Standard Roof Pitch Basics

Definition

• In North America, roof pitch is represented by a fraction such as 12/12 or 7/12. The numerators are inches and the bottom number is always 12 because pitch is based on one foot. The top number is the severity of the slope. The higher the number, the sharper the downward angle of the slope. A slope can also be measured in degrees. A 12/12 pitch is a 45 degree angle and 1/12 pitch is a 4.5 degree angle.

Determinations

• There are three factors that go into determining the slope of a roof. The first is the building codes in the area where the house is to be built. The codes may have a range of acceptable pitches or there may be specific pitch requirements for each house design. The second is the roofing material manufacturer specifications. Materials such as galvanized metal and fiberglass roofing may come with specific pitches that must be used for maximum efficiency. The final determining factor is any existing roof that you may be tying into. If you are building an add-on to a home or a covered porch, then that new roof needs to match the pitch of the existing roof to prevent problems with water leaking.

Measuring

• You can use two methods of measuring the pitch of an existing roof. The first is to run a level under the roof soffit out 12 inches horizontally. The vertical distance between the level and the roof is the pitch. To measure pitch from the top of the roof, place a 48-inch level on the roof until the level is perfectly vertical. Place a framing square perpendicular to the level with the framing square tongue facing up. Bring the framing square down the level until the horizontal square is touching the roof. Measure the distance on the level from the framing square to the roof and divide by two for the pitch.

Considerations

• Measuring roof pitch from the top of the roof may not be the most accurate way to get the correct number. If there are several layers of shingles on a roof, then measuring the pitch from the top of the roof will give a misleading number. If you notice layers of shingles on a roof, then measure the pitch from the bottom for accuracy.