How To Calculate Pitch/Slope of your roof
So what does the term 5&12 pitch mean?
The 5 means that for every 12 inches of horizontal run, the roof rises by 5 inches.
Knowing the pitch of your roof is vital to ordering the proper sized gable vents.
To calculate the pitch, all you'll need is an 18 or 24inch level, a tape measure, and pencil.
How To Calculate The Height Of Your Triangle Vent
If you know your roof pitch and you know the overall base width of the vent you are looking to order, but you don't know the overall height of your triangle,
here is how to calculate it. Make sure you are using inches and not feet.
Regular Triangle
Height=((Base/2) x Pitch)/12
 Divide your base width by 2
 Multiply that by your pitch
 Divide that by 12
Example:
Calculating the height of a 8/12 pitch triangle vent with a 48" base:
 48/2 = 24
 24 x 8 = 192
 192/12 = 16 = Height
The height of this triangle with a 48" base is 16".
Right Triangle
Right triangles are even easier, as you do not have to divide the base by two.
Height=(Base x Pitch)/12
Example:
Calculating the height of a 8/12 pitch right triangle vent with a 48" base:
 48 x 8 = 384
 384/12 = 32 = Height
So the height of this right triangle with a 48" base is 32".
