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I'm considering use of GRK R4 #10 3.125" structural screws in a applications where it appears that the primary load on the screw is shear. The package states 1016 lbs ave ultimate load in wood w/0.67 density - but needs a safety margin.

I have two specific application in mind:

1) A ledger board that supports a small platform where upto 300 lbs steps down onto.
2) 2x4 x 11" cleats for a stair treads - also supporting 300 lbs.

How many screws are needed?

  • I edited it a bit - but it was vague as I want to avoid discussions of code - and want to focus on understanding the use of screws and safety margins, etc. Its open ended question as the topic is endlessly complex and correct (but useless) answer is almost always "you need to hire an engineer" – pathfinder Apr 19 '18 at 18:50
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Let's make a few assumptions to be clear about the situation.

  1. A 300 lb. person stepping down at a conservative maximum of 2 g's (600 lbs. of force)
  2. Load split between at least two supporting members
  3. No more than 80% load on any one member
  4. No additional load from other persons

This means that any one member should be able to handle a load of 480 lbs. plus a safety factor.

In my experience* with SPF lumber, screws of fair quality pull out before they break, except when experiencing repeated bending stress due to displacement of the joined members. This means that in the case of a well-fastened framing member, no significant movement should occur.

In my estimation, a member fastened into SPF lumber using four 3" screws (where penetration into the supporting member is at least 1-1/2"), and which is supported against rotation through being fastened to an overlaying member, will carry around 500 lbs. without undue risk of failure.

This is too close for comfort with your requirements. I'd go to six screws, recognizing that this may be challenging with your stair cleats. I'd also look for a heavier screw, say in a #12--pullout strength increases exponentially with screw diameter/thread area. Piloting to prevent cracks is key, and vertical staggering is also important.

So for our 450 lb. load, 12 screws or more would be adequate to support each plane. Again, it's not the strength of the screw that's our weak link, but pullout in fairly soft lumber. With that in mind, consider going with three or four 5/16" or 3/8" lag screws per member instead. You'll need fewer and have more peace of mind.


* As a guy who's built a whole lot of stuff over the last four decades but done little engineering calculation

  • Hmmm...450 or 480 lbs. load? Does S, P, or F or grade of lumber or outdoor location affect your recommend? – Lee Sam Apr 19 '18 at 23:52
  • This is what I needed to hear. Way more screws than I want to use. I'll use 2x4s to support the beam in the ledger - this is where I'm comfortable. Regarding the cleats - they would be SPF or SYP if needed, but definitely will screw into SYP dense select structural. I want to avoid lag bolts as I expect to have to relocate the cleats at least once (tight space w/many difficult compromises). Longer term there would be a a riser ?? that connects adjacent treads vertically - thus sharing the load. I can add all this to the question.... – pathfinder Apr 20 '18 at 1:04

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