The following is an exerpt taken from the first edition of "Residential Sheet Metal Guidelines," published by SMACNA.

(The following is an exerpt taken from the first edition of "Residential Sheet Metal Guidelines," published by the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA). The 130-page manual is intended as a guide for contractors, designers, builders and homeowners.)

The roof is a major feature of any residential building, accounting for 25% to 40% or more of the exterior surface. Roofs carry most of the responsibility for protecting the building's interior and exterior from the elements, but in residential buildings they are also prominent architectural features. Compared to commercial and industrial buildings, residential roofs are not large, but they can often be far more involved.

The complexity of many residential roofs requires careful design of the roof drainage system for practical and aesthetic reasons. Widespread use of pitched roofs may influence drainage calculations and accurate sizing of all the system's sheet metal components, including valleys, gutters, outlets, and downspouts. In addition, residential buildings typically employ their own building materials, construction practices, and architectural features that may affect the installation of all these components.

Calculating drainage capacity

The first step in designing any roof drainage system is calculating the amount of water it is expected to handle. Taking data based on the total roof area of the building, then dividing it by the rainfall intensity factor established for the region, provides a figure for the drainage that can be used to accurately size gutters, downspouts and any other water-carrying components of the system.

Roof area

Experience shows that using the true surface area of a pitched roof often leads to over-sizing of gutters, downspouts and drains. Maximum conditions exist only when rain falls perpendicular to a roof surface, and rain comes at many angles. Moreover, roof area increases as pitch increases. To determine the design area for a pitched roof, multiply the plan area by the appropriate factor.

Rainfall intensity

Decades of U.S. Weather Bureau records have produced data on yearly rainfall that make drainage calculations more accurate for regional conditions across the country. Rainfall intensity is usually given in inches per hour for durations of five minutes or one hour. The design of different types of drainage systems may require one duration or the other. New Orleans, for example, may have an intensity of 8 in./hour (203 mm/hour) for a five-minute duration, yet record only 4.8 in./hour (121 mm/hour) for a one-hour duration over a 100-year period. These rates correspond to 0.133 in./minute (3.4 mm/minute) and 0.08 in./minute (2 mm/minute). Local codes may require drainage systems only be designed for the latter.

It takes 96.15 sq. ft. (8.93 sq. m) of surface with one inch per hour (25 mm/hour) of water to correspond with a 1 gpm (0.0631/s) flow rate. This flow rate is the basis for sizing downspouts and gutters in relation to rainfall.

Downspout sizing

When sizing downspouts, account for the following:

1. Downspouts should be constant in size throughout their length.

2. Downspouts should be constructed with conductor heads every 40 ft. (12.2 m) to admit air and prevent blockages due to vacuum lock.

3. Long or multiple offsets can reduce downspout drainage capacity.

4. Downspout size must not exceed the width of the gutter bottom.

5. Gutter outlet capacity should match the nominal gutter capacity.