Roof drainage systems have three functions: to remove the water from the roof, to prevent water damage to the building exterior, and to transport water away from the foundation. All the water that lands on a roof should be collected and distributed away from the building. The best time to inspect your roof drainage system is during and just after a heavy rain. The next time it rains, get on your rain gear, grab an umbrella, and go for a tour of your domicile's exterior.

First, look for water flowing over the rain gutter edges. Overflowing gutters may be too small; they may slope down and away from, instead of towards, the downspout (or leader) openings; or the downspouts may be clogged with leaves and debris. Next, look for indications of water flow behind the rain gutters. Leaks behind the gutters can cause decay in the wooden eave boards. Gutter leaks are especially problematic if the eave is small and doesn't extend out from the wall below, or if there is no eave and the gutters are tight against the siding. Rainwater can actually flow inside the building wall if leaky gutters are tight against the siding.

The roofing material should extend sufficiently over the rain gutter to prevent water leaking behind. It may be necessary to install "L" shaped sheet metal flashings to seal the connections between the roof surface and the rain gutters.

If you tap on a downspout and get a dull, heavy sound, the downspout is probably clogged with debris. A downspout can be cleaned out by putting a garden hose at the top of the downspout inside the rain gutter. If you do not have the equipment to do this safely, consult a gutter specialist for an annual debris removal. An inexpensive birdcage style screen can be inserted in the gutter at the downspout opening to keep debris out of the downspouts.

Uncontrolled rainwater is a primary cause of foundation problems. A concrete or plastic splash block placed at the bottom of each downspout can reduce erosion and direct the water away from the foundation. Ideally, the closest five feet of ground, walkways, and other surfaces to the building should slope downward away from the foundation.

Often the best procedure is to provide a subsurface drainage piping system to carry downspout water away from the building. Subdrains are often installed below the sidewalk and emerge at the street gutter. If you have a subsurface drain system, be sure to test it periodically using water from a garden hose. Locate the drain outflows and check them periodically for water flow in wet weather. Steep sites often require a specially designed drainage system.

Many older homes have clay tile subsurface drain piping, which often breaks or becomes displaced and may direct roof rainwater to flow beneath the foundation instead of away from it. It may be advisable to disconnect downspouts from old tile drain pipes, especially if water flows under the house in wet weather.

Low pitch or low slope roofs often do not have rain gutters and instead have surface-mounted drains or drain openings mounted in the perimeter parapet walls. "Parapets," common in Mediterranean style buildings, are walls that extend above the roofline. Parapet drains must be kept clear of any debris, as blockage could cause deep standing water on the roof. The standard in new construction requires a second drain above the primary drain to prevent deep roof flooding should the primary drain become clogged.

All flat or low sloped roofs should be designed to slope downward to the drain openings. Since no roof is perfect, a minor amount of ponding is considered acceptable. The roof surfaces should dry completely within 48 hours. Even minor ponding can cause premature roof surface deterioration.

In summary, the roof drain system, rain gutters, and downspouts should be checked periodically to assure they are functioning properly. Periodic inspections are especially important if there are trees or shrubs higher than the roof, which could cause debris blockage in the drains. Don't use a ladder or climb on a roof unless you are sure you can do so safely. It's usually best to call a gutter specialist to install, clean, or repair the roof drain system.