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Skylight Framing Calculator

Lay out the rough opening, headers, trimmers and interrupted rafters for framing a skylight into a rafter roof.

Skylight rough-opening width

Skylight rough-opening length

Rafter spacing (on center)

Header bearing allowance

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How it works

Rafters interrupted = ceil(RO width ÷ spacing) − 1 · Header length = RO width + bearing · Header lumber = header length × 4 (doubled top & bottom) · Trimmers = 2 doubled sides

Framing a skylight means cutting the rafters that fall inside the opening and carrying their load around it with a box: doubled headers across the top and bottom of the opening and doubled trimmer rafters down each side. The rough opening is a little larger than the skylight itself to allow for flashing and shimming. The number of rafters you interrupt depends on the opening width relative to the rafter spacing.

Worked example

A 46.5-inch-wide opening at 16-inch on-center spacing crosses ceil(46.5 ÷ 16) − 1 = 2 interrupted rafters. Each header is 46.5 + 3 = 49.5 inches long; doubled top and bottom that is four pieces, about 16.5 ft of header stock.

Frequently asked questions

Why do skylight headers need to be doubled?

Cutting rafters removes their support, so the load must transfer to the headers and then to the trimmers. Doubling the headers (and the trimmer rafters that carry them) gives the beam enough strength to span the opening. For wide openings a structural engineer may specify a larger member.

How much bigger is the rough opening than the skylight?

The rough opening is sized to the manufacturer’s specification — usually the skylight’s outside frame plus about a half inch of clearance all around for shimming and flashing. Always use the exact rough-opening dimensions from the unit’s instructions.

Can I frame a skylight wider than the rafter spacing?

Yes, but the wider the opening the more rafters you interrupt and the heavier the headers must be. Openings spanning more than one or two rafter bays often need engineered headers, so confirm the framing with the manufacturer or a professional.

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This calculator is for educational purposes. Double-check important results before acting on them.