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I will be fastening Unistrut vertically along studs to facilitate garage storage, and want some guidance on choosing the "best" lag bolts to use.

I want them large enough to handle hundreds of pounds of shear and tensile load, but small enough that I don't significantly weaken the 2x4 itself.

2x4's are only 1.5 inches wide. And mine are hidden behind drywall on both sides, so I'm not confident that I can hit them exactly centered. I could be off a bit if the wood has knots or the 2x4's are less than perfect.

e.g. If I was a half inch inch off from center, and drive a half inch bolt, it probably would pop out the side of the stud, right?

What's the maximum percent of a structural member's profile you'd want to drill out to install a bolt before you'd worry about reducing the strength of the member? For instance, I'm sure it would be self defeating to install a 1.25 inch diameter lag bolt in a 1.5 inch wide stud, even dead center, because the remaining wood would be so weak you could sneeze it in half.

UPDATE: I went on to test four options:

  1. 4" 'Traditional' lag bolts hex head
  2. 4" SPAX Lag bolts hex head
  3. 4" SPAX Pan-head Torx drive
  4. 4" GRK RSS Pan-head Torx drive

I made four test jigs out of 2x4's. I assembled them perpendicularly to form a t-beam, but with a 5/8 inch spacer between to create a gap in which I could measure deflection. I pre-drilled the 1.5 inch thick top beam so that it woud not grep any of the threads. I also used the same 5/8" washers with all bolts. The perpendicular 3.5 inch beam was only pre-drilled for the traditional lag bolt, as the other three did not require it. This perpendicular bean gives the threads the full thickness of a stud to screw into as it would in a residential wall.

My results were as follows:

  1. Traditional lag bolt easily stripped out the stud and lost traction. I was not impressed and it at no point felt "tight".
  2. SPAX Lag got tighter, but still stripped out the wood it was drilling into quickly.
  3. SPAX Lag drove itself in SO tight, that it sunk that 5/8" washer 3/4 inch deep into the top stud before I backed off.
  4. GRK RSS sunk into the top stud about 1/4 inch before starting to strip, but it was starting to strip the base stud with enough "notice" for me to stop.

I chose the SPAX Lag as hands down the best choice. I will be using 7 along each piece of unistrut to firmly anchor each to the vertical studs, and also to the sole plates and double plates. I am confident that because of the relatively small diameter of the shaft of the SPAX Lags they will not meaningfully degrade the strength of the studs. And between their strength and the strength of the unistrut, the entire wall will be reinforced.

At this point my greatest concern is that I don't tighten them down so much that it pile drives the unistrut right through the drywall like its playdough. The SPAX lags have enough pulling force I wouldn't put it past them at this point to just crush the drywall in their way.

The GRK RSS were very stout feeling as well. I got the impression they would be better served in harder woods, or into concrete. GRK RSS felt substantially beefier than SPAX panheads. But the SPAX Panhead lags just worked out better in pine framing 2x4's.

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The bigger question is how many vertical supports and how long are they? 1/4" lags 2” into the 2x4 has 510lb pull out strength (255lb/inch) put 2 or 3 in each piece of strut and each one can handle hundreds of pounds. 5/16” lags 266lb/inch; 3/8"lags 305lbs/inch. You don’t need a very big lag bolt when using multiples. My examples have a small safety factor added in and these won’t degrade the strength of the 2x4. I usually drill pilot holes, But not all brands of screws need pilot holes. The depth into the wood is the threaded part that provides the pull out strength over ½" sheetrock I would make sure to have at least ¾" -1” smooth shank that provides the sheer strength as the weak point is the start of the threads.

  • Good points. I'm trying to recall the last time I saw lags (or even standard construction screws) break our pull out. For utility shelving standards I often use #12 or #14 screws, considering that there are 4-6 per standard. Lags are overkill in that case. – isherwood Jan 7 '16 at 17:05
  • Thanks Ed! This helps a lot. Is there a formula you used to derive those lbs numbers? or can you link me to a table somewhere? Some of these struts are going to go from trim to ceiling (unfortunately not resting on floor). At what size would they begin to degrade the 2x4? I'm sure a 1 inch diameter is too much, and 1/4 is pretty safe. So where does the grey area begin? – Billy C. Jan 7 '16 at 17:47
  • these were from a very old code book. just now I went to the internet and typed -lag bolt strength in wood- there were several charts. you also want to look up pull out strength if the bolt is two short it will pull out I havent had a shear issue. I think at 3/8 it becomes excessive because with 7' length 4 1/4" 2 inches into the stud is going to get close to some unistrut max loading (at least the aluminum 1/2" deep) – Ed Beal Jan 7 '16 at 17:55
  • yeah. I've googled it before, and it seemed there were just too many different answers. I don't know what wood was used, but I would assume the cheapest pine they could get away with. – Billy C. Jan 7 '16 at 18:22
  • Based on averages of what I saw on those charts online, I was going to put 5/16ths lags screw at either end, then either two or three evenly spaced between, with as much consideration to getting it to lie flush to the wall as anything else. But then I started hemming and hawing over what margin I would have to sink each one centered enough in the studs, which I can't visually see. – Billy C. Jan 7 '16 at 18:30
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Studs in a wall are individual parts of a system. Unless you're talking about an unfinished load-bearing wall in a basement, for example, you have many studs, drywall or other sheathing, and possibly blocking or bracing all working together. Even cutting a single stud half way through doesn't substantially weaken the wall. There are many cases of 2" plumbing running through 2x4 walls with 2-1/2" bores in the 3-1/2" wide studs.

Your question should be simply, "what size lag screw is appropriate for my hardware and load?" If you properly pilot for the screws, you're unlikely to cause much damage to the studs, and you'll have a solid anchor.

If you found yourself needing a 1" bolt, you probably wouldn't be anchoring it to a 2x4 stud anyway. That bolt would tear the wall apart long before it would pull out. In that case, the load you're fastening to the wall is far too great and you'd seek other solutions.

  • Man, you're a question answering juggernaut lately! – JPhi1618 Jan 7 '16 at 16:31
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I use SPAX or GRK construction lags for that type of stuff. They are extremely strong, and don't require pre-drilling. I have never broken one off.

You can get them at Home Depot.
Construction Lags

  • And they're much narrower for the same strength, and so don't compromise the stud they're screwed into – Joel Keene Jan 8 '16 at 16:03
  • Where are practical strength ratings for Spax in pine 2x4's I could find an engineering doc with tables, but I think it was referring to just the metal, not what to expect of the fastener in wood: icc-es.org/Reports/pdf_files/ESR-1782.pdf And How do SPAX washerheads compare to HeadLok's? – Billy C. Jan 9 '16 at 10:46

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