Skylight in 2x Roof Decking

[jdgengineer]

Question I'm struggling with that I was hoping someone could assist that has run across it before. How do you typically frame a skylight in 2x roof decking with exposed dropped beams (like an eichler)? Typically, the decking and beam network is exposed so I don't see how you frame out the opening without introducing a bunch of ugly beams around the skylight (which is not usual from what I have seen). There is 4" of rigid insulation above the roof decking. Is it common to put the header framing above the decking around the skylight in the rigid insulation layer? Or is the decking typically just designed as a cantilever at the skylight (I haven't calced to see if this would work).

Just curious what others have seen so I don't come up with a totally unique detail for a relatively common problem.

 

[msquared48]

Put your headers above the decking if it can fit in the 4" space. It depends on thevdeck span and your snow load. Otherwise you will have to see some framing below.

Mike McCann, PE, SE (WA)

 

[XR250]

As M^2 said, put the headers above the decking, if needed. I usually use 4x4's and 4x6's flat and screw them into the decking.
Depending on the layup and span of the decking, you may not need headers. I have run across many of these without them and they have worked fine.

 

[JAE]

I think you can also armor the opening with angles (one leg flat and one leg turned down) to stiffen the opening.

This takes a bit more work.
1. Run two supporting angles across the deck span, spanning from the main roof beams that support the decking.
The angles are placed between decks in routed/widened joints.
2. Angle leg would not extend below the deck surface so from below you don't see them.
3. Run two cross angles between the supporting angles - again - one leg flat and the other turned down.
4. In these cross angles install screws downward into the top of the cut-off deck ends.
5. The cross angles are welded to the supporting angles (in the shop probably).
6. The size of the angle you can use depends on the deck thickness - so for a 3" deck you can use a 2 1/2" angle.

This is a lot more work and money - getting the deck to support itself would be preferred.


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[BUGGAR]

The skylight should sit above the roof by six inches or so; I think this is in the IBC under roofing. Regardless of code, this is good for drainage, and leaves room for framing above.

 

[spieng89]

I agree with most above. I use to detail and install roof systems and typically we would install wood blocking as needed above the decking to raise skylight to an appropriate height (stacking 2x4's is fine), then utilize an appropriate flashing detail to mitigate the possibility of water intrusion.

 

[jdgengineer]

Thanks for the help. I appreciate the input, and all is more or less in line with my thinking. One of the openings I should be able to get the headers to span across to the next adjacent beams hidden within the rigid insulation layer (say 4x6 flat suggested above). Two other skylights are in areas where the deck span is small and the deck can probably calc as a cantilever span to the skylight. The skylight opening will have a curb to raise the skylight up (as mentioned above), but this would only occur at the skylight location and therefore wouldn't allow that mechanism to serve as a beam to support the decking. However, for the openings that will remain unreinforced this will help tie the decking together locally at the skylight. Thanks for your help.