The Meaning of Pitch for a Roof

Significance of Pitch

• Roof pitch is an expression of how steep a given roof is. Roofs that are very steep are referred to as "pitched" roofs, simply meaning that the pitch is very high. Although there are aesthetic considerations in determining how pitched a roof will be, the primary deciding factor is the typical precipitation in the area of construction. Areas with high levels of precipitation commonly have structures built with very steep, or highly pitched, roofs. This allows the roof to redirect the precipitation off the structure quickly. Conversely, areas that see very little precipitation may have very low pitched, or sometimes even flat, roofs.

Engineering Pitch

• A roof's pitch is determined by the rafters underneath it, so the pitch plays a key role in how the overall construction of the structure happens. In most residential construction, the rafters are constructed off-site and then installed on the structure. The contractor must know the desired pitch before ordering the rafters.

  The pitch of a roof impacts more than just the roof itself. Very steep roofs will displace large amounts of precipitation, and that precipitation must be properly redirected to prevent flooding and water damage to the structure. This means having eave troughs capable of carrying the amount of water displaced by the roof and downspouts which drain properly away from the structure.

Measuring Pitch

• Pitch is represented by a simple fraction, expressed as rise over run in inches. More specifically, it is rise over every 12 inches of run, so a roof pitch of 3/12 will rise 3 inches for every 12 inches of run.

  The roof pitch can be measured from the top of the roof or the underside. To measure from the top, a level can be held out from the surface of the roof and a measurement taken straight down from 12 inches. This will give an approximate pitch, but this is prone to user error and also varies based on shingle thickness and the layers of shingles in place. If a measurement is taken from the underside of the roof, the process is similar, but it is done on the rafters themselves within the attic. This takes the thickness of shingles out of the equation and tends to yield a more accurate measurement.

Incorrect Pitch

• It is not uncommon, particularly in older homes, to find roof pitches that are not appropriate for the structure or the area in which it was built. As building codes have evolved over the years, this has become less and less of a problem, but improper pitch, subsequent improper water management and a lack of waterproofing membrane technology in older construction is a leading cause of leaks and landscape erosion.