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The full width of the dormer, measured across its own roof.
Half of this width is the rafter's horizontal run from the dormer's ridge to its side wall.
The dormer's own roof pitch, expressed as rise per 12 units of run.
Dormer roofs are often built steeper than the main roof to shed water effectively over their short run.
The horizontal overhang beyond the dormer's side wall.
The plan overhang of the DORMER's own roof past the dormer's cheek, which is usually far smaller than the main roof's — often 50 to 150 mm, sometimes nothing at all where the dormer is flush. Taking the main roof's overhang for a dormer is the standard mistake here and makes every dormer rafter too long, which is visible from the ground because the dormer's eave no longer lines up with anything.
Total dormer rafter length
5.083 ft
- Rafter length to ridge
- 3.91 ft
- Overhang tail
- 1.18 ft
They open the calculator with your figures already in it
Dormer Rafter Length Calculator: 5.08 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard carpentry rafter-length formula applied to a dormer's own (typically narrower, steeper) roof: rafter length = run × √(1+(pitch/12)²)
Inputs used
- Dormer Width
- 6.5 ft
- Dormer Roof Pitch (rise per 12)
- 8
- Overhang
- 11.75 in
Intermediate steps
- Rafter length to ridge
- 3.91 ft
- Overhang tail
- 1.18 ft
What this calculation does not cover
- The run is taken as half the entered width, so the figure only holds for a symmetrical gable dormer with its ridge on the centreline. A shed or mono-pitch dormer has a single slope whose run is its full depth, from the front wall back to the main roof, so entering that depth returns a rafter about half the length actually needed. For a shed dormer, enter twice the run you measured.
- Pitch is read as rise per 12 units of run, not as an angle in degrees, and the field accepts anything from 2 to 24, so a degree figure in that range passes with no warning. A 20 degree dormer entered as 20 is treated as 20 in 12, or 59 degrees, and the rafter comes out over 80 per cent too long. For reference, 8 in 12 is 33.7 degrees and 12 in 12 is 45 degrees.
- Overhang is treated as a horizontal projection beyond the dormer's side wall and is multiplied by the same slope factor as the run. A tail already measured along the slope, entered here, is inflated by that factor a second time, roughly 20 per cent at 8 in (203 mm) 12 and 30 per cent at 10 in (254 mm) 12. The tail also carries no allowance for the plumb or square fascia cut, nor any cutting waste for ordering.
- The length returned is a dormer common rafter only. Where the dormer roof meets the main roof plane the valley rafters are longer than this figure and are cut on a compound angle, the jack rafters running into them each differ in length, and a hipped dormer's hip rafters are longer again. None of those members are produced here.
- Nothing in this calculation tests whether the dormer is structurally adequate: rafter depth, section, spacing and permissible span all sit outside it. So does the trimmed opening in the main roof, where the main rafters cut away for the dormer are carried on trimmers and a header that normally have to be doubled. On a wide dormer that opening, not the dormer rafters, is usually the governing structural item, and both are sized from span tables or by an engineer rather than from a length.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Standard carpentry rafter-length formula applied to a dormer's own (typically narrower, steeper) roof: rafter length = run × √(1+(pitch/12)²)
Cite this page
Your workspace
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Tools and safety for this job
To cut framing lumber and frame the dormer and its cheeks. Generic types, no brands, no prices.
Protection this work requires
- Hardwood dust is a recognised carcinogen: extract at the tool and wear a P2/N95 respirator when cutting or sanding indoors.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
Frame the dormer and its cheeks: Framing a dormer means cutting into a roof that is carrying load. The trimmers, the front frame and the dormer's roof members are sized by the engineer's design, and this site never sizes them; the rafter lengths it works out are geometry for cutting, not a check that any member is adequate.
Show the 7 tools this job needsHide tools
Essential
Circular saw
Combination or framing square
Spirit level
Tape measure
Recommended
Framing nailer
Mitre saw
Optional
Hand saw
Also needed as materials: circular saw blades, framing nails, fuel cells, mitre saw blades.
what each carpentry tool is for, and the spec that decides which to buy where one does.