Floor thickness question, sandwich floor will 3/8 ply work?

[ajliquidation]

My 4x8 HF frame is done! Now time for my floor. Went to buy all my supplies and now I need some advice. I am going to do a sandwich insulated floor using plywood, foam, and 1x3's. I was looking at 1/2 inch ply wood and it seemed like overkill and 1/4 inch doesn't sound right. My question is will 3/8 exterior plywood work? Seems like a good middle ground and I want my base to be strong, thanks Adam in KC

[KCStudly]

For a lay down sleeper (non-standy/non-slouchie) 5 mm or 1/4 inch backed by foam is plenty. I'm well into my Benroy-ish build and have no regrets over using 5 mm marine grade Okoume (3/16 inch actual) top and bottom over a cedar frame filled with 1-1/2 inch foam, all surfaces glued. The Okoume is 5 plys and has very few/limited voids, so might be a touch firmer than typical 1/4 inch underlay.

I did use rosin paper and a waster piece of 1/2 inch ply (that later became my galley counter underlay) to walk on during the bulk of construction, but that was mostly for piece of mind and probably wasn't necessary (tho I would do it again).

[aggie79]

I used 1x2 framing "flat" and 1/4" ply top and bottom. This is plenty stout.

[Pmullen503]

3/8" ply, supported by the foam and 1x3s with be plenty stiff. My floor is 1/4" ply with 2x4s and EPS foam fill and I can walk around on it fine. The plywood is supported every two feet in both directions.

[ajliquidation]

Thanks everybody, the 3/8 plywood should be a good for me then. I'm afraid of overbuilding but more afraid of under building!

[MtnDon]

15 PSI foam like XPS can withstand the normal walking on loads with no problem. 1/4" plywood would be okay unless there were going to be point loads like walking on with stiletto heels. What is planned for on the underside of the foam? There is no need for 1x or 2x members except for around the perimeter to cover the foam and provide something solid for screws to bite into. The foam can carry 15 PSI. We pour concrete slabs on XPS foam all the time; 15 PSI works for a single story house.

[ajliquidation]

Mt Don... Great point sir, hadn't thought about it that way. Makes my 3/8s sound like overkill doesn't it!

[KCStudly]

Makes my 3/8s sound like overkill doesn't it(?)

Yup.

[kayakdlk]

I used 1"x2" and 1"x3" pine for a frame with 1/8" Balitic Birch plywood top and bottom with 3/4" foam in the middle. Once it was glued together I could stand on it in tennis shoes.

Happy Building
Dan

[daveesl77]

I just finished laminating my floor (5 x 9) on the HF frame. I used 1/2 ply, 1" (true) framing, 1/2 ply with all interior voids filled with 1" blue board. Everything glued and screwed. It is very, very strong and light enough. To be honest, it did not need to be this big of a build and 3/8 ply would be more than enough. Probably even going down to 1/4 would have worked, but 3/8 would be fine.

Here is a link to my floor build gallery
http://www.tnttt.com/gallery/album.php?album_id=3853

dave

[Gunguy05]

1/4 top and bottom sandwich with 3/4 and extruded foam and in the middle. More than strong enough. You are probably having the same issue that I did... Trust the experiences of others posted here. It will have you time, material, and most importantly weight. Good luck and keep us posted.

[javajaws]

On a related note - what foam should be used in between for this type of floor? The white stuff, pink stuff, or??? And should you be gluing the foam to the plywood (and if so, what's the preferred glue to use)?

[MtnDon]

Blue or pink XPS foam has the better crush rating and a better r-value.

I would use glue. There are foam friendly adhesives in tubes. That is important as some glues will dissolve foam. I use one of the loctite brands, they have a few that are foam friendly. I believe the one I use is PL300. Check the label or Loctite's website info. One surface must be porous (wood, concrete....). A light mist of water speeds the cure. Hold together with weights till set.

[javajaws]

I've cut the pink stuff before - and noticed it has a plastic sheeting on it that will delaminate over time. Do you'll remove this or glue right to it?

[daveesl77]

the foam boards do have a plastic sheeting over both sides and should be removed prior to gluing. It comes off very easily.

With my floor, I first coated the internal pockets and all wood with a 50% dilution of TB2 and let it dry for about 30 minutes (70+ degrees F in shade). I then went full strength using a 1/16 notched trowel to smear over all surfaces. I had mine set to where the top layer of ply (also initially coated with the diluted mix and dried) was cut and ready for attachment. I did leave very small air chambers to allow some air to get in during the curing process. These will be sealed in the next couple of days. Top layer was glued down to the framing and weighted. It has now set for about a week. I climbed on it yesterday, jumped on the overhangs and it is like a solid structure, absolutely no give or noise.

Like I said earlier, I way overbuilt mine. Could have definitely gotten away with 1/4 ply instead of 1/2.