This is probably a stupid question but here goes.
I’m putting up 1×3 strapping on sloped and flat ceilings. I’ve been using #9 2 in GRK R4 wood screws in pairs wherever a strap cross a rafter. These seem beefier than they need to be. I only ever need to shim things by 1/8 in at most. The rafters are old and I don’t want to abuse them any more than I need to. I’ve got #8 1-1/2 in GRKs but these seem too small. I can get spax #8 2 in screws. Do I need the #9 2 in or is that overkill and I should go smaller?
Replies
a number 6 would provide around 80# per inch of thread.
Number 8 would get you around 94# per inch.
Number 10 around 115# per inch of thread penetration
https://www.engineeringtoolbox.com/wood-screws-allowable-withdrawal-load-d_1815.html
https://spax.us/resources/technical-documents
Of course the actual value will depend on the density of the wood in your rafters, and would be reduced if the screw happened into a crack in the wood. In this case, the best use of these numbers is as a comparison between the sizes of screws.
If you ask me, smaller diameter is probably fine.
The load on each screw will depend on spacing of your straps and thickness of the drywall.
1/2 in light weight drywall at 40 lbs a 4x8 sheet is 40/(4*8)=1.25 lb per square foot. ~16 in o.c. spacing on straps over ~16 in o.c. spaced rafters, so 16*16/(12*12)*1.25=2.22 lb of drywall per attachment point. Nothing resting on the strapping or drywall. Don't know how much to account for vibrations, roof structure flexing, etc. Margin up to 5 lb per attachment point.
2 screws, #8 1.5 in, 3/4 in into rafters is 2*(1.5-0.75)*94 = 141 lb of withdrawal force. If only 1/2 in into rafter, 2*(1.5-1.0)*94=94 lb of withdrawal force. If using 2 in #8 screws, 2*(2-0.75)*94=235 lb.
On paper that sounds like serious overkill. Are there other factors assuming the wood is in decent shape? Tapering of the end of the screw?
Would only count the straight thread length
the actual diameter used depends on the design. spax makes a couple different kinds, for example.
you can also run a test or two. screw one in and see if you can pull it out. my guess is you will break the strapping before the screw comes out.