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Old 02-10-2019, 12:36 PM   #1
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1976 27' Overlander
Milwaukie , Oregon
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Routing edges of subfloor to allow greater thickness

Hey guys. Doing a full gut of a 76 Overlander, shell on. All of the subfloor needs replacement. The existing subfloor is 1/2”, which is kinda flimsy. I have been thinking about taking 5/8” or 3/4” and routing down the edges to fit the c-channel. What are the negatives in doing something like this? Obviously a little more weight. I will allow enough room for the interior skins to slide into the groove between elevated subfloor and shell. I don’t plan on using any of the original cabinetry, so the slight change in height will not impact that.

Any thoughts?
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Old 02-10-2019, 01:33 PM   #2
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Sounds good to me. I have done that for a small patch. I have see such a rout in places on my trailer that Airstream did at the factory. Seems the OSB was thicker than the channel from the start. The dado is on the top which makes sense.
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Old 02-10-2019, 01:46 PM   #3
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I used 3/4" Coosa board in place of my 5/8" plywood; 40-45% lighter than plywood; will not rot or promote mildew; just as strong, if not stronger than plywood, less than 1% water absorption rate. Very easy to machine to any profile.

I first did a 1/4" deep cut 1 3/4" to the inside of the blade on the edge fitting into the C channel; then machined the 1 3/4" to 5/8" thick; then machined an angle on the last 1 1/4" to aid insertion into the channel, since it's only the inside of the channel that needs support. Not the best picture below, but you'd get the drift.

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Old 02-10-2019, 01:56 PM   #4
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JoleneAS:
Airstream always installed 5/8 sub-floors.
1/2” is not possibly strong enough nor
Thick enough to accommodate the
Elevator bolts to hold everything together.
Back to the drawing board.
What ever you chose for sub floor be sure
to seal it and epoxy all around the edges.
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Old 02-10-2019, 03:15 PM   #5
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Your idea is just what I did on my ‘76 TradeWind Land Yacht. I used 3/4” tongue and groove “plytanium dry ply” and routed the edges down to the original ~1/2” thickness and it worked very well. Be prepared for a huge mess of sawdust when you rout the edges. I was more concerned about floor thickness and integrity than losing 1/4” of interior height. I also built flashing at the rear to avoid the wicking of water into the floor. See the photos and good luck with your project.
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Old 02-11-2019, 11:35 AM   #6
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I was able to pry up the c-channel and jam in 3/4" ply on my shell-on floor replacement section. This was about 4' of the end, so fairly substantial. So no need for routing?

Be sure to drastically seal the edges if you're using plywood. And use some nice, lightweight marine ply or that Coosa board ($$$).
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Old 02-11-2019, 11:54 AM   #7
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Quote:
Originally Posted by Del Gurney View Post
JoleneAS:
Airstream always installed 5/8 sub-floors.
1/2” is not possibly strong enough nor
Thick enough to accommodate the
Elevator bolts to hold everything together.
Back to the drawing board.
What ever you chose for sub floor be sure
to seal it and epoxy all around the edges.
My '77 has/had 1/2" subfloor. Tape measure and caliper confirmed.



Ian
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Old 02-11-2019, 12:28 PM   #8
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Stiffness goes up in proportion to the cube of the thickness. Well worth it to use the 3/4” nominal at whatever actual thickness it is.
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Old 02-11-2019, 12:38 PM   #9
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Prying the channel open is probably the easiest way to get 3/4" wood in there.

It's soft enough to open with a piece of 1/2" wood, like a fence picket, then the plywood fits in fine. After it's installed, tap around the bottom of the channel to close the gap back up.
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Old 02-11-2019, 12:50 PM   #10
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Wouldn't prying open the C-channel result in rivet holes not lining up? It doesn't seem like it'd be too hard to use 3/4" ply and route the outer edge to 1/2". I just wanted to make sure that this was a reasonable idea since there are posts on this sub stating "don't use non-factory thickness, you will have problems!" I suspect that is for people using original cabinetry and such.

And I agree with Iansk. My subfloor thickness was confirmed by caliper and tape measure at multiple areas. This subfloor has never been replaced, so the 1/2" was definitely factory on this model.
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Old 02-11-2019, 01:40 PM   #11
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If you indeed have 1/2" original flooring, I would recommend replacing it w 1/2" Coosa board. No routing, no sealing, light weight, easy to work with, stronger than plywood, and will never rot. It costs more but will give good results and make your project go faster. I had 5/8 plywood and replaced it w 1/2 Coosa and added 1/8 aluminum shim stock underneath the Coosa.

When removing my rotted plywood floor, it was apparent that the factory sealing of the surface and edges with black sealant didn't help so much. Once inserted in the c-channel, lots of holes get drilled for screws and bolts. It is no longer sealed. Plus, moisture can wick in to the plywood thru these penetrations, and then can't dry because of all the sealant. It becomes a moisture trap.

Good luck with your project!
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Old 02-11-2019, 06:19 PM   #12
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Huh... I hadn't thought about the risks of rot after drilling through the treated wood. That's a good point. Can you screw into coosa like wood though? Like if I wanted to install cabinetry and such?
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Old 02-12-2019, 12:38 AM   #13
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Coosa doesn't hold screws as well as wood. Threaded inerta are recommended.
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Old 02-12-2019, 07:23 AM   #14
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While coosa looks interesting, I personally can’t justify $2100 + tax + shipping to do my subfloor.

As for holes drilled I’d probably just dribble some spar urathane in there before bolt insertion.

Ian
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