Joining plywood for slideout subfloor - Montana Owners Club

freckleslettucehead · 03-07-2022, 12:15 AM

We are in the middle of replacing our 3'8"x8'8" subfloor underneath the slideout for the dinette area in our '06 Montana 3500RL. We recently bought this rig and discovered upon its arrival that the subfloor in this area was completely soft and rotted thoroughly. We have stilted the slide out and removed the subfloor, and we now have two pieces of ACX 23/32 plywood cut and need to join them end to end (could not get ahold of 12' plywood). We want to join the two pieces with floating tenons along the lengths end to end and are hoping that will be strong enough. Does anyone have any wisdom on this?
The other option that we have read people try is layering one or more plywood sheets with the joints staggered. If we were to go this route, are we able to layer anything over 23/32" plywood? Our original subfloor is 3/4". We aren't sure how much play we have there to add layers. We could also start over with thinner plywood layers. Hate to do that if we can avoid it, since the plywood is already purchased and cut.

Jeff n Susan · 03-07-2022, 04:12 AM

I would highly recommend bonding 3 pieces of 1/4" plywood with a good strong polyurethane glue. This would give you the maximum strength needed for that floor.

rohrmann · 03-07-2022, 10:57 AM

I would suggest milling a lap joint on the two edges you want to join together, maybe a few inches wide. This way, you will have a large area to bond together. I would also suggest using a high strength adhesive like this so there will be no way moisture will affect the joint in the future. https://www.amazon.com/Cascamite-Pow.../dp/B0001OZI98

jcurtis934 · 03-07-2022, 11:12 AM

Both of the above work. What is done in boat building is to put a taper on the ends to be glued together. This can be done by an electric sander or an electric hand held planer and cheap ones can be found at harbor freight, northern tool, etc. The joint can be glued by epoxy, resoucrinol, gorilla poly, etc. The taper joint will be stronger the longer the taper, but no need to get carried away. My brain seems to remember a 10 to one taper so for 3/4 play it would be 7 inches or so. You should be able to check my memory by looking up taper joint construction on the net.

Drifty1 · 03-07-2022, 12:22 PM

When I replaced the floor on our living room slide I used marine grade plywood. I used 2 layers of 1/2” and just staggered the two seams at 2/3 and1/3 so the seam is 1/3 from both ends. Then I used liquid nails to adhere the 2 layers and then screwed them together to make one sheet the length you need. Hope this makes sense to you.

phillyg · 03-08-2022, 10:59 AM

Make a lap joint, and add a sheet of 1/8" plywood, staggered, glued to the 23/32" with construction adhesive, gorilla glue, or epoxy. If you've already cut the existing piece and don't want to waste it, rout a groove in each end and use a piece of hardwood as a tenon, and glue it up well.

freckleslettucehead · 03-08-2022, 11:22 AM

Wow, incredibly helpful--thank you all!!

So, I'm thinking of doing both things, as phillyg outlined. Lap joint in the 23/32" piece we already have, glue that together, and then add 1/8" ply over the top of that with a staggered joint. I have a few questions about the logistics of that though.

1. The total width of the floor with 23/32" + 1/8" plywood is going to be almost a tenth of an inch wider than the original 3/4" floor. Seems negligible, but want to make sure we're not going to run into problems re-installing that slightly wider floor. Along those same lines, would it be better to do 1/4" ply on the top layer, for stability? Or stick with 1/8" to most closely match the original floor width?

2. The cut we had made for the 23/32" plywood is right down the middle. If we stagger the cut in the 1/8" plywood so that it is 1/3 of the way from the slide wall, is that sufficient? I realize that ideally, we would have cut the first piece differently so that we could stagger the joints of the two layers more evenly. Is this significant enough that we should just start over with a new base layer of plywood with the cut done differently? Or can we get away with using what we have?

3. I can't seem to locate any exterior grade plywood in the 1/4" or 1/8" range. If the bottom layer is ACX (or some other exterior grade ply), can the top layer safely be interior grade?

4. Last but not least, we were told by an RV repair professional that a heavy duty tarp would be fine in place of the Darco to wrap the subfloor. Anyone have any objection to that? Can't seem to locate any Darco aside from off the internet and I don't think we have time to wait for shipping.

Thanks again! You guys are great!

jcurtis934 · 03-08-2022, 04:34 PM

See no issue with inside layer being interior or the nicer looking cabinet grade stuff. I would seal the edges of your ply and probably the bottom outside layer with epoxy to add a lot of durability. At least seal the edges to protect against delamination which will happen even with exterior grade. I would use something other than tarp on the bottom, but that's just me.

chessera2527 · 03-13-2022, 01:50 PM

Quote:

Originally Posted by jcurtis934

See no issue with inside layer being interior or the nicer looking cabinet grade stuff. I would seal the edges of your ply and probably the bottom outside layer with epoxy to add a lot of durability. At least seal the edges to protect against delamination which will happen even with exterior grade. I would use something other than tarp on the bottom, but that's just me.

you can get duraplast at Home Depot that is also good for the bottom

chessera2527 · 03-13-2022, 01:54 PM

Quote:

Originally Posted by chessera2527

you can get duraplast at Home Depot that is also good for the bottom

it is coroplast not duraplast

MARK A · 03-13-2022, 02:27 PM

Quote:

Originally Posted by rohrmann

i would suggest milling a lap joint on the two edges you want to join together, maybe a few inches wide. This way, you will have a large area to bond together. I would also suggest using a high strength adhesive like this so there will be no way moisture will affect the joint in the future. https://www.amazon.com/cascamite-pow.../dp/b0001ozi98

this..👆

DemLin · 03-14-2022, 07:52 AM

Sorry, I don't have an answer to your question, but I have saved all the replies for future reference since we have the same unit. You said you just purchased it, may I be so bold as to ask what you paid for it?

freckleslettucehead · 03-14-2022, 07:18 PM

Quote:

Originally Posted by DemLin

Sorry, I don't have an answer to your question, but I have saved all the replies for future reference since we have the same unit. You said you just purchased it, may I be so bold as to ask what you paid for it?

Yes, we paid 20K. Seller was asking 26K. Honestly, knowing what we do now I think it was too much. But we do love the layout of the 3500RL and I think we'll be happy for it once all this is said and done.

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