Gutters are essential to the longevity of most homes. Without gurtters and downspouts leading rainwater away from the house, foundations become feedlots for mold that can sicken your family and rot your house. Once you've settled on a material, and a profile, you'll need to pick a size. This procedure for sizing your gutter system is based on roof size, your area's likely rainfall intensity, gutter volume and downspout size and frequency.
1. Calculate your roof's watershed area -- A roof's watershed area isn't obvious. Maximum rainfall is likely wind driven, so steep roofs may collect more water than flat roofs. To figure a roof's watershed area, multiply its surface area by the appropriate factor on the table.