A few inches of rain falling on the roof of a house can produce several thousand gallons of water runoff (EPA 2012). This runoff must be channeled away from the building foundation to keep the basement or crawlspace dry and to prevent water from seeping into the building interior where it may create moisture problems. If not drained away from the house, the immense volume of water coming off the roof can quickly saturate the soil surrounding the building and wick through the foundation to the interior. Once inside, this moisture can lead to a variety of problems, including mold and rot. Moisture in homes is both a cause of indoor air quality problems and an important factor affecting the durability of the building (Lstiburek and Brennan 2001).
Figure 1 - Saturated ground water. If not drained away from the house, the sheer volume of the water coming off a roof can quickly saturate the soil and wick through the foundation into the interior of a building.
As a whole-house best practice, it is important to design the house exterior with climate-appropriate flashing, overhangs, gutters, downspouts, and drainage planes to shed rainwater and conduct it away from the house (Baechler et al. 2010). A system of gutters and downspouts is particularly important to keep water away from the building foundation in areas with expansive and/or collapsible soils.
Expansive soils swell when they get wet and can put extreme pressure on foundations, even to the point of causing cracking or uplift of foundation materials. Collapsible soils are lose soils that shrink in volume when they become wet, causing the ground to collapse. At the very least, when these soils get wet around a building, they can create a trough that collects water, increasing the likelihood of that water seeping into the foundation. In extreme cases, some collapsible soils can sink to the point of dislodging or cracking a building foundation (Colorado Geological Survey 2011). The assessment of whether the soil is expansive or collapsible needs to be completed by a certified hydrologist, soil scientist, or engineer.
Controlling Roof Runoff
As outlined in the scope, builders have four options for dealing with rain water runoff from a roof:
Install gutters and downspouts that terminate at least 5 feet away from foundations. OR
Install gutters and downspouts that terminate to an underground catchment system at least 10 feet away from foundations. OR
An alternative option to gutters is to deposit rainwater to a grade-level rock bed with a waterproof liner and a drain pipe where water terminates on a sloping finish grade at least 5 feet from the foundation. OR
If a rainwater harvesting system is installed, properly design the drain to adequately manage the overflow, and meet discharge-distance requirements.
Gutters System Basics
Gutter systems consist of two parts: 1) gutter channels that run horizontally along the roof edge to collect runoff and 2) the downspouts that carry the collected water down to grade level. Gutters are needed in all but the driest climates of the United States. In cold climates, gutters will freeze and should be avoided. Instead, in cold climates, grade-level drainage systems should be installed (as described below).
Gutter channels are typically available in K-styles (also known as "ogee," as the shape resembles this molding type) and U-styles (or half round), and in 4-, 5-, and 6-inch sizes. The style is principally aesthetic; there is no substantial difference in performance. Size, however, does matter. A larger size will conduct more water at a faster rate, provided there are enough downspouts to drain the gutter channels (EPA 2012).
Figure 2 - Gutter channel. Stock gutter material's found in building supply centers are often listed as "K" or "U" styles. The difference in styles is purely aesthetic.
Both styles are readily available in vinyl, aluminum, galvanized steel, and copper. All materials weather well, although copper and galvanized steel have the advantage of withstanding impacts from ladders and tree branches better than vinyl and aluminum, which can crack or dent.
Install gutters along the roof eaves so they slope at least 1/16 inch per foot of run. This will provide positive drainage to direct water toward the downspouts.
Downspouts conduct water from the roof to grade-level. They should terminate at least 5 feet from the foundation, or to an underground catchment system located at least 10 feet from the foundation.
Figure 3 - Downspout termination. Unless downspouts are connected to an underground catchment system or storm sewer system, they should drain to daylight at least 5 feet from the foundation.
How to Install Downspouts
Install downspouts every 20 to 50 feet along the gutter. The more closely downspouts are spaced along the gutter, the more water can be conducted away during peak rainstorms.
Connect the end of the downspout to lateral piping made from non-perforated flexible ABS or Schedule 40 PVC pipe.
Extend the lateral piping at least 5 feet from the building. Use a level and tape measure to ensure that the pipe is pitched away from the foundation at a minimum 5/8-inch per foot (a 5% slope).
Terminate the lateral pipe to daylight (or to a catchment system or storm sewer). It is critical that a daylight drain empty onto a sloped grade that directs water away from the building, and does not allow water to drain back towards the building. A splash block placed at the end of the lateral pipe will help control erosion to this location (EPA 2012
Grade-level drainage system. In cold and very cold climates, where snow can accumulate on roofs, the water in gutters can freeze; therefore, gutters should be avoided in cold and very cold climates. Instead, install a grade-level drainage system that conducts roof run-off away from the foundation.
Figure 4 - Grade-level drainage. Gutters can freeze in cold and very cold climates. Therefore, in these climates, install a grade-level drainage system, as shown here.
How to Install Grade-Level Drainage
Install a perimeter foundation drain at the footing level. The standard foundation drainage system shown in the graphic above consists of Schedule-40 perforated piping and clean stone that is isolated from the surrounding soils with filter fabric.
Backfill the foundation drainage system with a well-draining backfill material.
Stop backfilling 16 to 18 inches from grade level. 3a) Lay-in a piece of EPDM rubber roofing or other impervious membrane to isolate the grade-level drainage system from the foundation drainage system. 3b) Install Schedule-40 PVC or ABS perforated pipe, or similar perimeter drainage piping. This piping should extend past the corner of the foundation at least 5 feet and drain to daylight or connect to a catchment system or storm sewer. 3c) Backfill around the drainage pipe with clean stone surrounded by filter fabric. Bring the sides of the EPDM isolation membrane up the sides to create a contained channel so water is drained off by the perforated piping and does not overflow and saturate the foundation backfill.
To finish off, cover the drainage system with 4 to 6 inches of clean stone.
Leave 8 to 16 inches of space between the finished surface and the first course of siding to prevent splash back from damaging siding materials (EPA 2012
Underground catchment system. As an alternative to draining roof runoff to daylight, the downspout can connect to a catchment system, storm sewer or rainwater-harvesting system. The illustration below depicts one type, known as a drywell, that can be installed, but many types of both underground and above-grade catchment systems are available. When designed to collect roof runoff and properly drain overflow, rainwater harvesting systems, which retain the water in a tank for use in watering landscaping, may also be used to meet roof drainage requirements.
Figure 5 - Catchment system. A roof runoff catchment system, such as the drywell shown here, must be located at least 10 feet from the building foundation.
How to Install a Catchment System
Connect downspouts to lateral piping that extends at least 10 feet from the foundation.
Provide overflow protection to alleviate the water load during peak rain events. Locate overflow pipe at a branch in the downspout, or tee-off near the top of the catchment basin. If it branches off the downspout, as shown in this illustration, it must extend at least 5 feet from the building foundation and drain to daylight.
3a. Install a clean-out in the catchment basin. In a drywell, as shown here, this clean-out consists of a perforated pipe through the center of the gravel that allows the basin to be flushed out periodically. The clean-out should also connect to the lateral pipe to allow leaves and other debris to be removed.
3b. Isolate the catchment basin with a geotextile filter fabric to prevent soil intrusion from clogging the system.
3c. Fill the basin area with coarse gravel. One to 1½-inch gravel is typical. Small gravel will tend to clog too quickly (EPA 2012
A Note about Kick-Out Diverters for Climates with Heavy Rainfalls
In climate zones that have heavy rainfalls which can overwhelm gutter capacity, a diverter (also known as “kick-out flashing”) should be integrated with the house wrap. Step-by-step instructions for kick-out diverters can be found in Chapter 12 of Baechler et al. (2010).