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Ridge Skylight - Hole in Wood Diaphragm

Ridge Skylight - Hole in Wood Diaphragm

Ridge Skylight - Hole in Wood Diaphragm

I have a 26' long (ridge runs parallel to this dimension) x 24' wide wood diaphragm, sheathed with 1x6 wood decking. The roof is about a 6:12 pitch consisting of 2x4 trusses at 24" oc (scissor type truss). The arch/owner would like to install a ridge skylight that is about 22' long x 2' wide.

I'm thinking that this essentially creates (2) separate diaphragms (on each side of the skylight) and therefore if we provide some sort of strap and blocking on each side of the skylights than this should be acceptable. Any further thoughts on this?



RE: Ridge Skylight - Hole in Wood Diaphragm

I'm afraid this creates 0 diaphragms, as a diaphragm spans from horizontal supporting element to horizontal supporting element.

RE: Ridge Skylight - Hole in Wood Diaphragm

I think APA Research Report 138 has a method for calculating forces around holes in diaphragms. It's avaialble on their website:



RE: Ridge Skylight - Hole in Wood Diaphragm

Depending on how the ridge vent opening is framed, there may still be some continuity of the diaphragm across the opening. Plus, there is still about 6 feet or so of ridge line diaphragm at either end of the opening.

Personally, as the opening is at the zero shear line of the diaphragm, plus the questionable capacity of 1X6 planks to act as a diaphragm much over 100 plf, I would consider the situation to be somewhat problematic.

Mike McCann
MMC Engineering

RE: Ridge Skylight - Hole in Wood Diaphragm

Thanks for the replies.

Say the ridge skylight went all the way across the roof, I still think you'd be OK just assuming (and detailing) you have (2) diaphragms, no? I mean basically you have (2) (less) deep beams instead of (1) deep beam so to find your tension/compression you would just use a reduced depth based on each 'beam'. Also your shear length at the ends would be slightly less.

If you think about analyzing a post frame (pole barn) roof where they have purlins @ say 2' on center and a metal roof - when analyzing/designing the connection of the purlins to act as chords, you can (and is done frequently) assume that each slope acts as a separate diaphragm and that each purlin is 'loaded'. I'm thinking that this would be similar where each slope acts independently.


RE: Ridge Skylight - Hole in Wood Diaphragm

What about the perpendicular direction?

RE: Ridge Skylight - Hole in Wood Diaphragm

That is a very good question. In my particular case this is actually an interior diaphragm (i.e. the 2 end walls in the sketch are actually interior bearing walls)and probably the reason I had over-looked this in my argument. However, you bring up a very good point to my theoretical question and I'd have to think about it some. Because it is located at the ridge, there would be very little shear. You may be able to justify some sort of week asix bending in the rafters/trusses but having said that I think it's is a valid reason not to put a full length opening at the ridge.


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