight savings. Also the new axles that I bought from Inland have about a 10% load rating increase. In my case the original GVW for the AS is 7100lbs. I do not intend to use the 10% but that is about 710lbs. of safety margin. Since my trailer was largely gutted I can do pretty much anything I want on the inside to help make up for the weight increase. So while it is a little disappointing I think I can live with it.


PWDF4 12-04-2004 06:31 AM

I'm A New Member With A 1969 18' Caravel. I'm Trying To Replace The Entire Floor. I Like The Sound On The Polyboard. I Contacted The Canadian Company (through Web Site ) For My Local Dealer Listings. They Gave Me 4 And I Called 2 Of Them. They Have Never Heard Of Polyboard. I Contacted The Company Again And They Told Me That All Ge Polyshapes Did't Handel The Polyboard. I Live In Louisiana And While We Are Having Cool Days, I Would Like To Finish My Project. I Just Hope That The 5/8" Is Stiff Enough, If I Ever Find It.

malconium 12-05-2004 12:10 AM


Originally Posted by PWDF4
I'm A New Member With A 1969 18' Caravel. I'm Trying To Replace The Entire Floor. I Like The Sound On The Polyboard. I Contacted The Canadian Company (through Web Site ) For My Local Dealer Listings. They Gave Me 4 And I Called 2 Of Them. They Have Never Heard Of Polyboard. I Contacted The Company Again And They Told Me That All Ge Polyshapes Did't Handel The Polyboard. I Live In Louisiana And While We Are Having Cool Days, I Would Like To Finish My Project. I Just Hope That The 5/8" Is Stiff Enough, If I Ever Find It.

Was the original plywood in your AS only 5/8" thick? Mine was 3/4".

Evidently the GE Polymer Shapes office here in Portland, OR is kind of leading the way with the Polyboard. Maybe there is a way to work out having them ship from here in Portland to a GE Polymer Shapes office closer to you. Did you check out the GE website? I may not have published it below.

I would be happy to help get things moving from this end if there is anything I can do to help. Maybe I should see if I can get the local guy here to publish some sort of application note that can be sent out to other GE sites so they at least know that there may be some of us AS folks interested in the product. Just let me know if you want my help.


malconium 12-13-2004 12:45 PM

Progess update and some observations...
This last weekend I was finally able to spend a little more time on my Polyboard installlation. I now have 6 of the 7 4x8 sheets installed. All I have left is one 4' section between the wheel wells. The installation seems to be going fine still. The floor seems plenty stiff enough for me when I walk on it. I am 6'2" and 230 lbs and the floor seems fine to me. I will still have to go back over the floor and finish adding all the fasteners. I mostly added enough to make sure that the panels were held in place while I move to the next section. I also added all the necessary screws in any places where I was gluing spacers or splines to the floor panels.

As I think I mentioned earlier I am doing some of the installation with the belly pan still in place and some with it off. In the area where the belly pan is still intact I am using a combination of elevator bolts and self-drilling/self-tapping screws. The elevator bolts are in places I can reach from the top side before I put in the next floor panel (along the panel edge). Everywhere else I am using the screws. I think if I had considered the technique soon enough that I might have added 1/8" thick strap metal under the cross-member flanges and used larger screws less often. The thicker metal would have worked pretty much like a nut on a bolt. As it is I think the screws that I am using are holding fine - I just feel like I should add them more often than I would add elevator bolts. I am using Tek brand screws that I found at Home Depot. I am using two different sizes. The shorter ones are designed for up to 1" material fastened to metal. These are about 3/16" in diameter and work fine where there is only one thickness of Polyboard on top of the frame. The other screws are long enough for up to 1-5/8" (I think) to metal. They are larger in diameter than the others and definitely beefier. I am using these where there are 5/8" spacers at the floor panel joints. In these areas there are essentially two thicknesses of Polyboard and I need a longer screw. I also need the longer screw where the floor panel sits on top of the wheel well flange. The flange is just thick enough that one layer of Polyboard and the flange is just too much for the short screw.

Most of the cross-members are thinner material and the self-tapping screws do not have any trouble penetrating. There is one cross-member over the middle of my under-floor fresh water tank that is 1/8" thick angle iron. I found that pre-drilling a pilot hole for those screws helps a lot. I also needed the longer screws for that member. They are definitely holding solidly there too. That is one thing that led me to believe that using a strap of 1/8" metal under the other cross-members would give a better bite for the screws.

I did notice that the elevator bolts would not pull down entirely flush with the top surface of the Polyboard. I tried to do that on one of the bolts and actually snapped it off. They stick up about 1/16" of an inch. So if you want the top surface to be absolutely flush for installing a finish floor I think you would have to counter sink the bolt held just a little bit. The self-tapping screws can pull into the surface of the Polyboard enough to be flush.


74Argosy24MH 12-13-2004 01:08 PM


Originally Posted by malconium
Most of the cross-members are thinner material and the self-tapping screws do not have any trouble penetrating. There is one cross-member over the middle of my under-floor fresh water tank that is 1/8" thick angle iron. I found that pre-drilling a pilot hole for those screws helps a lot. I also needed the longer screws for that member. They are definitely holding solidly there too. That is one thing that led me to believe that using a strap of 1/8" metal under the other cross-members would give a better bite for the screws.

You probably did better by not using the reinforcement. When you use tek screws on too thick a piece of metal they snap. The threads start to bite before the bit has penetrated and it is all over.


malconium 12-13-2004 06:43 PM


I could certainly see how that could happen. I did pre-drill pilot holes on the 1/8" angle iron cross member and was glad I did. While that somewhat defeats the idea of using a self-drilling screw it worked fine. I think if I were to add a strip of metal that I would take a closer look at some screws I found that are specifically used for holding down plywood on the floor of semi-truck trailers. My understanding was that they are intended for frame members that are thicker than the typical AS cross-members. That is why I didn't buy any of them when I found them. They are definitely available in 1/4" diameter too. The have a part of their flat head that flairs out a little wider too as I recall. Sort of part way between the Tek screw which has a more or less normal flat head shape and the elevator bolt with a very definitely flared out head.


74Argosy24MH 12-13-2004 07:26 PM

Good move passing on the floor screws, you would have had to drill holes for them also. They are self tapping, not really self drilling.


malconium 12-14-2004 12:07 AM


The drilling issue is not too big of a deal as I see it since I have to drill for elevator bolts where ever I use them. I passed on them more because the hardware store salesman indicated he thought they needed thicker metal than what is normally on the cross-members of an AS.


malconium 12-14-2004 11:58 AM

Water leaks just puddle up...
Over this last weekend we had some terrific winds. I have had a large tarp draped over a rope that goes down the length of our AS about 2" above it. I did that initially because I had way too many leaks to deal with at the time. I was trying to work on the floor replacement. Well the tarp blew over the top of the AS and was flapping around like a big sail in the wind. It was too windy to put it back in place so I took it down, folded it up and put it in the garage. While it was very windy there was no rain so I spent a little time applying Vulkem on the inside of the outer shell in places that I could remember seeing leaks. I had allready fixed a few places that had leaks too such as missing screws on the awning mounts, etc. Well I am happy to report that the rain we had last night only got through in a couple of small places that I can now check out further.

The main thing I wanted to report though was that the water leaks just created puddles on the Polyboard floor. Before with the plywood (both the original and the preasure treated plywood) the water would spread out into a bigger circle and soak in to the wood. I am very pleased that this does not happen with the Polyboard.


malconium 12-29-2004 08:11 PM

I am finally done!
Yesterday I finally finished fastening down all of the new Polyboard sub-floor! The floor looks fine and seems plenty stiff enough to me. The white Polyboard is a pain to keep clean looking though. A lot of the work on the floor was done when it was either raining outside or at least damp and somewhat muddy. I cut each Polyboard panel to shape and installed the aluminum re-enforcing strips on the bottom side in my garage and then carried them out to the AS for final installation. It was impossible to keep from tracking some muddy dirt onto the new floor. If it were plywood the ground-in dirt might just add to the patina of the wood. With the Polyboard every foot step shows. I was able to wash it up enough using some Simple Green and a mop. I am sure I could get it cleaner if I worked on it a bit harder but it is the sub-floor after all.

As I mentioned earlier I used a combination of elevator bolts and self-drilling/self-taping screws to hold down the floor. I also used a Polyurethane glue on all of the joints where there was an overlap of Polyboard with a Polyboard spacer or shim. These joints I also used decking screws to hold them together while the glue set up. I think the floor is pretty securely attached.

Unfortunately I will have to wait until much warmer weather to see if there are any issues with expansion of the Polyboard. As far as I am concerned that is the only unresolved question regarding the use of Polyboard for the floor of an AS.

I would be happy to answer any questions anyone has regarding the techniques I used or my experiences with the Polyboard.


PWDF4 01-05-2005 07:37 PM

I Have Installed 11' Of Polyboard. I Only Have About 34" In The Back To Go. I Bought The Gray Instead Of The White To Keep Things Cleaner. I Am Installing 2" Channel Iron Every 2' Across And Down The Center. Every Other Frame Cross Member Is 1/2" Low. On These I Have Welded 1/2" X 1" Metal To Make All Beams The Same Highth. I Have Painted The Frame With Por-15 Paint To Protect It. After I Get All The Floor Down And Only Then Will I Secure It.

malconium 01-06-2005 01:01 PM


From your description I am not entirely clear which direction the new steel u-channel is running. Are you talking about adding new members from side to side in between the existing cross-members? Also it would be an interesting piece of information for other Polyboard prospective installations to know how much extra weight you are adding with the steel. Did you decide to run the Polyboard sheets lengthwise or from side to side? What type of fasteners have you decided to use for holding the Polyboard in place? How do you like the product overall? Any other details or even photos of your progress would be of interest to me and perhaps to anyone else considering Polyboard.

Thanks and happy Polyboarding,


PWDF4 01-11-2005 01:23 PM


malconium 04-23-2005 12:43 PM


Wouldn't the marine grade plywood cost in the same general range as the Polyboard? Why not consider using the Polyboard for the whole thing? You would want to add some stiffeners to the Polyboard but that would not be much different work wise than adding all the splices between the plywood and the Polyboard apron. The last time I checked the price of Polyboard was rising some. I think the 3/4" 4x8 sheets where into the low $80's per sheet.


Over59 04-23-2005 02:25 PM

1 Attachment(s)
How are you going to attach the lap together? Glue? Screws?. I think you'll be better off with the 5/8 marine ply or A/C exterior all the way. Known vs experimental. I'm using 3/4 marine saturated with thin epoxy around the edges for the whole floor with the shell off. The last floor made it 45 years. That will be good enough for me.

You know of course that it often starts out as "just a patch". Then it's going to be "a shell on", and then.

Over59 04-25-2005 05:56 AM

As I understand it you are lapping poly and ply together. Does the poly hold glue or screws?

C Johnson 04-25-2005 11:08 PM

I have my "C" channel out and there are no weeping holes in it. Just the holes for the elevator bolts, rivets, and #10 screws that hold it to the floor. The ends, however, are open.

malconium 04-25-2005 11:11 PM

While it is true that the Polyboard does not hold screws quite as well as plywood does they do still hold. Certainly long enough to let the glue on your splines set. The type of glue that I used seems to stick very well to the Polyboard and is also supposed to be excellent for wood. I used Elmers Polyurethane glue. A very similar product is called Gorilla Glue. Both are available at Home Depot. You need to wet the surfaces before you put them together as the glue activates with a little bit of moisture. It also expands to fill any voids and usually squeazes out of the joint some. It will stain your hands too if you get any on you. I think a well glued spline joint with about a 4" strip of wood or Polyboard as the spline should make a joint between the Polyboard and plywood that would be as strong as either of them.

It would be possible to install everything with the Polyboard using bolts instead of screws if you have the belly pan off by the way. The main place where I used screws was to connect the Polyboard to the splines where I am relying on the glue to do the real work. I did use decking screws in the u-channel down to the Polyboard too except at the ends of the outriggers. There I used self-taping/self-drilling screws down into the ends of the outriggers. In the original install there wern't any screws into the outriggers that I could find (which suprised me). I used larger screws than what were originally into the plywood too. Since the c-channel part of the u-channel slips over the edge of the floor (at least for those later models that have that type of channel) the screws are only holding the channel laterally

Regarding the Polyboard expansion issue I installed my Polyboard sheets from side to side. I also typically cut my sheets in half so I was basically installing 4' x 4' tiles. The specs for the Polyboard suggest that you leave a 1/4" gap on the ends between 8' sheets. That seemed to me to mean that 1/8" gaps should be about right for between 4' pieces. As a practical matter I think you are supposed to leave a little bit of a gap between plywood sheets too. A 10" piece of Polyboard would have a pretty small amount of expansion compared to an 8' piece at 1/4".

I did not see any intentional weep holes in the u-channel when I took my AS apart. I did decide to add some. I drilled 1/4" holes with at least one in each wall section. I added a piece of fiberglass screen over each hole that is held in place with a little vulkem around the edges.
The Polyboard glued to the plywood should be strong enough laterally in my opnion. Also while every part of the AS does add some to the strength it is also my opinion that making sure to fasten the u-channel to the ends of the outriggers should go a long way toward resisting any outward movement of the body even without any contribution from the sub-floor.

I do wonder it there could be problems using 9" tiles over Polyboard because of the expansion. Maybe the tiles would actually have a similar rate of expansion though. I believe I read somewhere here in the forums that the original tiles were installed so that the joints in the plywood fell at the same places as the joints in the tiles.


Torii 04-27-2005 08:31 AM

an objective comparison please...

Thanks for the great info on your project! I have a few soft spots in my 75' Tradewind. In the fall I will have a look and see what kind of damage I am dealing with. If needed, a new floor will be my winter project. You mentioned several times in this thread that "The floor looks fine and seems plenty stiff enough to me". You have the distinct advantage of having done your floor in the same unit using plywood, how would you say the polyboard campares to plywood in terms of strength and stiffness. (which one is stiffer) I am 6'4 and weigh about 220. If I do replace my floor I would like to have one that does not flex under my weight if possible.

malconium 04-27-2005 12:46 PM


Originally Posted by T Man

Thanks for the great info on your project! I have a few soft spots in my 75' Tradewind. In the fall I will have a look and see what kind of damage I am dealing with. If needed, a new floor will be my winter project. You mentioned several times in this thread that "The floor looks fine and seems plenty stiff enough to me". You have the distinct advantage of having done your floor in the same unit using plywood, how would you say the polyboard campares to plywood in terms of strength and stiffness. (which one is stiffer) I am 6'4 and weigh about 220. If I do replace my floor I would like to have one that does not flex under my weight if possible.

The plywood was definitely nice and stiff without having to add anything additional to stiffen it. The Polyboard does need something like what I did with the aluminum channel. I am 6'2 and weigh in at 230 lbs. I think the Polyboard as I have installed it is plenty stiff enough for me at my weight. I have been working back and forth in the AS installing insulation and various other things ever since having installed the Polyboard and I do not feel that there is any kind of a strength problem. I have had my portable air compressor running on the floor without problems too. The Polyboard seems to be pretty tough stuff. It just is not quite as stiff as plywood and needs the extra help. I am still very pleased with the results.


malconium 04-28-2005 11:38 AM

There is some flexing...
I went out to the AS yesterday just to make sure what I was saying about the Polyboard stiffness was correct. I can detect some small amount of visible deflection of the floor when I stand in between the major cross members. I do not detect it at all when I walk around though. My aluminum bracing is probably not as stiff as if someone were to use some steel. I still am fine with the small amount of deflection but it could cause a problem if someone wanted to glue down 9" tiles. Both the small amount of deflection and the possible expansion/contraction could very well cause problems with flooring of that type. I am intending to use some sort of floating floor so do not feel this will be a problem for me at all. I guess the bottom line is that plywood does provide the best stiffness if that is an issue such as for flooring type. I still very much like the Polyboard for the fact that it is entirely waterproof.


Torii 04-28-2005 12:07 PM

Waterproof = GOOD
I agree that the fact that it is waterproof is a HUGE plus for the polyboard. If I end up ever replacing the floor in my Tradewind I will probably use it because sooner or later new leaks will develop and it would be nice to know that when they do that my floor will not be rotting as a result. I wonder if welding steel supports to the frame in about the same location and frequencies as Malcolm's aluminum bracing would result in a stiffer Polyboard floor? Of course that means even more weight added.

PWDF4 - I think you mentioned adding steel supports... How did that work out? Any flex in your floor? How much weight did you add to your trailer?

C Johnson 04-29-2005 12:07 AM

I think adding extra steel would significantly increase your weight. If you plan to upgrade your axles tho, it may be ok. Just my 2 cents.

ankornuta 01-09-2006 09:20 PM


Originally Posted by malconium
I have already replaced the floor once but made the mistake of using the new variety of treated plywood before I found out that it is way too corrosive to aluminum and steel.

Malcom, I know you posted this a long time ago, but this statement confuses me. Did you use the ACX 5/8" plywood that so many people use for floor replacements? What's this corrosion you found out about?

malconium 01-12-2006 06:32 PM


Originally Posted by ankornuta
Malcom, I know you posted this a long time ago, but this statement confuses me. Did you use the ACX 5/8" plywood that so many people use for floor replacements? What's this corrosion you found out about?

The first time I replaced my floor I used chemically treated 3/4" plywood. My unit uses 3/4" but I guess some models used 5/8". The problem with this solution was that the chemicals used to treat the plywood is way too corrosive to metal and especially aluminum. I could already see signs of problems when I pulled out the treated plywood to replace it with the Polyboard. Normal non-treated plywood does not have this problem (including ACX grade) only the chemically treated varieties.


ankornuta 01-14-2006 07:40 AM

Oh... good, I'm glad to hear that ACX won't do that! I was thinking that you were referring to ACX having this issue. I plan to start laying down my new ACX plywood floor next weekend after finishing the body lift this weekend. I've been shocked by the speed of my progress :) (that's the good kind of shock, of course).

guy99 01-14-2006 07:48 AM

Has anyone used MBO (Medium density overlay) for flooring. It is the material used for outdoor signs and is reported to be very water tolerant.

guy99 01-14-2006 04:17 PM

To clarify, MBO is a plywood product with waterproof glue and with a resin impregnated 'paper' adhered to one side (or both).

markdoane 01-14-2006 07:47 PM


I didn't use mdo. I think the advantage would be that mdo is edge sealed, which is something everyone does anyway with penetrating epoxy, and the one smooth side.

Otherwise, it is just B-C exterior grade plywood.

guy99 01-14-2006 09:39 PM

The entire surface, or both surfaces, are water proof (resistant?) because of the layer of resin/adhesive used to bond the 'paper' to the surface of the plywood.

Also, the mdo I have used for other projects had no voids. I don't think B-C grade plywood is without voids. Flooring underlayment is though.

tallone 01-22-2006 05:30 PM

in boat building we use acx and then coat with West epoxy to waterproof, works great.

Makarov 02-01-2006 01:18 PM

Hey guy99,

My first choice in material for floor repair would be marine plywood.
But who can afford the stuff?
So I would use MDO.
MDO is awesome suff. Made to stand up to the elements and seems to be stiffer that like thickness ACX.
Most of the ACX I see is just bearly good enough- voids, warped, plug repaired.
The MDO is high quality, rock solid, dead flat, stiff and reasonably priced compared to ACX.
There is a reason freeway signs, billboards and concrete forms are made from it.
It lasts.


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