The overhang on your flat roof is the portion of the roof that extends out past the walls of your house. Flat roofs are easier to measure than their sloped counterparts because you don't have to adjust your calculations to take into account those very same slopes. You can simply measure the distance from the wall to the edge of the overhang, and that will be the same measurements used for the top of the roof as well.
Set up your ladder underneath your roof's overhang and ensure that the footing is secure.
Climb the ladder and get to where you can easily reach the overhang without stretching.
Hook the measuring tape onto the outer edge of the overhang and bring the tape all the way in until it is flush with the wall. Examine the tape at this point and note the marking on the tape. This will tell you the depth of your overhang.
Calculate the length of your overhang by measuring the length of the wall directly beneath the overhang, then adding the depth you found in the previous step to both sides of the wall.
Repeat these steps for all the overhang areas around your house.
If you wish to get the surface area of all your overhangs at once, it might help to draw out the shapes on a piece of notepaper and label each area so you do not miscalculate. It is always easier to do these types of calculations if you can see exactly what you are doing.