Dormer Rafter Calculator

A dormer rafter calculator treats the dormer as a small independent roof and solves it with the same right-triangle math as the main roof. For a gable dormer, the dormer width W is its clear span, so each dormer common rafter runs (W ÷ 2) × √(1 + (Y/12)²) long at the dormer's own pitch Y — a 6'-0" wide gable dormer at 8/12 gives 3.0 × 1.20185 = 3.606 ft = 3'-7-5/16" per side, with a ridge height of 3.0 × 8/12 = 2'-0" above the front wall top. For a shed dormer, the run is the front-wall-to-ledger distance R, the rafter length is R × √(1 + (Y/12)²), and the calculator checks that the rise (R × Y/12) stays below the front wall height so the roof plane actually lands on the main roof. Main-roof pitch enters through the valley geometry, not the dormer length. Verify all framing against local code.

Take the horizontal run R from the dormer front wall to the point where the dormer roof plane meets the main roof plane (the ledger line), apply the slope factor, and cut. Worked example: R = 8.0 ft at a 4/12 dormer pitch — factor = √(1 + 0.33333²) = 1.05409, so rafter length = 8.0 × 1.05409 = 8.433 ft = 8'-5-3/16" to the nearest sixteenth. The rise over that run is 8.0 × 4/12 = 2.667 ft = 2'-8", which must be less than the front wall height above the main roof plane; if the front wall is only 2'-6" tall, the plane exits below the ledger and the pitch must steepen or the run shorten. Keep the factor at five decimals and round only the final cut length. Verify local code before cutting.

It sizes the three members a shed dormer adds to a main roof: the dormer rafters themselves (run × slope factor at the dormer pitch), the front wall height (which sets the window head and the ceiling line), and the ledger tie-in where the dormer roof dies into the main rafters. A practical layout is a 24' long × 8' deep shed dormer at 4/12 over a 12/12 cape: 8'-5-3/16" rafters, a 2'-8" rise, front walls 3'-0" above the main roof plane to clear a 6'-8" window head inside, and valley boards on both cheeks. The calculator's shed mode produces the first two numbers instantly and flags the front-wall-height conflict before you cut. It is a layout tool, not a structural approval — verify local code.

Valley-first, in this order. Snap the valley lines where the dormer roof planes intersect the main roof planes. Run a valley board (a 2x laid flat or on edge per local practice) along each line — its bevel is governed by both pitches, so a 12/12 main with a 4/12 dormer cuts differently than 12/12 with 8/12. Fit jack rafters from the dormer ridge or front wall down to the valley board; they shorten progressively toward the valley, so cut from a single pattern and shorten per station. Then stand the cheek walls whose top plates follow the valley. General hip/valley geometry is covered step by step in the hip and valley rafter guide — this page stays on the dormer body itself. Verify local code.

Tall enough to make the interior work, low enough to keep the dormer subordinate — and checkable with two numbers. Interior: a 7'-6" finished ceiling needs the front wall at about 3'-0" above the main roof plane once rafter depth and drywall are subtracted. Exterior: many design reviews expect the dormer ridge (gable) or roof line (shed) to stay below the main ridge; keeping the shed dormer front pitch at 3/12–4/12 over a 10/12–12/12 main preserves the hierarchy. The calculator flags the hard constraint: front wall height must exceed the dormer rise over the run, or the roof plane exits below the ledger line. On a 4/12 dormer with an 8' run that minimum is 2'-8" — anything less forces a steeper pitch. Verify local code and any HOA design rules.

Not by cutting it in. Trusses are engineered as complete assemblies — every web and chord carries a designed load path, and cutting any member voids the engineering stamp. The safe routes are: order an engineered dormer-framing package from the truss manufacturer (they re-stamp the affected bays), use a framed-in-place girder redesign by a licensed engineer, or confine the dormer to bays framed conventionally from the start. The failure mode from field-cut trusses is slow, not dramatic: months later the drywall cracks over the cut webs as the sag develops. On a rafter-framed roof the same dormer is a straightforward ledger-and-valley job. Verify with the truss supplier and local code before any roof opening.

Follow the type, then the main roof's hierarchy. Shed dormers work at 3/12–4/12: steep enough to shed and shingle, shallow enough to stay subordinate on a 10/12–12/12 main. Gable dormers typically match or sit slightly below the main pitch — an 8/12–10/12 dormer on a 12/12 main keeps the proportions plausible and the cheek walls reasonable. Hard limits come from the covering, not taste: asphalt shingles want 2/12 minimum, most metal panels 1/12–1/2:12 depending on profile. The math the calculator enforces is the ledger rule — dormer rise over run must stay below the front wall height. Pitch conversion between X/12 and degrees is on the roof pitch calculator; this page sizes members only. Verify local code.

The cheek is the dormer's side wall, and its framing has two rafter-family members: the cheek rafter (the outer common of the dormer roof, running ridge-to-front-wall on the dormer pitch) and the valley jack rafters that run from the main roof down to the valley board on each side. The cheek wall itself is a right triangle with a vertical front edge (the dormer front wall), a horizontal bottom (the main roof plane it stands on), and a sloped top that follows the dormer rafter. Two compound angles matter: the top plate cuts where dormer pitch meets main-roof plane, and the valley-board bevel where the two roof systems intersect. For a 6'-0" wide gable dormer at 8/12, the cheek rafter is the 3'-7-5/16" common from the calculator's gable mode. Verify local code.

Measure the dead zone it replaces. A typical cape knee wall gives less than 5' of height over the outer 4' of the room — effectively dead. A 24' × 8' shed dormer at 3/12–4/12 converts roughly 190 sq ft of floor to full-height area and nets about 120 sq ft of genuinely usable space after walls and circulation; that is the largest per-dormer gain available. A single gable dormer 6'–8' wide nets about 35–45 sq ft plus a vertical window wall — less floor gain, but the best light per dollar. Two gables bracketing a center shed (the 'doghouse row') multiplies valleys and flashings, so the layout that looks symmetric on paper is the one that costs the most to keep dry. Verify local code for egress-window sizing in finished dormer rooms.

Five repeat offenders. First, cutting trusses — the only unforgivable entry on this list. Second, front wall too short: the dormer roof plane exits below the ledger line and the whole layout redraws; the calculator's ledger check exists for this. Third, ledger dado overload: notching main rafters more than about 1.5" deep at the dormer tie-in removes fiber at mid-span where bending stress peaks — bolt the ledger to rafter sides instead. Fourth, valley weathering: treating the dormer valley like a gable edge instead of running valley metal under step flashing; both planes drain into that trough. Fifth, jack-rafter waste: cutting every jack to the first one's length instead of shortening per station toward the valley. The calculator handles the geometry of mistakes two through five; the truss rule is policy. Verify local code.