Question about placing a temporary load on a garage truss roof
Wife and I have become campers in our retirement. Love it except for the 5th wheel hitch that sets full-time in the bed of my pick-up. I would like to remove it at the end of each camping season and that is the source of my question.
My garage has a typical truss roof design, 2 x 4 construction on 24″ centers. I would like to hang a small electric lift from the ceiling temporarily each year to install and/or remove the 300 – 350 pound hitch from my truck. The spot that I would propose to do this would be near one of the outside walls where the trusses rest on the top plate.
I reason that by laying a 2 x 8 across the tops of several lower truss chords and fastening the plank to each chord to hold the lift, I could do this safely without damage by spreading the weight to across several trusses. When the task was done, I would remove the screws from the plank, which would remain there until needed again.
I have done my own remodeling type work over the years but this one has caused me to pause and seek out thoughts from those knowing more than I. Is my plan doable?
Thanks for any help!
Replies
I wouldn’t do it. You’d be introducing a force to a truss bass-ackwards from its intended support.
Now, inserting a couple posts temporarily under the trusses at each end of a built up “beam”, lower the load onto a cart and roll away. I might do that.
Thanks for the reply. I thought my idea of laying a plank across several trusses might be enough, but I hear what you're saying. Will look into your suggestion of adding a couple temporary posts beneath the plank on either end. Should be simple enough .
Thanks for the reply !
Thankfully, we can separate our hitch into two separate pieces, and then move it with much less effort, using a dolly or hand truck.
I would not be comfortable applying the excessive downward force on the bottom truss chord using the idea you're considering. As you noted, the trusses are constructed using 2x4s, and the truss joints are fastened only with nailing plates. A concentrated lift, focused on a relatively small area of the truss structure, would potentially exceed the design specifications and either damage the building materials or separate/loosen the truss assemblage.
I hope this was helpful.
I think the right answer is that it all depends. On where you are, what time of year, and how the trusses are specified. Some trusses are specified for some attic floor loads, others not so much.
trusses on a two foot center that are specified for a heavy snow load, (say 50 PSF) are meant to transfer distributed loads to the walls. a truss with a 20 ft span for such a case would support 2000 pounds. It really would call for detailed understanding of the specifics (more than just 2x4....) of the trusses involved.
If you can identify the maker of the truss, they may be able to provide advice on a proper location and mounting for a point load. I would install the hoist permanently and not try any temporary mountings. Capacity should be prominently displayed.
A local engineer may also be able to help and propose an acceptable option.
Thanks for the reply. I had not considered the design load of the truss system. Not certain if I can find the design specs but will try. Am I understanding your comment that if the trusses were designed for a heavy load they would be sufficient to sustain the additional load imposed by my lift, given my plan for installation. (assuming design load in PSF x 2' truss spacing x truss span?)
Thanks for your help. I do appreciate it!
Perhaps consider anchoring it close to the peak of a truss? Or better yet install a block between two trusses near the peaks and hang from that? Use structural screws with enough shear strength.
I like Calvin's idea of temporary posts down to the garage floor at each end of the beam. I would attach the beam semi permanently to each truss with screws so it can't roll or move causing the posts to become non vertical.
You could put a bottom plate if the ceiling is high and you are worried about the bottom of the posts kicking out or getting bumped.
It COULD be done with the proper load distribution.
I once used a series of short beams suspended with screw eyes from the the floor joists over my garage to pull an engine + transmission out of a Toyota. The weight was distributed equally to (4) joists using (3) short 2x4 or 4x4 beams. I have no idea what the weight of the combined engine + transmission was. I do know it worked without any adverse effects.
Trusses make it a bit more complicated because the members are smaller and pretty much loaded to capacity (assuming full design live load). It's best to apply the load to the top chord of the trusses rather than the bottom chord (which is typically designed only for tension) as Carpenter suggested.
Thanks for the last couple of replies. Interesting thoughts about applying the weight to the top chord of the trusses rather than the bottom. In my very limited understanding of how truss systems work that would seem to distribute the weight out and down onto the top plate of the walls (?), making application of the weight a safer bet than my original thought.
No matter where you apply the load, it has no choice but to get transferred to the bearing walls. It's the forces in the truss members that can be adversely affected when load(s) are applied to members that were not designed for it. The top chords are designed for both the compressive forces resulting from the overall loading AND the load applied by the roof sheathing essentially perpendicular to the chord (e.g. local bending & shear). The remaining truss members are typically designed only for the tension or compression forces resulting from the top chord loading. Always better to apply loads to top chord only unless the truss had been design for bottom chord loading (say for attic loads).
thank you so much for your suggestion. really appreciate that.