Fastening 3/4-in. Plywood Sheathing
I’m working on a house where the architect has specified 3/4″ plywood for the exterior sheathing. I talked to him and he offered two reasons. First, it’s a high-end custom home, it has lots of nice soundproofing features like 5/8″ drywall and cork underlayment under the floors, he feels it will give the house a more solid feel. But more important, there is 2″ of continuous exterior insulation, then furring strips and either shiplap or shingle siding. He’s worried that with the length of the nails needed to go through the furring and the insulation that regular 1/2″ sheathing isn’t going to have enough holding power.
My thought is just to make sure the nails go into the studs. Unless we go with fasteners beyond what a nailgun can shoot we’re limited to 3-1/2″ anyway which wouldn’t fully penetrate 3/4″ , my understanding is nails grip best when they penetrate sheathing. And with 3/4″ plywood topping $75 a sheet these days it’s a significant upcharge. There is a structural engineer involved and his section says “minimum 1/2″ sheathing” so there’s no permit issue with ignoring the architect.
Any thoughts?
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Replies
DC,
I'm with you. I can't think of any legitimate reason to ue 3/4" for the sheathing.
> There is a structural engineer involved and his section says “minimum 1/2″ sheathing” so there’s no permit issue with ignoring the architect.
Ignore a registered design professional at your own peril. Regardless of whether it's warranted or not, if it's on a drawing or spec and you ignore it, you're probably violating a contractural obligation, opening you to liability.
"Ignore" was a poor choice of words. "Overrule" is probably more fitting. Permit approvals are very slow right now and avoiding having to get a revised permit is a serious concern.
I doubt you will be using nails to attach furring strips to the framing if there is 2" exterior insulation. Most likely it will be done with screws. Sometimes they recommend using 3/4" sheathing so even if the screw misses the stud, there will be sufficient holding power.
Sounds like the architect is getting paid based on a percentage of the construction costs :) Since the engineer has signed off on 1/2", I would let the client make the call.
Your ability to use nails may be dictated by the siding you choose. I know that with their 5/16" siding Hardie requires 2" embedment of the fastener into the stud when using 3/4" furring strip over 2" of insulation, so you are looking at a 5" HeadLOK screw. With their Artisan shiplap, they require 2x furring strips so even longer fasteners. If you are using cedar shingles it may not be an issue.
I've used 3/4 ply for exterior. Besides cost, the only issue/benefit to it is that you can't tell if you miss a stud. The screws feel just as solid driven in. If you are in an area with a lot of wind, I would take extra care and re-screw any shiners.
For sound control you want mass. 3/4 ply is not that heavy. You are better off with either OSB or denseglass. Each time you double the weight of the material you add about 3dB of attenuation. This is not that much, most people find 6dB to be noticeable, and most sheathing options won't be double the weight of 1/2" ply.
3/4 ply is about 60lb, 3/4 osb is 72lb, 5/8 denseglass 80lb, 7/8 OSB is 90lb.
If you want to spend extra money for sound, your best bet is 7/8 OSB. It should be about the same cost as 3/4 ply.
If you want cheap and good sound control, 5/8 densglass is the way to go. Pretty quick to nail up on wood studs with 1 3/4" roofing nails. Does itch a bit.