How do I calculate the roof pitch from the base measure or the upright?
Well...
Start with the common that every roof pitch ends with /12.
Example: 6/12 pitch or 8:12 pitch.
The 12 means the measure is based on how many inches the angle rises for every 12 inches or 1 foot of the width.
To keep everything simple, we will use a 24 inch base on the vent.
So for every 12 inches of base, the pitch is how many inches the rafter rises in the center.
If the center height is 12 inches then you might think this would be a 6 pitch with the base measure of 24 inches.
No. The rise is only calculated once, not for both sides.
So, you must first divide the base or width in half.
Now the base for only that rise area is 12 (half of 24 inches).
Now put the rise of 12 inches right up the center in junction with the half base width to make 12/12 as the pitch.
A more difficult one would be a base width of 70 inches and center height of 30 inches.
Same way...divide the width by 2 first.
So, 35 is the workable base.
Now 35 divided by the height of 30 is 1.16666
Finally, 12 divided by 1.16666 is 10.28 for the pitch.
So, this is a 10.3/12 pitch roof, rafter and gable wall.
To calculate off the height of the vent, just reverse the calculation.
Warning, using the height to calculate the pitch is even more unreliable for the actual roof pitch.
Example: 6/12 pitch or 8:12 pitch.
The 12 means the measure is based on how many inches the angle rises for every 12 inches or 1 foot of the width.
To keep everything simple, we will use a 24 inch base on the vent.
So for every 12 inches of base, the pitch is how many inches the rafter rises in the center.
If the center height is 12 inches then you might think this would be a 6 pitch with the base measure of 24 inches.
No. The rise is only calculated once, not for both sides.
So, you must first divide the base or width in half.
Now the base for only that rise area is 12 (half of 24 inches).
Now put the rise of 12 inches right up the center in junction with the half base width to make 12/12 as the pitch.
A more difficult one would be a base width of 70 inches and center height of 30 inches.
Same way...divide the width by 2 first.
So, 35 is the workable base.
Now 35 divided by the height of 30 is 1.16666
Finally, 12 divided by 1.16666 is 10.28 for the pitch.
So, this is a 10.3/12 pitch roof, rafter and gable wall.
To calculate off the height of the vent, just reverse the calculation.
Warning, using the height to calculate the pitch is even more unreliable for the actual roof pitch.
Feel free to call with any additional questions...901.281.2887