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Overbuild onto existing roof trusses

Overbuild onto existing roof trusses

Overbuild onto existing roof trusses

(OP)
Hi, hope someone can help.
We are designing a project where we will be overbuilding new roof rafters over a portion of a garage that has engineered roof trusses at 24"o.c. The house is not that old, so I am assuming the trusses were designed for 30psf snow load/15psf dead. The new perpendicular gable roof will be a true rafter design with ceiling joist ties holding together 2x10 rafters except at the overbuild. We are providing a 14"LVL ridge board (not beam). The overbuild connection will sit on the existing sheathing of the trusses, and we will provide a flat 2x12. In theory, we are not adding any additional load to the trusses other than the dead load of the framing members. Will this be a problem? I do not have calcs form truss manufacturer. Perhaps add blocking to help the 2x12 flat span the 24" between the trusses
Thanks for any help.

RE: Overbuild onto existing roof trusses

Done all the time. Usually not a problem unless the overframed area is very large. If the area is large, and depending on the framing scheme of the overframing, it could concentrate any snow and dead loads to specific locations. I have just used 2X posts on a 4 foot grid in the overframed area to mitigate that scenario in the past.

Mike McCann
MMC Engineering

RE: Overbuild onto existing roof trusses

(OP)
Thanks so much, I really appreciate it.
Jay

RE: Overbuild onto existing roof trusses

And make sure the deck stays, to brace the top of the current framing, or provide an equivalent bracing system, and a way to get diaphragm forces to the foundation.

RE: Overbuild onto existing roof trusses

Typically trusses are designed with enough dead load to handle some valley framing. So I doubt it would be a problem.

In theory, the valley framing is supposed to be supported every 4' so the overframing load is evenly distributed over the trusses below. But I've never seen it done in practice.


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