Rafter Length Calculator

Find the run, which is half the building span for a gable roof, then the rise, which is the run multiplied by the pitch over 12. The rafter length is the square root of run squared plus rise squared. A 24 foot span at 6:12 has a 12 foot run, a 6 foot rise and a rafter of 13.4 feet.

Rafter length equals the square root of run squared plus rise squared, which is the same as the run divided by the cosine of the roof angle. With a pitch p in 12, the length per foot of run is the square root of 1 plus (p/12) squared, so a 6:12 roof gives 1.118 feet of rafter per foot of run.

For a gable roof, the run is half the span, less half the thickness of the ridge board. A 24 foot span with a 1.5 inch ridge board has a run of 12 feet minus 0.75 inches, which is 11.94 feet. For a shed roof the run is the full span, since the rafters go from one wall to the other.

Roof pitch is the rise of the roof over its run, usually written as inches of rise per 12 inches of run. A 6:12 pitch climbs 6 inches for every 12 inches of horizontal distance, which is an angle of 26.57 degrees. Higher numbers mean steeper roofs.

Take the inverse tangent of the pitch divided by 12. A 6:12 pitch is atan(0.5) = 26.57 degrees, 8:12 is 33.69 degrees and 12:12 is 45 degrees. To go the other way, multiply the tangent of the angle by 12: 30 degrees is 6.93:12. The pitch table above lists the common values.

The run is 12 feet and the rise is 6 feet, so the common rafter is 13.42 feet, about 13 feet 5 inches, with no overhang or ridge board. Allowing for a 1.5 inch ridge board and a 12 inch overhang, the length rises to 14.46 feet, or 14 feet 5 9/16 inches.

At 4:12 the run is 6 feet and the common rafter is 6.32 feet. At 6:12 it is 6.71 feet and at 12:12 it is 8.49 feet. These are measured along the rafter from the centre of the span to the wall, before overhang and ridge adjustments. The span table above covers more pitches.

A 20 foot span has a 10 foot run. At 4:12 the rafter is 10.54 feet, at 6:12 it is 11.18 feet, at 8:12 it is 12.02 feet and at 12:12 it is 14.14 feet. Add the tail for any overhang and subtract half of the ridge board.

A hip rafter runs along the diagonal of the corner, so its run is the common run times the square root of 2. Its length is the square root of that diagonal run squared plus the rise squared. For a 12 foot common run and a 6 foot rise the hip is 19.4 feet, longer than the 13.4 foot common rafter.

A valley rafter has the same length as the hip rafter when the two roof planes have equal pitch and meet at a 90 degree corner. Use the common run times the square root of 2 for the horizontal distance, then add the rise by the Pythagorean theorem. Unequal pitches or other angles need a different calculation.

The rafter tail is the part of the rafter that extends past the wall plate to form the eave overhang. Its length along the slope is the horizontal overhang times the square root of 1 plus (pitch/12) squared. A 12 inch overhang on a 6:12 roof needs a tail of 13.4 inches along the rafter.

Add the tail to the common rafter. Convert the horizontal overhang to a length along the slope by multiplying by the length factor for your pitch, which is 1.118 for 6:12. The calculator does this when you enter the overhang and shows the total rafter length and the length without overhang.

Yes. The rafters butt against the sides of the ridge board, so each one runs to the board's face and not to the centre line. Subtract half the ridge thickness from the run. A 1.5 inch ridge board takes 0.75 inches off the run and, at 6:12, about 0.84 inches off each rafter.

Divide the building length by the spacing, round up, add one for the end, and double the result for a gable roof, with a rafter on each side. A 30 foot building at 16 inches on centre needs 24 rafters per side (23 spaces plus the end one), so 48 in total. Closer spacing means more rafters.

Common spacing is 12, 16, 19.2 or 24 inches on centre. The right choice depends on the rafter size, the span, the roof load and the sheathing, which are set by your local code and lumber span tables. This calculator uses your spacing only to count the rafters and does not size the lumber.

Multiply the run by the pitch and divide by 12. A 12 foot run at 6:12 gives a 6 foot rise. In angle terms the rise is the run times the tangent of the angle. The rise tells you how high the ridge sits above the wall plate and helps plan ceiling heights and trusses.

A rafter is a single sloped member that, with its partner on the other side, carries the roof loads onto the walls and is cut and assembled on site. A truss is a factory-built frame that includes rafter-like top chords and a bottom chord joined with connectors. This calculator finds the length of a rafter.

A common rafter runs at a right angle to the ridge and the wall plates, from the wall plate to the ridge board. It is the standard rafter in a gable roof. Hips and valleys, jack rafters and cripples are the other types and they are cut from the common rafter's geometry.

The building span is the width between the outside walls. The rafter's run is only half of it for a gable roof. People also speak of a rafter's span as the horizontal distance it crosses between supports, which is a structural question about load, lumber size and spacing and not a length. Check your local code for those.

No. It finds lengths, angles and counts from the geometry. Which lumber size and spacing a given span needs depends on the wood species, grade, roof load and local building code. Use your code's span tables or an engineer for that, then enter your chosen spacing here for the rafter count.

A shed roof slopes one way, so the run is the full span from the low wall to the high wall. At 4:12 over a 12 foot span the rise is 4 feet and the rafter is 12.65 feet, or 13.7 feet with a 12 inch overhang. Choose the shed roof option in the calculator.

The two sides slope away from a central ridge, so each rafter spans only half the building width. Use half the span, less half the ridge board, as the run, then apply the pitch. The calculator's gable option does the whole sum and counts the rafters on both sides.

The plumb cut at the ridge is the roof angle measured from vertical: for 6:12, 26.57 degrees from the vertical edge. The level or seat cut at the wall is 90 degrees minus that angle, or 63.43 degrees. On a speed square, set the pivot at the pitch number on the common scale and mark along the edge.

Divide the length in inches by 12 for feet, and read the remainder as inches. The calculator shows the decimal feet and the result in feet, inches and sixteenths. A rafter of 14.4646 feet is 14 feet 5 9/16 inches. Round up to the next stocked board length when buying.

Buy the next standard lumber length above the calculated length, since stock comes in 2 foot increments. A 14.46 foot rafter needs a 16 foot board, and a 13.4 foot rafter also needs a 14 or 16 foot board depending on what you have in stock. Add a little for the cut at each end.

Divide the run by the cosine of the angle. A 12 foot run at 30 degrees gives 12 / cos 30 = 13.86 feet. Pick the degrees option for the pitch in the calculator, enter the angle, and it applies this directly and also reports the pitch in x:12 form.

A hip rafter sits at an outside corner where two roof planes meet, forming a ridge that slopes down to the corner. A valley rafter sits at an inside corner where two planes meet, forming a trough that carries water. For equal pitches at a 90 degree corner, the two have the same length.

For a 90 degree corner with equal pitches, a hip or valley rafter is the square root of 2 + (pitch/12) squared per foot of common run. At 6:12 the factor is 1.5 and at 12:12 it is 1.732. Multiply the factor by the common run to find the hip length, as in the table above.

No. The pitch is set by the rise and the run, and the overhang is an extension of the same slope beyond the wall. It lengthens the rafter's tail, not its pitch. The calculator's total rafter length adds that tail to the common rafter.

Set the pivot at the pitch number on the common scale, such as 6 for 6:12, and mark along the edge for each foot of run. Each mark advances 12 inches of run and the same distance along the rafter, which is 13.42 inches for 6:12. The calculator gives the total length directly.

The unit run is 12 inches, the standard horizontal distance used to describe pitch. A rafter's unit length is the sloped distance for that 12 inches: 13.42 inches at 6:12 and 16.97 inches at 12:12. Multiply the unit length by the run in feet for the rafter length.

Use a 15 foot run for a gable roof, less half of the ridge board. At 6:12 the rise is 7.5 feet and the common rafter is about 16.77 feet before overhang and ridge adjustment. Enter your actual pitch and overhang to get the final cut length.

A 12:12 pitch is a 45 degree slope, so the rafter is the run times the square root of 2, or 1.414 feet per foot of run. A 10 foot run gives a 14.14 foot rafter. The pitch table above lists the length factor for other pitches.

Yes. A lean-to is a shed roof, so choose the shed option and enter the full span from the low wall to the high wall as the run. The calculator returns the rafter length, the tail for your overhang and the roof angle for that single slope.