Roof pitch decides what covering you can use, how steep the roof looks and how much material you need. This roof pitch calculator takes the horizontal run and the vertical rise and returns the pitch in the x:12 form that building codes use, the angle in degrees, the rafter length with and without overhang, and the counts and areas you need to order materials.
What Is the Roof Pitch Calculator?
Roof pitch describes how steep a roof is, and it is quoted in the x:12 form that building codes and manufacturers use. A 6:12 pitch rises 6 inches for every 12 inches of horizontal run, and the calculator returns that number along with the equivalent angle in degrees and the slope as a percentage.
It also works in the other direction, which is what you actually need when shopping. The rafter length tells you what to cut, the rafter count and total length tell you what to order, and the sheathing area tells you how many sheets of plywood to buy.
How Roof Pitch and Rafter Length Work
Pitch = (Rise in inches ÷ 12) ÷ Run in feet, multiplied by 12
The rafter length follows from the Pythagorean theorem, where the run and the rise in feet are the two legs and the rafter is the hypotenuse. Add the overhang to the run before calculating if you want the true cut length.
The rafter multiplier is the factor that converts the horizontal run into the sloped length, and it is the number that lets you convert a roof plan area into the actual surface area you have to cover. It is 1.0 for a flat roof and about 1.118 for a 6:12 pitch.
Common Pitch Categories
A pitch of 2:12 to 4:12 is a low slope, usually covered with roll roofing or a standing seam metal roof. 4:12 to 6:12 is the standard asphalt shingle range. 6:12 to 9:12 covers most of the traditional styles, and above 9:12 is considered steep.
The steepness matters practically because it determines which coverings are permitted, whether you need enhanced underlayment, and how much the roof area is affected by the pitch multiplier when you order materials.
Getting the Measurements Right
The run is half the span on a standard gable roof, so a 20 foot wide building has a 10 foot run to the ridge. On a hip roof the common rafters have that same run while the jack rafters are shorter, and the calculator gives you the common length.
The rise is measured from the top of the wall plate to the ridge, not from the floor. A 6:12 pitch over a 10 foot run means a 60 inch rise, because the pitch is 6 inches for every 12 inches of horizontal distance. If you are working from a floor to ridge figure, subtract the wall height first or you will end up with a pitch that is far too steep.
Frequently Asked Questions
What is a 6:12 roof pitch?
A roof that rises 6 inches for every 12 inches of horizontal run, which is an angle of 26.57 degrees and a slope of 50 percent. It is one of the most common pitches for asphalt shingle roofs.
How do I convert pitch to degrees?
Divide the rise by the run to get the slope, then take the arctangent. A 6:12 pitch gives a slope of 0.5, and the arctangent of 0.5 is 26.57 degrees.
What rafter length do I need for a 10 foot run?
For a 6:12 pitch the rise over a 10 foot run is 60 inches, so with a 12 inch overhang the run including the overhang is 11 feet and the rafter length works out at about 12.08 feet. The calculator gives the figure with and without the overhang.
How many rafters do I need?
Divide the building length by the spacing, add one for the end, and multiply by two for the two slopes. At 24 inches on centre over a 20 foot building that is 11 rafters per slope, or 22 in total.
Does a steeper roof need more roofing material?
Yes. The rafter multiplier converts the plan area into the true sloped area, so a 6:12 pitch needs about 12 percent more material than its footprint, and a 12:12 pitch needs about 41 percent more.