Roof pitch is the measure of a roof's slope or level of incline. Pitch is expressed as a fraction representing the roof's rise (height) over its width (span). A house roof that is 38 feet wide with a 1-foot overhang has an overall span of 40 feet. Measured from the eaves to the peak, the roof's rise or height is 10 feet. The pitch is therefore 10/40. Reduced to its lowest form, the fraction becomes 1/4. The roof therefore has a 1/4 pitch.
Geography and Pitch
Geographical location is a prime factor to consider when planning a new structure and deciding upon the best roof pitch. If you live in an area that experiences heavy snow, you'll want to reduce the accumulation of snow and ice loads on the roof. A steep pitch will allow snow to slide off the roof more easily. Two prime examples of a high-pitched, geographically determined roofing style are the A-frame cottages found in Snowbelt areas and the high altitude ski chalets located in mountainous terrain.
Costs and Pitch
Low-pitched roofs are easier to work on and cheaper to construct than medium or high-pitched alternatives. These low-pitched roofs are common on large commercial complexes. Asphalt shingles will not adequately protect low-rise roofs against water damage so you must use specialised water-resistant membranes. Routine maintenance is necessary more often to ensure the integrity of this material. Steep-pitched roofs make an aesthetically pleasing statement in residential applications but cost more for installation and maintenance due to the need for specialised safety equipment.
Roof Pitch Terminology
Architects, contractors and roofers may express roof pitch differently. The 1/4 pitch referred to in the Introduction above may also be denoted on a builder's plan as 1-4; 1 to 4; 4,1 or 4, 1 on 4. If you are framing a new home, garage or garden shed, the pitch of the roof will be marked on the plans. Look for a triangular-shaped drawing with a fraction inside expressed in inches such as 1/4 or 3/4. The higher the fraction, the steeper the pitch.
Measuring Existing Roof Pitch
You can most easily determine the pitch of an existing roof using a pitch and angle finder. This special tool can also find level or plumb. For pitch determination, place the pitch finder outside on top of the roof, on a gable end soffit or inside the attic under a rafter. There are also printable roof pitch charts to visualise the angle of inclination with a 1-foot run at the base and 12 inches of vertical rise.