How to calculate roof pitches in 3D from 2D drawings
I was asked again today how to do a takeoff of a 2d roof drawing so that the measurement you get is accurate in the 3D roof.
It's a simple mathematical table, and you can download and print it out here.
Download roof_pitch_factors.pdf
Update: Chris from Southern Store Fixtures has requested a clarification.
The image belows shows a roof in two parts (one part is the first floor and one part is the second floor.) Let's measure the second floor part.
Now the blue area (bottom drawing - click to enlarge) shows a flat square footage of 981.8 square feet. But this is in 2 dimensions. Since we know the slope of the roof (6:12), we look it up on the table and find 1.12. Multiply 981.8 * 1.12 to get 1099.6 square feet, which is the true 3D area of this roof.
Interestingly, it doesn't matter how many ways the roof slopes, or how complex it is - if all planes slope at the same pitch, you can apply the same factor to all of them. If the roof has multiple different pitches, just measure each part separately and apply the appropriate factor. (And one other note - roofs are usually measured in squares - 1 square is 100 square feet, so divide by 100!) Hope that helps!
I still don't get it. Is there anyway to see an example?
Posted by: chris | December 19, 2007 at 03:13 PM
Hey, that roof looks familiar!
Posted by: Mike Gemmell | December 21, 2007 at 11:26 AM
Hey, that roof looks familiar!
Posted by: Mike Gemmell | December 21, 2007 at 11:27 AM
As this is the correct way to convert 2-dimensional surface area into actual 3-dimensional surface area if you are trying to draw a 3D roof from takeoffs of a 2D the process is just as easy. Most 2D plans will show a slope as shown in the table above, this is nothing more than a ratio of rise/run. If a roof shows a pitch of 6:12 (6" of rise to every 12" of run"), it is half as tall as it is wide, 3:12 would be a quarter and so on and so forth
Posted by: Tony Greene | January 02, 2008 at 02:34 PM