A roof pitch multiplier, also called a roof pitch factor, is a number that is multiplied by the area covered by a sloped roof to produce the area of the actual surface of the roof. The roof pitch multiplier is determined by finding the square root of ((rise/run)² + 1). The slope of the roof provides the rise and the run to be plugged into the equation. A roof pitch of 4/12 has a rise of 4 and a run of 12.
(For help converting any slope from one way of expressing it to another, see Roof Slope Expressed as Rise/Run, Degrees, Percentage.)
The roof pitch factor is also used to calculate the length of common rafters. In this case, the multiplier is multiplied by the run of the rafter (the horizontal distance that will be covered by the rafter). For precision, the thickness of the ridge board and any eave overhang should also be taken into account when determining the correct length of the rafters.
This table provides the roof pitch multiplier for roofs of various slopes. For a detailed explanation of the roof pitch multiplier, see "How to Find the Area of a Sloped Roof".
(Always measure for every rafter, ¼-inch makes a difference.)
One more thing: if you’re using this table, you should consider getting yourself a construction calculator. This one is very good.*