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I'd like to install a new skylight in my dining room. The skylight is 30x30 and will require one ceiling joist to be cut and (2) new double headers to be installed. Do the new headers need to span from the uncut joists (B) or can I simply install them as bracing, between cut and uncut joists (A).

If I go with method (A), the drywall hole I cut in the ceiling is a one-time thing. If I go with method (B), I think I need to extend the cut of the drywall another 3" on each side, remove the joist section, install headers, and then add drywall to cover the headers. Make sense? (A) seems simpler to me.

All work will be done from below because attic is too small to do work from above.

ceiling joists

  • B is more structurally sound, although I don't know enough of the reason to give a complete answer – depperm Aug 22 at 15:33
  • Just curious.. what are you doing about the rafters? – JACK Aug 22 at 15:46
  • The rafters are 32" OC (flat roof). Fingers crossed for a perfect fit! – dabi Aug 22 at 16:39
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Option B is the better choice.

(A) may eliminate some drywall patching, but will not provide the same strength as it different load forces at work.

Option A will create a pivot point that will be more torsion/twisting force. Each connection wanting to act like a hinge under weight and pull away from their connections, which will take less energy than shearing.

The header spanning across the full space (B) as a single structure is mostly bending (Compression and tension) force against the middle that will be pressed on the connection with outer joists (and some shear force where nailed to joists).

Think about the pressure on your roof will be pushing at least in part down on that middle joist.

See https://www.teachengineering.org/lessons/view/wpi_lesson_1 for an overview of load types.

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    B is the better choice, and if you're carful you can probably cut the drywall as in A, cut the center joist shorter with little damage or a small area of damage and perform B. – JPhi1618 Aug 22 at 15:46

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