Rafters Calculator
Calculate rafter system for gable roof: rafter length, spacing, cross-section. Snow and wind load consideration.
Width of the house at the gable end, in meters
Height from cornice to ridge, in meters
Length of the house along the ridge, in meters
Distance between rafters on centers, typically 60–100 cm
Overhang past the wall, typically 0.3–0.7 m
Calculation formula
L = √((W/2 + O)² + H²); N = (2 × ⌈Lhouse·100 ÷ spacing⌉ + 2); Ltotal = N × L
Rafter length L is the hypotenuse of a right triangle: half the house width plus overhang as one leg, roof height as the other. The number of rafters per slope equals the house length divided by the spacing, plus the end rafter. A gable roof has twice as many rafters total.
About the rafters calculator
The rafters calculator returns the length of a single rafter, the total number of rafters for a gable roof and the overall lumber footage. You only need to enter the house width and length, the roof height, the rafter spacing and the overhang.
The calculation is useful when designing the rafter system and buying 50×150 or 50×200 mm boards for the roof framing of a private house, bathhouse, garage or attic. With the rafter length and count in hand, the lumber budget falls out immediately.
All math runs locally in your browser — nothing is sent to a server. The calculator is free, requires no sign-up, and has no usage limits.
Calculator benefits
Length and count at once
Rafter length, total rafter count and overall lumber footage in a single pass.
Overhang included
The overhang is already part of the rafter length, so the result is ready for ordering boards.
Any spacing
All common spacings are supported — 60, 80, 100 cm and any value in between.
FAQ
Which rafter spacing should I pick — 60, 80 or 100 cm?
60 cm is used under heavy roofing and in heavy-snow regions. 80 cm is the universal default for metal tile and corrugated sheet. 100 cm works for light roofing (onduline) on short spans. The wider the spacing, the larger the rafter cross-section must be.
What rafter cross-section to use — 50×150 or 50×200 mm?
A 50×150 mm board covers spans up to 4–4.5 m with 80 cm spacing. For 5–6 m spans, or at 60 cm spacing under heavy snow, choose 50×200 mm. Beyond a 6 m rafter, 100×200 mm or built-up sections are typical.
How do I account for snow load per the building codes?
Per SP 20.13330 ‘Loads and Actions’, the snow region sets the nominal load — from 0.8 kPa in zone I to 4.0 kPa in zone VIII. A pitch coefficient μ between 1 and 0 applies for slopes of 25–60°. In most temperate regions the design load lands at 180–240 kg/m², and the rafter section and spacing are sized for that.
Do I need a mauerlat (wall plate) under the rafters?
For a masonry house (brick, aerated or foam concrete) a wall plate is mandatory — typically a 100×150 or 150×150 mm beam that the rafters bear on, transferring roof load to the wall. In a wooden house the top log course or the top of the frame wall serves as the wall plate.
How is the overhang accounted for in the rafter length?
The rafter length must include the overhang: add the overhang (usually 0.3–0.7 m) to half the house width, then take the hypotenuse from there. The calculator does this automatically — just enter the overhang in the matching field.
How much extra lumber should I buy?
Add 10–15% to the calculated footage for cuts, end-trimming and waste. For a typical 8×10 m gable house at 80 cm spacing this works out to roughly 1–1.5 m³ of extra board.