altering pitch on garage roof
 

how do i work out the pitch and degree of fall on a 20 foot garage roof which will be covered with box profile sheeting.

Pitch is based on inches of rise per 12inches of run. Example: 5/12 pitch has a rise of 5 inches per 12 inches of run.

Hmm
 

If you are going to use conventional roofing like shingles or metal seamed, you should have a 4 pitch or more.(steeper)
If you are using anything for flat roofs, keep the pitch at 4 or less. (flatter)
Hope that makes some sense to you.:)