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Cutting Joists for skylights

Posted by jstoesz (My Page) on
Thu, Aug 15, 13 at 11:58

I am hoping to gang two, 24in wide, skylights together and install a sloping well on 3 sides. This will necessitate cutting at least two ceiling joists, but perhaps more with how sloped I want to make the well. The more the merrier as far as I am concerned to open up the ceiling in the kitchen. But I don't want a sagging ceiling or roof. Obviously when cutting individual stringers you can simply head them off with doubled 2X6s, but can you do this on more than one if you sister the uncut stringers on each side? How many can I span with headers and sistered stringers.

The rafters and stringers/joists are 2X6 on 16 centers. The roof has 14 ft run from center bearing wall where the stringers overlap and are nailed together. The roof pitch is 30 degs. There are no hips or valleys, simple straight run roof.

Follow-Up Postings:

RE: Cutting Joists for skylights

you can span more, but you need to sister several additional joists to your end joists to treat them more like a beam. Then the opening can be framed out like a built up beam tieing into the new built up joists. Roof loads, snow loads (if any) are other things not taken into account here.

RE: Cutting Joists for skylights

I think I get what you are saying, and I plan to do that. I figure, if I remove 4 joists, I need to add 4 joists sistered at the edges of the opening (two on each side).

I am not so worried about supporting the plaster and live loads, but curious about top plate spread/rafter sag. Can you mitigate this with strapping on a diagonal from center of shortened joists at the top plate to the new doubled/triple sistered joists? Maybe this is just crazy talk from an engineer (non-structural). But if I interrupt the joists in tension over a 6 foot opening (breaking the rafter truss triangle, won’t that 6 ft opening begin to spread regardless of how many joists I put on either side? Is everything is stiff enough where this is not a concern?

RE: Cutting Joists for skylights

not sure I understand your question or concern exactly. Most loading on the rafter is compression rather than tension. You have tension when the wind is blowing hard from a specific direction on one side, while the other side is in compression. The roof sheathing creates the diaphram and locks it all together. There really isnt a whole lot unique about what you are wanting to do or trying to do. Its a typical roof framing opening for dormers, just no dormer.

RE: Cutting Joists for skylights

You can and you must support the now cut ends of the rafters.

Here is another thread discussing something similar.

Here is a link that might be useful: GardenWeb

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