Spike's Calculators

- the roof slope (grade) in a percentage
- the angle of the roof in degrees
- the slope factor
- hip valley factor

6 - 12 pitch 6 / 12 = 0.5 * 100 = 50 % 4 - 12 pitch 4 / 12 = 0.333 * 100 = 33.3%Angle of the Roof in Degrees

Convert radians to degrees by multiplying the radians by

Convert degrees to radians by multiplying the degrees by π/180

where p = 3.14159265358979 and then some!

For most purposes, we are dealing with here

The slope factor is a pretty handy number for use in construction. With a slope factor you can calculate the square footage of a roof. To get the slope factor from the roof pitch you need to know the formula:

The rise = 6" in 12" of run 6 squared = 36 12 squared = 144 36 + 144 = 180 The square root of 180 = 13.41460786 divide this by 12 rounded to 4 digits = 1.1180 The slope factor for a^{6}/_{12}roof pitch = 1.1180

With this factor you can calculate the square footage of the roof without having to be physically on the roof. Measure the total width of the home including the two eave's overhangs and the total length including the two gable overhangs.

A home with a total width of 20' and an overall length of 45' and a^{6}/_{12}roof pitch Multiply these two amounts to get a square footage value. 20 * 45 = 900 sq ft. Multiply this amount by the slope factor for a^{6}/_{12}pitch. 900 * 1.1180 = 1006.2 sq ft.

The slope factor can also be used to calculate the length of a common rafter. All you need to know is the slope factor for your roof the length from outside wall to outside wall plus the eave overhang. And this can all be calculated standing on level ground.

the building width from outside wall to outside wall = 20' 8" the pitch of the roof =Split the the building width in two and add the eave measurement:^{6}/_{12}the overhang of the eaves = 18"

it is easier to work with by converting the feet value to inches first

20' * 12 = 240" + 8"= 248" / 2 = 124" + plus the eave measurement 18" = 142"multiply this length by the slope factor of a 6 /12 roof = 1.1180 to get the rafter length

142" * 1.1180 = 158.756"

Inches + Fraction: 158

Inches + Usable Fraction: 158

Feet + Inches:13'

The length of the rafter is 158

If you are using a ridge board you will have to subtract half the thickness of the ridge board from this measurement.

A hip or valley factor is calculated with a slightly different formula.

6 squared = 36 16.97squared = 287.9809 36 + 287.9809 = 323.9809 the square root of 323.9809 = 17.999469....... 17.999469 / 12 = 1.49995 or rounded to 1.5000The factor to calculate the length of a hip or valley rafter = 1.5000