Roof Rafter Length Calculator

Rafter cutting is the most geometry-intensive task in residential framing. Each rafter has three critical cuts — the plumb cut at the ridge, the birdsmouth at the wall plate, and the tail cut at the eave — and getting any one wrong means the rafter does not fit.

1. Calculate the rafter length from the numbers above. The calculator gives you the total length along the slope, including the overhang. This is the measurement you need for ordering lumber — always order the next standard length up from the calculated value because you cannot splice a rafter mid-span.

2. Mark the plumb cut at the ridge end. Set your framing square with the pitch rise on the tongue and 12 inches on the blade. The line along the tongue is the plumb cut. For a 6/12 pitch, the tongue reads 6 and the blade reads 12. This cut sits against the ridge board (or against the opposing rafter in a ridge beam system).

3. Measure and mark the birdsmouth. From the long point of the plumb cut, measure the rafter length along the top edge to the wall plate location. The birdsmouth is a notch cut where the rafter sits on the wall plate — it consists of a seat cut (horizontal, equal to the plate width, typically 3.5 inches for a 2x4 wall) and a plumb cut (vertical). The seat cut must not exceed one-third of the rafter depth to maintain structural capacity.

4. Mark the tail cut past the birdsmouth. From the birdsmouth, measure the overhang length along the top edge and mark another plumb cut. This is the tail cut — the end of the rafter at the fascia board. Some builders cut the tail plumb (vertical) and others cut a combination plumb and level cut for a decorative tail profile.

5. Cut a pattern rafter first. Cut one rafter carefully, test-fit it on the actual building, verify the ridge fit, birdsmouth seat, and overhang dimension, then use it as a template for all remaining rafters. Never cut the full batch from calculations alone — minor framing variations always exist.

6. Verify with a chalk line. After installing several rafters, snap a chalk line along the top edges. Any rafter that sits high or low relative to the line has a birdsmouth error. Fix it before sheathing — plywood over a misaligned rafter creates a visible hump or dip in the finished roof.

The pitch multiplier converts horizontal run to actual rafter length along the slope. It is the most important number in rafter layout because it determines how much longer the rafter is than the run.

Pitch	Rise/Run	Angle	Multiplier	Length Added
2/12	2" per foot	9.5°	1.014	+1.4%
3/12	3" per foot	14.0°	1.031	+3.1%
4/12	4" per foot	18.4°	1.054	+5.4%
5/12	5" per foot	22.6°	1.083	+8.3%
6/12	6" per foot	26.6°	1.118	+11.8%
7/12	7" per foot	30.3°	1.158	+15.8%
8/12	8" per foot	33.7°	1.202	+20.2%
9/12	9" per foot	36.9°	1.250	+25.0%
10/12	10" per foot	39.8°	1.302	+30.2%
12/12	12" per foot	45.0°	1.414	+41.4%

The jump from 6/12 to 12/12 nearly doubles the extra length — from 12% to 41%. This is why steep roofs use significantly more lumber than low-slope roofs of the same span width. A 12/12 pitch roof with a 12-foot run needs a 17-foot rafter versus 13.4 feet at 6/12.

For projects where pre-fabricated trusses make more sense than stick-framing, the truss price calculator compares truss cost against the labour and material of site-cut rafters.

The birdsmouth is the notch cut into the rafter where it sits on the top plate of the wall. It looks simple — a right-angle notch — but its depth is critical. Cut too deep and the rafter loses bending strength at the bearing point. Cut too shallow and the rafter does not seat firmly on the plate.

The one-third rule governs birdsmouth depth: the seat cut (the horizontal cut that sits on the plate) must not remove more than one-third of the rafter depth. For a 2x8 rafter (7.25 inches actual), the maximum seat cut is about 2.4 inches. For a 2x10 (9.25 inches actual), it is 3.1 inches. A standard 2x4 wall plate is 3.5 inches wide, which means a 2x8 rafter birdsmouth cannot cover the full plate width — the seat cut stops at 2.4 inches and the rafter overhangs the inner edge of the plate by about an inch. This is normal and structurally acceptable.

The plumb cut of the birdsmouth (the vertical face that sits against the outer edge of the plate) is cut at the same angle as the ridge plumb cut. Together, the seat cut and plumb cut create a clean right-angle notch that locks the rafter onto the plate. The bearing point transfers the rafter load vertically into the wall stud below.

On steeper pitches (8/12 and above), the birdsmouth gets shallower because the seat cut angle is more acute. This naturally protects the rafter from over-cutting. On low pitches (3/12 and below), the nearly horizontal seat cut can easily be too deep — measure carefully and mark the one-third depth limit before cutting.

If the roof span is long enough to require structural support at mid-span, consider the shed roof truss calculator for truss configurations that eliminate mid-span bearing walls.

Site-cut rafters and factory-built trusses both frame roofs, but they serve different project types.

Rafters (stick framing) give you full use of the attic space. With no web members filling the triangle, the attic is open for storage, HVAC equipment, or future conversion to living space. Rafters are cut and installed one at a time on site by a framing crew. The trade-off is labour intensity — a skilled crew needs 2-3 days to cut and install rafters for a typical 1,500-square-foot roof, versus a single day for trusses. Rafters also require a ridge board or ridge beam, collar ties, and possibly rafter ties depending on span and pitch.

Trusses are engineered and manufactured in a factory, then delivered and crane-set in a single day. They span longer distances than rafters of the same lumber size because the web members distribute forces efficiently. A truss can span 30-40 feet without interior bearing walls, where rafters of the same size might need a mid-span support beam. The trade-off is that the web members fill the attic triangle, making it unusable as living or storage space.

For homes where attic access matters (bonus room over garage, future attic conversion, or simply accessible storage), rafters are the right choice despite higher labour cost. For production homes, large commercial roofs, and any project prioritising speed, trusses win on economics. The cost difference varies: trusses typically run $3-$7 per square foot of roof area including installation, while stick-framed rafters run $4-$10 per square foot depending on crew rates and complexity.

When ordering either option, the roofing shingle bundle calculator estimates the shingle quantities once the roof area is known.

Rafters push outward against the walls (a force called thrust), and three different framing members resist that thrust in different ways.

Rafter ties connect opposing rafters near the top plate (within the bottom third of the attic height). They act as tension members, preventing the walls from spreading apart under roof load. IRC R802.5 requires rafter ties when the rafter slope exceeds 3/12 and the rafters bear on the wall plates. Ties are typically 2x4 or 2x6 lumber, nailed to each rafter with at least three 16d nails per connection. Every pair of opposing rafters should have a tie unless a ceiling joist serves the same function.

Collar ties connect opposing rafters in the upper third of the attic height. They resist wind uplift forces, not lateral thrust. Collar ties are smaller (2x4 minimum) and can be spaced at 4 feet on centre rather than at every rafter. They are not substitutes for rafter ties — a roof with collar ties but no rafter ties will still push the walls outward.

Ridge beams eliminate the need for rafter ties entirely by supporting the rafter peaks on a structural beam instead of a non-structural ridge board. A ridge beam is a heavy timber or engineered beam (LVL, PSL) sized to carry the full roof load and transfer it through posts to the foundation. Ridge beams are common in cathedral ceiling designs where rafter ties and collar ties would be visible and unwelcome. The beam size is significant — a 20-foot span at 6/12 pitch with typical residential loading might require a 5.25" x 11.875" LVL.

The choice between rafter ties and a ridge beam is primarily an aesthetic and functional decision. Rafter ties are cheap and effective but fill the attic space. Ridge beams are expensive (the beam alone can cost $500-$2,000) but create clean, open ceiling lines.