I'm thinking about installing a new skylight, and I'm skeptical about some of the information I've seen about how it should be framed. The skylight I'm looking at is a Velux 30 9/16 wide unit, which would require cutting a single rafter (16" OC). Most of the info I've found indicate that the framing should look something like this:
This looks easy enough to do. However, I'm concerned about how well the headers will support the load of the center rafter.
With windows and doors, the header is supported on the ends by jack studs, which transfer the load to the bottom plate. However, with this skylight framing, the headers are only supported by the adjacent rafters via nails. I've seen recommendations of 3 16d nails on each end of each header, so 6 nails on each end of the double header. Still, the shear strength of 16d nails is only about 100-150 lbs (based on a convenient but not necessarily reliable Google search). Even with 6 of these securing each end of the double header to the adjacent rafters, it sounds weak considering the weight of the roof and potential snow load.
What am I missing here? Is this really the correct way to frame the skylight?
From ’97 UBC:
Sec. 2326.12.5. Framing around openings. Trimmer and header rafters shall be doubled or of lumber of equivalent cross-section, when the span of the header exceeds 4 feet (1219 mm). The ends of header rafters more than 6 feet (1829 mm) long shall be supported by framing anchors or rafter hangers unless bearing on a beam, partition or wall.
From 2009 IRC:
R802.9 Framing of openings. Openings in roof and ceiling framing shall be framed with header and trimmer joists. When the header joist span does not exceed 4 feet (1219 mm), the header joist may be a single member the same size as the ceiling joist or rafter. Single trimmer joists may be used to carry a single header joist that is located within 3 feet (914 mm) of the trimmer joist bearing. When the header joist span exceeds 4 feet (1219 mm), the trimmer joists and the header joist shall be doubled and of sufficient cross section to support the ceiling joists or rafter framing into the header. Approved hangers shall be used for the header joist to trimmer joist connections when the header joist span exceeds 6 feet (1829 mm). Tail joists over 12 feet (3658 mm) long shall be supported at the header by framing anchors or on ledger strips not less than 2 inches by 2 inches (51 mm by 51 mm).
Thanks for the references. The RO width of the skylight is 30", so I think this is the most relevant section:
If the trimmers are new framing members the length of the RO, that makes sense to me since those trimmers can transfer a huge load between the headers. However, this Velux skylight (which is a very common size) would not fit between the 1st and 3rd rafters due to the additional 1.5" framing on each side. So, I doubt this is the case.
The examples I've seen (like the photo in my original post) just use the existing 1st and 3rd rafters and rely on fasteners to transfer the load. I'm still unclear whether or not this meets code (are those existing rafters considered trimmers?). On the other hand, it seems to me that it must meet code, because why else would Velux make a skylight for a 30" RO?
Hi, again. "Are the trimmers just the existing 1st and 3rd rafters/joists? Or, are the trimmers new framing members the length of the RO that run from the top header to the bottom header (similar to the jack studs in a window or door)?"------- The singles are the existing IF the new hole is within 3' of the outside or bearing walls. Otherwise you need to add one on each side to make them double. Give a quick call to your local B.D. to verify, as I don't know hat year Code you are under or the local amendments added.
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