What does a 6:12 roof pitch mean?

A 6:12 pitch means the roof rises 6 inches vertically for every 12 inches of horizontal run. This equals approximately 26.6 degrees and is one of the most common residential roof pitches.

How do I calculate rafter length from roof pitch?

Compute the pitch factor as the square root of 1 plus the square of (rise divided by 12), then multiply the horizontal rafter run by that factor. For a 6:12 pitch the factor is about 1.118, so a 10-foot run needs an 11.18-foot rafter before overhangs.

What is the minimum roof pitch for asphalt shingles?

Standard asphalt shingles require a minimum pitch of 2:12. Between 2:12 and 4:12, shingles need a special underlayment system (double coverage) to stay watertight on the low slope.

How do you convert roof pitch to degrees?

Take the arctangent of the rise divided by 12. A 4:12 pitch is about 18.4 degrees, 6:12 is about 26.6 degrees, and 12:12 is exactly 45 degrees.

What is the roof pitch factor used for?

The pitch factor converts horizontal measurements into sloped measurements. Multiply the horizontal run by the factor to get rafter length, and multiply the plan (footprint) area by the factor to get the true roof surface area for material orders.

Why does roof pitch matter for a covered stair?

A porch roof or stair-tower cover sits directly above the flight, and its low edge is usually right over the top of the stairs. Too shallow a pitch can drop the cover below the required headroom at the landing, so the pitch has to be checked against stair clearance.

Roof Pitch Calculator

What Is the Roof Pitch Calculator?

Calculate roof pitch ratio, angle in degrees, pitch factor, and true rafter length for roof construction planning. Roof pitch is expressed as rise over a 12-inch run: a 6:12 pitch rises 6 inches for every 12 inches of horizontal run. The calculator converts that ratio into the angle in degrees, the pitch factor used for rafter math, and the actual rafter length from any horizontal run you enter. The pitch factor is the multiplier that turns a flat run into a sloped rafter: it equals the square root of 1 plus the square of the rise divided by 12. Multiply the horizontal run by the pitch factor and you get the rafter length, measured along the slope, before overhangs and cuts. Although this is not a stair calculator, roof pitch math is the same rise-over-run trigonometry that drives stair angles, and it matters on stair projects more often than people expect. Covered exterior stairs, roofed stair towers, and porch roofs over deck stairs all put a roof directly above a flight, and the roof pitch controls rafter lengths, the choice of roofing material, and the headroom under the low side of the cover at the top of the stairs. Most residential roofs sit between 4:12 and 9:12. Standard asphalt shingles need at least a 2:12 pitch, and anything below 4:12 needs special underlayment detailing, so pitch is a material decision as well as a geometry one.

How to Use This Calculator

Enter the roof rise in inches per 12 inches of run (for example, 6 for a 6:12 pitch), or enter the total rise and total run and let the calculator derive the ratio.
Review the pitch ratio, shown in the standard X:12 format used on plans and in lumber yards.
Review the angle in degrees, calculated as the arctangent of rise over run. A 6:12 pitch equals about 26.6 degrees.
Note the pitch factor. This multiplier converts any horizontal run into the sloped length of the rafter.
Enter the horizontal run of your rafter, measured from the ridge to the outside of the wall plate, to get the true rafter length along the slope.
Add the overhang or tail length separately. The calculator gives the rafter from ridge to wall; the eave overhang and the plumb or birdsmouth cuts are additions you plan on top of it.

Roof Pitch Calculator