Roof Area Vs. Gutter Size

Roof Area

• Three main factors play a role in determining the size of the gutters and downspouts you should use for your home. The first of those is the roof's watershed area, which will give you an idea of the amount of rainfall that can fall on your roof at any given time interval. This number is based on the pitch of your roof multiplied by a square footage of your roof. A roof with a higher slope will have a higher watershed factor because it will rid the roof of water more quickly than a slightly sloped or nearly flat roof. Watershed factors range from 1 to 1.3, with 1 being the number used for a flat roof or one that only rises 3 inches every 12 inches. One that rises 12 inches every 12 inches will have a factor of 1.3.

Rain Instensity

• The intensity of the rainfall in your area also factors into the size of the gutters and downspouts you will use. These numbers vary by region and the maximum expected rainfall that will occur in that area in a five-minute span. This number is generally measured in an inches per hour format. The resulting number is then divided into the 1-inch per hour watershed capacity of your gutter and downspout system.

Downspouts

• The size and location of downspouts also factors into the size of guttering you choose for your home. The Copper Development Association notes that each downspout should drain somewhere in the neighborhood of about 50 feet of guttering. The association also indicates that you should try to avoid placing downspouts in any place where the water might have to go around the corner to reach the gutters in the first place.

Pitch

• The pitch of the gutters also plays a role in determining how much the maximum watershed intensity will be for a given area of roofing. In instances where it appears that the watershed capacity will be close to but less than the maximum rainfall intensity, the gutters should be pitched slightly more than normal to account for any possible unanticipated rainfall intensity. The normal pitch for a gutter is one-eighth inch for every 1 foot of guttering.