Will attaching to ceiling joists with lag screws weaken trusses?

Question

I have a 12ft high ceiling in my garage. My garage measures 13’ wide x 21-1/2’ long. The ceiling is finished w/ drywall but has no insulation or a room above it. The joists appear to be 2x4s (I can’t get to them to measure) and I want to install a channel strut trolley system (with electric hoist), to lift and raise a hard top from my Jeep Wrangler. The hard top alone weights approx. 150lbs and I’ve estimated the total weight of everything (to include the hard top) to be approx. 235 lbs +/- another 5-10 lbs. My plan is to build an H-type system and distribute the weight and run lag screws into 6 different joists. I plan to install 2 strut channels parallel to each other and about 8ft apart. Then I want to use a 3rd channel strut w/ trolleys for the hoist, completing the H shape.

I’ve been getting conflicting replies when I ask about screwing lag screws into the joists. I’ve seen many YouTube videos where there doesn’t seem to be an issue, but people I’ve spoken to say that I will weaken the structure of the joist. I’m confused and could use some advice or insight on if I should scratch my project or do it and hope for the best and pray that my garage ceiling doesn’t come crashing down onto my Jeep.

Answer 1

The probable problem with this idea is the channel's poor bending stiffness (these are those Unistruts that the electricians are always playing with, right?). If the parallel channels were very stiff, then the channels would remain perfectly flat under their 250#/2 = 125# load at their midspans. Perfectly flat means that all 6 joists deflect the exact same amount and therefore experience the exact same increase in stresses.

Instead, under your kinda flimsy channels, you can imagine the parallel channels each deforming into a rippled shape, with wave peaks and wave troughs located midway between joists. The wave heights will taper off the further you look from the load. As an interesting thought exercise, think about this ripple pattern but with a total of only 4 joists. In particular, look at an outer joist and the channel wave crest beside it: The flimsy channel is pushing up on the end joist, not pulling down! So in addition to the 125# load itself, now the two interior joists also have additional load from the outer joists pushing down! Holy geez! (I lied a little about the wave peaks and wave trough locations--they're very near midway between joists, but they don't quite hit the midpoints.)

Ceilings are typically designed for 5 psf dead load, where 1/2" drywall plus 2x4 framing on 16" centers leaves 2.5 psf of unused capacity. That extra capacity translates to a parallel channels load at each joist of

(1/8)(2.5psf)(16in)(1ft/12in)(13ft)^2 / [(2.5ft/6.5ft)(1/4)(13ft)] = 57#,

where each of the parallel channels can exert half of this load on the joist. By my reckoning, then, you would need to span (250#)/(57#/joist) = 5 joists with near perfect balance in your lag screw load at every joist.

Sounds like you never go up in that space, so I guess you could gobble up some of the attic's live load capacity for this project. Without knowing more about the attic, it's somewhat safe to assume a 10 psf live load capacity up there. That implies a per-joist available strength of

(1/8)(12.5psf)(16in)(1ft/12in)(13ft)^2 / [(2.5ft/6.5ft)(1/4)(13ft)] = 290#.

Under this schematic a single joist is adequate, leaving a live load capacity in the single joist of

12.5psf - (2.5ft/6.5ft)(1/4)(250#)(13ft) / {(1/8)(13ft)^2[(1joist)(16in/joist)](1ft/12in)} = 1.4 psf.

That's equivalent to 12# applied at midspan, so don't do that.

One consideration when designing a structure is whether loads will occur simultaneously. For instance, you wouldn't design a structure to survive a 50 year return interval wind storm striking at the same time as a 50 year return interval seismic event. If you do gobble up some live load capacity, then just remember so that if a couple of guys are going up there to insulate or something, you can set the hardtop down on the ground or something.

Oh, and the AWC maintains a connection calculator. that will spec your lag screw withdrawal strength for you (use ASD). If you don't know your joist species, then use Spruce Pine Fir (SPF) for your main member. For the side member use steel. Don't mess with any of the factors at the bottom.

Answer 2

Using lag screws in ceiling joists (whether engineered trusses or hand-framed rafter ties) is not a problem. See every modern overhead garage door in existence, along with all the canoes, bikes, storage racks, and other junk we hang in our garages.

See also the many natural knots and other flaws in framing lumber.

The key is in piloting properly so you're not severely splitting or taking chunks out of things. For soft wood, pilot to just below the screws' shank diameter and a bit deeper than the expected penetration. Don't remove more than say 1/3 the joists' width where you need large or multiple screws at a point.

Be sure you have adequate penetration. For your purposes, 2" into the framing should do well.

And obviously don't overload any one joist. Where needed, use substantial stiffeners across multiple joists and fasten your hardware to that instead of directly to the joist. The actual capacity of your ceiling depends on its type, joist interval, etc.