Would using glue between the joists and sheathing increase rigidity of the floor?

Question

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?

Answer 1

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 III_s 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.

Answer 2

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.

Answer 3

We (Climate, Inc.) manufacture high performance polyether adhesives designed to strengthen light-frame wood structures. The characteristics you want are excellent elongation (around 1000%), class 50 (50% expansion/contraction) along with 250-300 psi shear strength and excellent adhesion. If your adhesive has these characteristics, then you get very good energy dissipation along with excellent resilience. The product should be able to move with the structure (like a shock absorber) without compromising bond lines. We recommend the adhesive to be used at wall/roof sheathing and truss connections. This will increase the strength of a home by up to 300%. You could plan on similar results with a deck application (although I'm unconvinced about the OSB here). Cost is very modest for the ROI.