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Old 12-09-2015, 04:51 PM   #1
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New Subfloor Sag

Hey all,

I had 3/4" marine plywood subflooring installed for us about a year ago. After taking some time off from our renovation, I recently noticed one of the panels doesn't seem to be solidly attached to the frame. When I stand on it, there's a noticeable sag.... which cracks anything I apply to level the panels to each other.

I began removing sections of belly pan and soon realized the problem area is "land locked" at the front corner, which means I may need to remove the belt trim.

I've never done this, and am concerned that the ext. skin, C-channel and belly pan may pop out of alignment when I try to reattach everything. Any warnings or advice?

Video here, (sorry that it's mostly out of focus):
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Old 12-10-2015, 09:42 AM   #2
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I'll tell you up front that I am no expert on 50's era construction, so can only give general advice. Looking at your video, I see that the flooring across thr front seems to be sectioned in at least three pieces. It is strange to me that if you had the whole floor professionally replaced, that it would be so cut up (as opposed to a single piece). Maybe your installed did the floor replacement without lifting the shell, and had to cut the front panel up to slide it under the walls. This makes me question how the C or U channel at the base of the wall is fastened to the outriggers through the floor as well. As you mention, that front section is pretty closed up, and without access to the top and bottom of fasteners, I wonder what method was used.

As to your question about disassembling the front belly skin, you should be able to remove the trim, drill out the rivets, and drop the bellypan without much fear. The weight of the shell should keep it firmly pressed against the floor, and the U channel in the base of the wall should keep it from changing shape radically. You will probably also need to remove the lowermost interior skins so you can see the upper ends of the fasteners that should be attaching the shell to the outriggers, and see how the belly skin interacts with the U channel.

good luck!
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Old 12-10-2015, 01:07 PM   #3
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Thanks Belegedhel, that's what I presume as well. The interior skins are off, and the new flooring definitely runs all the way out to the edge, so to the best of my knowledge no corners were cut, quality-wise. I did notice they didn't insulate under the dinette in the first section behind the tongue -- that is bugging me.

Furthermore, the portions of the belly pan where they did use new fiberglass matting (no paper on either side) is a soggy mess after one year of just sitting in the driveway!

On the other hand, we lovingly installed our own wall insulation - two layers of Prodex insulation interspersed with 3 layers of air pockets - so I may simply upgrade the belly pan insulation too. Perhaps 2" rigid pink foam, which I hear doesn't absorb water.

But first, I'll try and fix this bouncy subfloor. Thanks again for your opinion!
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Old 12-10-2015, 02:49 PM   #4
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I attachedtwo inches of rigid foam board with a bottom-most layer of reflectix to the underside of my floor. Had to open up the belly after a couple of years, and everything was still in place and holding up well.

good luck!
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Old 12-10-2015, 08:22 PM   #5
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Thanks, that sounds like a good solution ~ after I track down why that section is sagging, of course!
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Old 12-10-2015, 11:48 PM   #6
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You can drill out the rub rail rivets then the front corner piece of belly pan can be removed without issue. Just use the same rivet hole when you reinstall.

I would guess that you have a sagging issue because the front piece of plywood has been cut into 3 pieces and the corner piece has no support from the frame.
I believe it should be one solid piece to have full structural integrity.
As you may or may not know, the floor is an integral part of the whole trailers structure.

You may be able to secure it by scabbing pieces across the cuts from underneath the floor. all though you may not have the room between the cut and the frame to secure a patch.
It is difficult to tell were the cuts were made without further under side photos.
I'm not sure how long patching would hold up in the long run. However it would be easier than replacing the front section of ply, but not better.

you say in your video that the floor was "professionally installed" you may have recourse from the "professional" who did this work.

I also used 2" solid foiled foam for my under floor insulation. I used adhesive and screws with fender washers to secure it to the underside of the floor. It has proven to be solid and secure. You will like it better than fiberglass batting.

Best of luck

-Dennis
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Old 12-11-2015, 12:15 PM   #7
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Thanks for weighing in, Dennis. I imagine this guy did the right thing -- while I don't recall specifically having a discussion about it, I may have asked him to try and install the new sub without removing the body. I'm regretting it now, obviously - particularly since it was a full renovation anyhow with a gutted trailer. Wouldn't have been that much additional work...

I like your idea of patching from underneath, but like the idea of patching from above even better since I'm still at the subfloor. It's a 3/4" panel, so using my router, couldn't I create a large channel to accept a rectangular 1/4" steel plate with countersunk holes that I would bed in epoxy and screw the panels into alignment?

If I'm missing something from an engineering standpoint, feel free to holler. For what it's worth, these front corners will be under dinette seats, meaning the walls they're attached to will also bear some of their weight.
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Old 12-18-2015, 10:12 PM   #8
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I have a 1986 25' frame and found that the front of the trailer had a 32" span between subfloor supports. I added two 2X3" rectangular steel tubes to reduce the span to 12". I could then fasten the plywood to the frame using #14 frame screws.

I don't seem to have pictures but I do have a video of the frame repair work showing the new steel added to the front (towards the end). I was wondering if you have the same thing going on. Just my two cents.

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Old 12-19-2015, 04:48 PM   #9
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Quote:
Originally Posted by sgschwend View Post
I was wondering if you have the same thing going on....
Cool, thanks for sharing that! In my case the bouncy portion sits on the outer portion, where there's no frame on the outside edge. I'd have to add a triangle bracing under that corner from the first outrigger back to the main rail.

I think in my situation, I'm better off trying to tie the bouncy subfloor section into the large, solid center section. But you sure did a great job fixing your sag - that's certainly a lot more support than the factory gave it.
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