If the roof is walkable you can simply measure each plane both width and height. This way you will get a base square footage of the roof. Now if you have a hip roof you’ll have to square up any triangular sides with a little geometry and you’ll have to build in a waste factor based on how cut up the roof planes are.
For a Gable, Gambrel or Shed type of roof you’ll need to build in a 10% to 12 % waste factor for the Shingles. For a Hip roof you’ll need to do a little human computing depending on the roof and how chopped up the slopes are. I am going to suggest that you use 15 to 18 % as a waste ratio above your base measurements. All of this is considering that you are not scared of heights and can climb the roof slopes with great self confidence.
Now lets get into the way to measure your roof right from the ground you’ll need to determine the roof slope or pitch so you can get your conversions right. To do this right you will need to climb a ladder high enough to use a 1 foot level to determine each slope or the roof pitch.
The Slope = Rise (inches) divided by the run in feet. Follow this example:
1. Determine the rise in inches……. The level will help you determine the rise. If the level is 1 foot and the bubble is level at the roof plane you can use your tape measure to line up with the bottom side of the level and when you get to your inch mark and lets say you are on the 7 inch mark on your tape measure then you re at a 7:12 roof slope and that will work all the way up to 24 inches of which would be a vertical wall.
2. Measure your foundation and draw the roof as best as you can to represent what is visible to you and then add your overhangs to your base foundation dimensions. Now calculate the foundations and any add on like a carport that has a roof over it into a total square footage number.
These are your roof pitch and conversion factors as follows:
These are numbers I use when needed. I got them from the asphalt roofing manufacturers associations residential asphalt roofing manual published back in the early 1990’s.
I still add the waste ratios beyond the actual pitch conversions. NOTE, Different shingle manufacturers have different conversion factors and these are meant only to be a general guideline for getting your materials right for the roof. 100 square feet equals 1 roofing square and the bundles will have the coverage listed on the pack. Standard Strip 3 tabs will usually read 33.3 square feet. I feel this will hep you get the job done of measuring your roof.