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3/4 T&G Plywood vs 1 1/8 for a Su...

alan_joseph_samson's picture

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Gordon, I think it is called a torsion box , not a box beam per se. But, yes, you offer a great response to this question.

And, for me it stimulates the following question:

Can the size of the joists be made smaller when both sides of the floor are sheathed so as to form a "torsion box"?

My goal is to achieve a house made of torsion box walls, floors, roof, etc, so as to connect with Simpson Strong-Tie type rigid connectors.

My purpose is strength.

Aside from that, the question of sound transmission when partying:

I know in the old days the 30lb felt was the best one got. Has it improved any?

Hope this helps.

(post #172051, reply #3 of 10)

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We build steel frame homes. Our floor joists are 8" deep, 16 gauge to 12 gauge, steel C sections spaced 16" o-c. Our floors are designed for a deflection of L/480, twice as stiff as a wood construction. We can obtain this stiffness using 3/4" plywood but the floor feels much stiffer with the 1 1/8".

Just my $0.02.

...Mark

(post #172051, reply #5 of 10)

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The 1-1/8" is much more solid than 3/4". With the 1-1/8" it sounds and feels like you are walking on a concrete floor. Also, the tongue and groove is much stronger with the 3/4" so you get less deflection at the seams.

(post #172051, reply #7 of 10)

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I use 3/4" T&G for the subfloor, and then 5/8" particle board for underlayment with 15# felt underneath. No underlayment is used if a finished hardwood floor is planned. I use 5/8" exterior grade plywood underlayment in the bathroom and kitchen. After the house is framed and dry and before the tub or enclosure is installed I put down the 5/8" underlayment. In my opinion this makes an excellent floor for both carpet and linoleum and fits under the category of a sound building practice (i.e., not just minimum code).

(post #172051, reply #2 of 10)

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Gordon, I think it is called a torsion box , not a box beam per se. But, yes, you offer a great response to this question.

And, for me it stimulates the following question:

Can the size of the joists be made smaller when both sides of the floor are sheathed so as to form a "torsion box"?

My goal is to achieve a house made of torsion box walls, floors, roof, etc, so as to connect with Simpson Strong-Tie type rigid connectors.

My purpose is strength.

Aside from that, the question of sound transmission when partying:

I know in the old days the 30lb felt was the best one got. Has it improved any?

Hope this helps.

(post #172051, reply #9 of 10)

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Is there any perceptible benefit to substituting 1 1/8 in T&G plywood subfloor material for the standard 3/4 in.? The floor joists are " I-beam type" spaced 16 inches on center. The additonal cost per sheet is approximately $8. Will I notice the difference walking, bouncing and partying on the floor? Thanks in advance.

Larry

(post #172051, reply #1 of 10)

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With 16 inch spacing, 3/4" is plenty strong. The 1 1/8" stuff on 16 inch centers would be noticeably stiffer but to what end. The 3/4" makes an excellent subfloor. Any bounciness is going to come principally from the sizing of the floor joists. The original plans for our house called for 2x10s for floor joists. I was going to substitute 9 1/4" Trust Joists. I talked to the the Trust Joist engineer and he suggested that I go to the standard 11 7/8" joist over a heavier version of the 9 1/4" joist. Both had similar ratings for floor loads but the deflection for the 91/4" joists was much greater. The price was the same and I ended up with a much stiffer floor. If you have a problem where the floor may be on the bouncy side, I would go with the 3/4" subfloor and then glue and screw 1/2 CDX to the bottom side of the joists to a make a box beam. This would be a lot stiffer then just putting on the 1 1/8" subfloor.

(post #172051, reply #4 of 10)

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Alan - I think torsion box is a vernacular term and that APA uses the term stressed skin. Putting a skin - plywood in this case - furthest away from the the axis with web members or honeycomb or foam uses the strength of the plywood and the joist or webs become almost irrelevant. APA (apawod.org) has the design data for this. You can get much stronger and stiffer floors (decks? membranes? may be better terms) with less depth, but acceptance by the a.h.j. would be a chore, I suspect.

I have built several for short term use - usually with 1 by webs; foam is more common now in production facilities but 1by or 2by would still be easier for site built. Glueing is critical, as well as splices between sheets of plywood and laying out. And construction adhesive is no good for these - too little shear strength. But, for instance, you can build a deck 16' long with maybe 1X4's and 1/4" bottom, 3/8" top - and one person can carry it and walk across it! I've been thinking about building one for my pump jacks but whenever I get around to it, it seems easier to set up one more pump jack.

I once had a spread sheet that would do calcs but don't think I could find it today - many computers and versions of software ago. There are a lot of calcs and as I recall some are reiterative and I was still using slide rule, but apa document is very clear.

(post #172051, reply #6 of 10)

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You can indeed use smaller joists in a "torsion box" or box beam design, but as another poster pointed out you may have some problems getting it accepted by the inspector without engineering analysis and approval. It was also correctly stated that splicing the plywood correctly is very important in such a design. I offered the suggestion not as a structual element to replace some other element in the floor system, but as a way to improve the stiffness of an existing adequate floor design. In the latter case splicing of the plywood makes it better but it is not required. Attaching the plywood so that its long axis is parallel with the floor joists will improve the stiffness.

In a previous house that we owned, the floor joists were on 24 inch center and spanned about 14 feet. The floor could easily carry its design load, but it was very bouncy. When we finished the basement ceiling below this floor i.e., installed 1/2" drywall, there was a noticeable improvment in the floors stiffness. Drywall is certainly not a structural element, where as plywood would be. I have no doubt that using 1/2 inch plywood or even 3/8" plywood would have made the floor significantly stiffer than the dry wall did. In a box beam or stressed skin design, the outer skins become a principal structural element in the system. The construction of this system becomes very important to insure proper performance. As stated earlier my suggestion was to enhance rather than replace.

(post #172051, reply #8 of 10)

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Gordon makes good points. If you're just looking for some reduction in deflection, 4X8 sheets at the middle of the span, face grain parallel with the joists, would do a lot. (e.g.: 14' span would be sheathed in center with 3' each end of joists unsheathed. I'm not sure but would probably wager you'd get better results if the seams between adjacent sheets were centered between joists because of the better glue joint. Think of them as "i" beams.

Of course there are always truss rods. :)

(post #172051, reply #10 of 10)

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I used 3/4 T&G ply where the finished floor will be hardwood and 1-1/8 T&G where quarry tile will be set in thinset, both glued and screwshank nailed over 11-7/8 I-joists, spanning 14'. There is a noticeable difference walking across these subfloors. When stepping between joists, the 3/4 feels "softer", which in some cases may be desirable, but not under tile. I'd go with 1-1/8 before gluing and screwing overhead...arrghh.