huangyhg

2x4 flat roof
i was asked by a contractor to look at a skylight installation in an existing building. he was asked by the owner to install two skylights identical to two others already in place. as he got into the job, he got leary and looked for some engineering help.
i have never seen a roof system like this before. it is a flat roof that is simply wood 2x4's stacked side by side on edge with 3/4" plywood on top. the span between support beams is 14 to 16 feet, but there is no rhyme or reason as to where joints are. it looks like they just started layng 2x4's and staggering joints, so some are midspan, some 1/4 span, some are over beams etc. the 2x4's must be nailed to each other (i'm guessing), but when an opening was cut to install the skylight, the cut 2x4's just fell apart, so they definitely are not glued together (or nailed at a very close pattern).
i don't see how it would be possible to install a skylight since the adjacent 2x4's are not laminated together, the point load from the header across the skylight will have to be taken out by a single 2x4 next to the skylight, or at best you could run some long lags to pick up as many 2x4's as you could get (which my preliminary calcs indicate would have to be a bunch!).
so here's the question... has anyone seen a roof system like this? am i missing something in the design approach? apparently someone previously thought it was a good idea to just cut a hole and add a skylight, and those have been in place for years and have seen some heavy loading (wet pacific northwest snow). so when i see something that someone else did that looks way wrong to me, i always wonder if i'm missing something obvious.... any thoughts?
thanks,
tom w
obviously not an engineered design, but didn't fall down (yet) due to perhaps composite action of the plywood nailed to the 2x4's. for liability reasons, i would back away from that job. there is no way i could rationalize a method to justify that roof.
sacrebleu,
thank you for your opinion. it reaffirms my initial reaction as well. as i said, sometimes when something looks so obvious it's nice to bounce it off someone to make sure you're not missing something.
i'd steer away from that job as well. if i was going to do it, i'd put a big disclaimer that i neither examined or reviewed the existing roofing (the 2x4 w/ plywood), but simply framed the opening. i'd provide edge framing beams that span from the support beam to the other support beam around the skylight.
consider the long simpson screws though the sides on the skylight hole, to tie the adjacent studs into a header.
thanks for the input.
aggieyank, if i was going to do it, i was thinking along the same line as you, to provide edge beams that will support the roof in the area of the new opening ... but i think i might just put my tail between my legs and run!
thanks
tom w
i worked on a job that had all the floors made that way. i wanted to lag the 2x4's together, but i came to the same conclusion that it would take too many.
another thing i looked at was using wt shapes and angles to frame the opening, then you could saw cut a groove and fit the steel flush with the wood. we ended up adding extra wood beams to frame out the openings.
thanks lowlax,
i agree that adding beams to frame the openings and carry those loads back to existing beams or exterior walls would be the way to go.

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