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Q.When you remove one rafter to install a skylight, do you have to double the two rafters at the sides of the opening? What about the headers?

A. When you’re framing a skylight that requires you to remove no more than two rafters, a common rule of thumb says to double the rafters on either side of the opening and use double headers. This is often required by building inspectors, but in fact, it’s a very conservative guideline that often results in unnecessary framing. Only in rare cases are the doubled headers required. And in many cases, particularly when 2x10 or 2x12 rafters are chosen for insulation thickness rather than for strength, doubled rafters may not be required.

In my experience, the inspectors also often insist on joist hangers at the headers, but these are also rarely needed. Usually, an adequate number of 16d nails, as many as 8 nails for a 2x12 connection, depending on the loads) can handle the reaction forces.

Watch out, though: As the opening size increases to the point where you’re removing three or more rafters, even doubling the perimeter framing may not be sufficient.

—R.R.
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rather than structural requirements, it may be unnecessary to double trimmer rafters. It would rarely be necessary to double the opening headers.