Floor question-

[RogHodge]

I’d like to get some input about my floor construction:
the plan is torsion box/hollow core door/ whatever you call that-
Thin as reasonable ply on top and bottom- probably 1/8 – the question concerns the skeleton or ribs- what material and orientation- My thought is that the spacing on the ribs will determine the flex and safety- I’ll place them across the frame- like side to side and on 12â€

[Miriam C.]

I hope I am remembering weights right:

1/8 birch is about 11# per sheet

Weight of Plywood?
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The easiest way to determine an honest weight of a 4' x 8' sheet of plywood?

20-25 pounds per 1/4" of thickness.

3 pounds per square foot per inch of thickness according to APA Plywood Design Specification.

You can add foam to add stiffness. You will need to have enough thickness in the framing for screws/nails to hold and use really good glue.

Stand your spars so the thickest part is upright and supporting weight.

[aggie79]

Hi Rog,

I don't know how you plan to attach your walls, but poplar will hold screws better than yellow pine or douglas fir, or the edge veneers of plywood. As far as thickness/depth, 3/4" will be sufficient if you skin both sides.

On my floor, I used 1/4" plywood, but only glued (PL Premium) the wood crossmembers to the plywood, and did not glue the insulation. On my roof, I used 1/8" plywood top and bottom, but glued both the spars and insulation to the plywood. Even on the flat area of my roof, there is less deflection in the 1/8" plywood between the spars than there is in the 1/4" plywood between the crossmembers. So, I would say apply glue to the whole assembly.

One thing I have encountered with the 1/8" baltic birch plywood, is that it is so thin that it is "wavy" at times. The 1/4" is flatter. This may be something to consider in that the 1/8" plywood may be a little harder to build "flat" unless you build your floor on a workbench. Also, the "thinness" of 1/8" plywood gives some challenges on holding fasteners - particularly staples from an air nailer - unless you use some additional type of backing material.

If you use furring strips, you may want to bring a tape measure with you. In our area, a 1x2 is 3/4" x 1-1/2", but when you buy "furring strips" the actual dimensions are smaller than 3/4" x 1-1/2".

Good luck,

Tom