Pitched roof area calculator.
Working out the pitched roof area can be done with the aid of a simple calculator using the overall foot
print area times by a factor (usually the cosine).
There are plenty of online pitched roof calculators that will work everything out for you including rafter
lengths, hip lengths etc.
For those that would like to work it out simply using a scale rule & a calculator for yourselves here is the
description of what to do:-
If you have a roof plan and no section through the building showing the length of the rafter, provided you know
what the rafter pitch should be it is possible to calculate the rafter length from the horizontal distance from the
fascia board to the ridge board.
The rafter length will always be longer than the horizontal distance from the fascia board to the ridge board
and will vary with the angle (pitch) of the roof. The steeper the rafter the longer it will be.
There is a direct mathematical relationship between the horizontal distance and the rafter length for any given
angle. Like Sine, Cosine and Tangent in trigonometry there are other relationships, one of which is Secants.
Natural secants are tables of the relationship between the angle and the hypotenuse of a triangle when the
horizontal (adjacent) distance is one.
When using the tables, you look up the angle (pitch). The figure found should be used to multiply the horizontal
distance, and the result will be the rafter length (see table).
If you are measuring the total area of a pitched roof, and all rafter pitches are the same, it is not essential
to measure each roof slope separately and add them together, especially if there are a lot of small roof
By measuring the length and width of each rectangular part of the building, and using the natural adjustment
factor figure for the rafter pitch, all three can be multiplied together and the answer will give the total
roof area. A hipped roof will have the same roof surface area as a mono pitch, or duo pitch (gable to gable), for
the same building footprint.