I’m closing in a 24×30’ pole barn with a raised wood framed floor (massive slope, so a slab would be $$$ and I’d rather it be wood anyway). Joists are 2×10 on 24” centers. I know 3/4” is minimum plywood for 24”oc, but I’ve also read a lot on here about pros and cons of using multiple layers of plywood instead of one thick one.
My idea: 1/2” CDX on joists (glue and screw as it should be), thin layer of rigid foam board, then 1/2” better looking plywood as top surface (screwed through foam and into bottom ply).
My questions: previous posts on this site have proven that 2 layers of 1/2” ply have less deflection than 1 later of 3/4”, even with no glue. So would this be a safe way to span 24”oc joists? The only “tests” I’ve read about were on 16”oc, but in my head, less deflection over any span should mean less deflection over any other span.
Also, the rigid foam board is multipurpose – decent insulator on an otherwise uninsulated floor, helps reduce squeak with multiple layers of ply, and (even if marginal) an added bit of deflection resistance, given that it does have its own rigidity to an extent. Actual question: will the foam/ply sandwich do what I need it to do over 24”oc? The joists are in, up to code, and not moving, so we can skip the “should’ve done 16”oc”. The top ply would be the “finish” floor – this is nothing but a closed in barn to be used as a shop building. Weather tight, but not trying to keep it comfortable room temp year round, so insulation isn’t an issue. Also I don’t really care too much about little squeaks or appearance.
2 layers of ply with rigid foam in between – acceptable sturdy sandwich? Also, with foam in between, would you screw the top layer straight into joists or only in the field? Pros/cons? Thanks!
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I think it will be much stiffer, not because of the added stiffness of the foam on it's own, but because of the added depth of section provided by the foam. Like a stress-skin panel. Glue it up well and it should act as one thick sheet.
FWIW we used to use a PPG paint called 'Breakthrough' on stage floors - tough stuff, a bit gnarly to work with
That's too much span. Make a mockup and try it. If the plywood was touching and screwed together it might work but not with insulation in between.
Florida, I’m not sure I understand. If all are glued and screwed to act as one, why would it be less rigid than one piece of 1” ply? The insulation in between would be thin, but it’s the kind used in home subfloors every day, so I wouldn’t imagine its presence would somehow weaken the overall rigidity? Maybe I’m not understanding the line of thinking. Thanks!
"My idea: 1/2” CDX on joists (glue and screw as it should be), thin layer of rigid foam board, then 1/2” better looking plywood as top surface (screwed through foam and into bottom ply). "
You didn't say you'd be gluing the foam to the plywood but I doubt it would make much difference. When you say 'thin" I'm assuming 1/2" to 3/4" foam. Assuming you glued the whole sandwich together which in my opinion would be very expensive and very labor-intensive it might work but it might not. I still think 24" span would be too much. I also think that it's a hard way to go about getting not much insulation at a high cost. 3/4" plywood will span 24 inches but will be springy in the mid-span.
As I said before, make a mock-up, should be cheap to do, and see how it deflects in the real world. Since you've already framed it in put down 1 X 4" straps on 16" centers and use 3/4" ply.
I hadn’t thought about doing straps/sleepers on top of joists - interesting idea. If 1x4 would be enough... By that thought, how about ripping some 5/4” deck boards 2” wide and using them in strips 16”oc? I’m curious as to what thickness/width would actually be necessary? Obviously thicker/wider is always better but cost is a consideration, and it’s just a shop building, not living space. Thanks for the thoughts!
You could certainly do that but I don't see any return unless you have old deck boards that are free. That's a lot of sawing labor.
By the way, I wish you'd do a test of the foam and layered ply. I could be totally wrong and it would be interesting to know. We used to have a place here that made SIPs and they sprayed glue on the foam and plywood and then left it in a bag press overnight to bond.
I'm in the process of building a similar floor, but mine is 16"oc, 5/8 Advantek OSB first layer, 4" iso foam, and finally 1/2" cdx ( a shop floor, and the batch of plywood actually isn't bad looking). I'm with Florida on how to approach your idea. A mock up could save you a lot of grief. I have walked on 2" foam spanning 24" by itself (140 lbs) and it was stiff enough for me to be comfortable. It wouldn't be a big deal to use a roller to apply glue to the foam and make the it all a sound unit like a stress skin. I think glue and rigid foam has more uses than anyone has explored. I've used XPS ripped into stud dimensions to help add drywall support and avoid thermal bridging in corners, and I'm sure foam has other semi structural uses.
How about some solid wood 1 1/2'' tongue and groove and shoot insulation from underneath ?
For my 24x30’ barn, that’s way more quality wood than I’d want to buy for flooring. At 4” wide, I’d need 2100+ board feet. Also, I’m not necessarily trying to maximize R value with floor insulation. That might seem crazy, but frankly it’s a closed in pole barn being used as a shop for very small amounts of time, relatively. That’s why I’m thinking a relatively thin layer of rigid foam board will at least be an air barrier, prevent squeaks, and have some insulating effect as an added bonus. It’s an open crawl space in Alabama. My feet (wearing shoes) don’t really care if the floor is cool in winter. Just trying to get the thing weathered in without spending overkill. I’m still curious about the idea of straps/sleepers 16oc, but I wonder what thickness/width would be necessary
As for the strapping, 1x4 or 3/4" depth deflects based on its depth (like all spans do). Doubling its width merely doubles (x2) its strength while incremental increased in depth cubes its deflection strength. So 5/4x2 will be stronger than any 1x4 (unless it's set on its thin side). For the cost and effectiveness of the glue and minimal insulation, look into 9/8 t&g subfloor plywood and save a ton of/on time. I spec 3/4 max on 16" o.c. floors and 9/8 on 24" oc.
Thank you for giving some cred to the science of thickness vs width and deflection! I did consider 1” or even 9/8” ply, but with this being an open crawlspace in super humid Alabama, I figured I’d need a plywood made for withstanding exterior conditions (basically if it doesn’t have an X in its name, I’d worry it’d eventually crumble). That’s why my original thought was 1/2” CDX and 1/2” (or yes even 5/8”) of a more attractive ply on top of it. Another reason this came to mind that I haven’t mentioned - the only help I have is my 10 and 12 year olds, and while they’re willing to help, 9/8” plywood is HEAVY and maneuvering 24 sheets from store to trailer to joists just might be more than I (and my mini crew) want to wrangle. Hence the 2 layer line of thinking. This is why I want you experts to chime in!
I believe the deflection of wood, or any material, follows the Square law not the Cube law as mentioned previously. If you use 1 layer of 1/2" plywood you get an amount of deflection for a certain load. If you use 1" plywood you get 1/4 of the deflection or 4 times the strength for the same load. If you use doubled sheets of 1/2" plywood, not glued and screwed together, it acts like 2 separate sheets and you get 1/2 the deflection or 2 times the strength. If you glue and screw the double sheets together they will act as a single layer of 1" plywood and you get 1/4 of the deflection or 4 times the strength for the same load. It will act like a 1" sheet of plywood.
Now if you sandwich a foam layer in between it will not be rigid. the foam doesn't have much strength and it will act like 2 independent sheets. BAD IDEA if you want strength! Unless you need insulation?
I hope this helps your understanding!
Your humble servant
Marvin F. Malm PE (ret)