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Consider a standalone, 4 post, 7' high deck, with joists 12" on center, with OSB subfloor sheathing fastened with screws. Would using glue between the joists and sheathing increase rigidity of the floor and/or overall structure? Or would it be a waste of time?

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    A deck. So, it gets rained on. And you're sheathing the "subfloor" of the deck with OSB. Have fun in 2-5 years when it turns back to woodchips...
    – Ecnerwal
    Feb 28 at 14:48
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    @Ecnerwal, we sometimes refer to floor structures as decks in the field. For example, "we're building the floor deck today". I don't think this is a recreational outdoor deck.
    – isherwood
    Feb 28 at 15:05
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    "standalone; 4 post; 7 ft high" all vote the other way.
    – Ecnerwal
    Feb 28 at 15:12
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    With joists spaced 12" OC I'm not sure you're going to get a significant increase in rigidity by adding glue. That's already 25% narrower spacing than normal. Why are you using 12" OC instead of a more standard 16"?
    – FreeMan
    Feb 28 at 15:49
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    @FreeMan I assume that the span is long or that stiffness requirements are higher, as for a spa load. It's not particularly relevant to the question at hand, which is about improvement to a given scenario.
    – isherwood
    Feb 28 at 15:53

2 Answers 2

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Glue increases the joist stiffness assuming that you use the correct glue. That's the only structural benefit. ASTM D3498 is the specification for the glue that you would use. Basically the stuff has good strength even with slight gaps, cold weather, and wet joists. Here's a marketer's write up of D3498: http://www.titebond.com/App_Static/literature/adhesives/FF1210_ASTM_D3498-19_Sheet.pdf.

The idea with the glue is to develop the floor sheathing and joists into a composite section. Instead of rectangular joists, you instead have a bunch of tee-shaped joists.

Glue can additionally prevent squeaks, although using screws probably already achieves that. Personally, I would rather spend the glue money on an extra 2" of joist height instead.

In Response to isherwood's Comment:

How does glue provide the stated benefit when used in addition to standard fasteners? Are you suggesting that there's that much movement without it that the sheathing is decoupled?

I ran numbers on a hypothetical 10 ft long 2x8 spaced on 12" centers with 5/8" thick floor sheathing. 10 psf dead load plus 40 psf live load. Assuming that the whole 12" flange is active (in steel design, there's an upper limit on the amount of slab you can include), I get a "shear flow" at the beam/sheathing interface of 30 #/in. Based on the 75# mode IIIs strength of a #8 by 2-1/2" screw, the 30 #/in implies (75#/fastener)/(30#/in) = 2.5 in/fastener spacing. Clearly the typical fastening schedule for floor sheathing doesn't have the strength to develop the joist plus sheathing into a composite section.

Moving away from the ends where the maximum shear occurs, however, there's certainly some sub-interval where the fastener spacing transitions to sufficient. Over this sub-interval of the joist, the screws are adequate, so you're right that the screws provide some stiffening. In this hypothetical example, that sub-interval is only 2 ft long.

Will the joist ever actually see the 50 psf total load? Maybe not. If it ever does, though, all but the middle 2 or 3 fasteners will move a little inside the joist, the joist will sag, and the old stiffness will never come back.

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    Don’t forget blocking between the joists and some form of bracing on the posts. Feb 28 at 14:47
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I'd bet real money that you couldn't tell the difference in bounce between two identical floors, one with subfloor adhesive and one without. That's not why it's used. In theory, bonding the sheathing to the joists increases their total rigidity, but 90-some percent of that is already accomplished by good mechanical fastening (ring-shank nails or screws). There's very little left to be gained with glue.

It's worth using adhesive for noise prevention, but not for stiffness. For stiffness, use bridging, which boxes the framing into a unit. Or just increase joist depth slightly. If floor height is an issue they can often be lowered with respect to the beam or ledger.

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  • especially when you look at the bonding strength of something like PL400 subfloor adhesive. The stuff seems so soft and easy to remove I can't imagine you get much strength out of it. you certainly get the resistance to squeak though. If you used PL premium or a construction adhesive foam you might tear the plywood/joist before you decouple it. Feb 29 at 17:27

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