Get common rafter length from building width, roof pitch and overhang, plus ridge and count.
Quick answer: A common rafter runs from the outside wall to the ridge, so its length is half the building width multiplied by the pitch multiplier, plus the overhang. For a 24 ft wide building at 6/12 with a 12 in overhang: 144 in of run times 1.118 is 161 in, plus 12 in gives 173 in, or about 14 ft 5 in.
What are you working out?
Rise over 12 in run
in
A 6/12 roof climbs 6 in for every 12 in across.
ft
The full span; each rafter covers half of it.
ft
in
12–18 in is common; measure from the wall to the fascia.
in
16 in or 24 in centres are the usual choices.
Pitch
6/12
26.6° · slope 50.0%
Rafter length
14 ft 5 in
Run plus overhang, measured along the slope
Cut list
Run per rafter (half the span)144.0 in
Common rafter length14 ft 5 in
Rafter count at 16 in centres31 (one per position, both slopes)
Ridge board40.0 ft
Roof surface1,073 sq ft
Pitch
Angle
Slope
Area ×
4/12
18.4°
33.3%
1.054
5/12
22.6°
41.7%
1.083
6/12
26.6°
50.0%
1.118
7/12
30.3°
58.3%
1.158
8/12
33.7°
66.7%
1.202
9/12
36.9°
75.0%
1.250
10/12
39.8°
83.3%
1.302
12/12
45.0°
100.0%
1.414
Low-slope roofs below 3/12 need a different system entirely — roll or membrane roofing rather than shingles — and steep roofs above 8/12 usually add labour for staging. Both change your material list, not just the numbers here.
Pitch is rise over 12 in of run, written 6/12. The area multiplier converts a flat footprint into sloped roof surface — at 6/12 you need about 12% more shingles than the footprint suggests.
Core facts
Run
half the building width
Rafter length
run × multiplier + overhang
Ridge board
same length as the building
Count
length ÷ spacing, plus one
What is Rafter Length Calculator?
A common rafter is the sloped member that runs from the top plate of a wall up to the ridge board, and its length depends on three things: half the building width, the roof pitch and the overhang. Half the width is the run; multiplying the run by the pitch multiplier gives the sloped distance from the wall to the ridge; adding the overhang gives the board length before you cut the birdsmouth and the plumb cuts. The calculator also returns the ridge board length and how many rafters you need at 16 in or 24 in on center. It gives a theoretical length, so cut one rafter and test-fit it before you cut the rest of the stack.
Common uses for Rafter Length Calculator
Cut a pattern rafter for a gable roof from the building width
Work out how many rafters a 32 ft building needs at 16 in
Add a soffit overhang to the rafter length before ordering stock
Compare 16 in and 24 in spacing and the lumber each needs
Estimate ridge board length for a simple gable roof
Run, rise and the plumb cut
The run is half the building width, adjusted for the thickness of the ridge board. The rise comes from the pitch. The plumb cut at the top matches the roof angle, and the birdsmouth at the bottom sits over the wall plate. Cutting the plumb cut at the wrong angle shows up as a gap at the ridge, so mark the angle from the pitch rather than eyeballing it, and cut a test rafter before the whole stack.
Overhangs, and why they change the order
The overhang is the part of the rafter that extends past the wall to form the eaves. Plans usually dimension it horizontally, but a rafter is cut along the slope: on a 6/12 roof a 12 in horizontal overhang is about 13.4 in along the rafter. The calculator adds the figure you enter directly to the sloped length, so if your plan measures the overhang horizontally, enter the converted number.
Spacing, spans and what the code checks
Rafter spacing of 16 in or 24 in on center comes from the span tables, which depend on species, grade, lumber size, roof load and whether a ceiling is attached. The calculator counts rafters at the spacing you choose; it does not check whether that size and spacing is adequate for your snow and wind load. Check a span table or have the design reviewed, and confirm with your local building department.
Frequently Asked Questions
How do I calculate rafter length?
Divide the building width by two to get the run in feet, convert it to inches, multiply by the pitch multiplier and add the overhang. For a 24 ft building at 6/12: the run is 12 ft, or 144 in; times 1.118 is 161 in; plus a 12 in overhang gives 173 in, about 14 ft 5 in.
Do rafters span the whole building?
On a gable roof two rafters meet at the ridge, so each one covers half the width plus its overhang. A 24 ft wide building therefore uses rafters around 14 ft 5 in long at a 6/12 pitch, not 24 ft. A single-slope shed roof is the exception, where one rafter spans the full width.
How many rafters do I need?
Divide the building length in inches by the spacing and add one. A 32 ft building at 16 in on center is 384 divided by 16, plus one, which is 25 rafters per side. Double that for a gable roof, and add rafters for hips, valleys and any framed opening.
Do I measure along the top edge or the center line?
A calculated rafter length is measured along the top edge of the board, because that is the plane of the roof. The birdsmouth and the plumb cuts are laid out from that line. Cut one rafter, set it in place to check the seat and the ridge fit, then use it as a pattern for the rest.