The consequences of poor water management around the outside of the building can lead to building durability and health and safety issues, including mold and indoor air quality problems. Directing water down and away from the building through proper installation of the building envelope is essential to a water management strategy for the home. In addition, patio slabs, porch slabs, walks, and driveways must be designed and built with the appropriate slope that will force water to flow away from the edge of the home. It is also important to understand that if the setback requirements limit the available space for the grade, additional water management systems must be used in the form of swales or drainage pipes.
Creating Proper Slope
Because patio slabs, porch slabs, walks, and driveways are typically made of nonporous materials, water will freely flow across them. If these surfaces are not properly graded to allow water to naturally flow away from the home, water will pool around the foundation, resulting in potential water intrusion, structural integrity issues, and a welcoming environment for pests and insects.
Figure 1 - Standing water around the home. This image shows how water can collect and pool at the foundation of a home built without proper sloping to direct water away from the home.
As part of the site water management strategy, patio slabs, porch slabs, walks, and driveways should be designed to drain water as follows (EPA 2011):
- Install the surface with a slope at least 0.25 inch per foot for the entire length of the slab. Or,
- Install the surface with a slope at least 0.25 inch per foot for at least 10 feet if it is not possible to slope the entire length of the slab.
Code note: Be sure to check with local building codes to see if a greater slope is required in your area.
Figure 2 - Proper slope. Slope all patio slabs, walks, and driveways away from the house at least 0.25 inch per foot for either the entire length of the slab or for 10 feet. If setbacks limit the space to less than 10 feet, install swales or drains designed to carry water away from the foundation.
Slope Exceptions for Creating Proper Drainage in Limited Space
At times it may be challenging to slope a patio slab, porch slab, walk, or driveway to the requirements listed above. This is most common when setback requirements limit the available space for the grade to less than 10 feet. When this happens, one of the following must be installed:
Creating a Swale. A swale is a trapezoidal channel that receives stormwater overflow and allows it to flow away from the home. Swales are typically planted with specific types of vegetation to both improve the aesthetics of the design and to help remove any pollutants found in the stormwater runoff.
Figure 3 - Trapezoid swale design. Swales are typically trapezodial in design with specific vegetation planted to improve aesthetics, filter stormwater runoff, and prevent erosion.
To Construct A Swale:
Vegetated swales are typically located along property boundaries along a natural grade, although they can be used effectively wherever the site provides adequate space. Swales can be used in place of curbs and gutters along parking lots. While each site is unique, constructing a swale to divert water down and away from the house should be done in a consistent manner following these steps (EPA 2006):
Do not construct vegetated swales (swales that will have plants introduced to improve pollutant filtration) in gravelly and coarse sandy soils that cannot easily support dense vegetation. If available, alkaline soils and sub-soils should be used to promote the removal and retention of metals.
Do not compact the soil in the swale trench. Soil infiltration rates should be greater than 0.2 millimeters per second (one-half inch per hour); therefore, care must be taken to avoid compacting the soil during construction.
Choose plants that will aide in filtration. A fine, close-growing, water-resistant grass should be selected for use in vegetated swales, because increasing the surface area of the vegetation exposed to the runoff improves the effectiveness of the swale system. Pollutant removal efficiencies vary greatly depending on the specific plants involved, so the vegetation should be selected with pollution control objectives in mind. In addition, care should be taken to choose plants that will be able to thrive at the site. Examples of vegetation appropriate for swales include reed canary grass, grass-legume mixtures, and red fescue. The best plants to use in a swale will depend on the region and location. Local cooperative extension programs based at land grant universities often have a list of plants that will work well in a swale. See www.extension.org
for the extension office in your climate zone.
Construct the swale in a parabolic or trapezoidal cross-section with side slopes no steeper than a 1:3 ratio. The overall slope of the swale (the longitudinal channel or from water-entry point to water exit point) should slope consistently between a 2 and 4 percent grade. Slopes less than 2 percent can lead to pooling; slopes greater than 4 percent can lead to excessive water velocity and contribute to soil erosion.
Build the swale large enough (deep and wide) to comfortably divert the volume of water occurrence (rain fall or snowmelt runoff) for a 6-month frequency, 24-hour storm event. The exact intensity of this storm must be determined for your location and is generally available from the U.S. Geological Survey. Swales are generally not used where the maximum flow rate exceeds 140 liters/second (5 cubic feet per second).
Special Notes about Swales
Swale construction can be relatively easy and a cost-effective approach to storm water mitigation; however, there are several things to keep in mind:
Know the factors decreasing the effectiveness of swales, including compacted soils, short runoff contact time, large storm events, frozen ground, short grass heights, steep slopes, and high runoff velocities and discharge rates.
Use non-settling compact soils, as determined by a certified hydrologist, soil scientist, or engineer.
Schedule a site visit to provide in-fill and final grading after settling has occurred (e.g., after the first rainy season).
If landscaping does not allow for a swale to be built, drains can be installed to divert water from downspouts away from the home. A drain is simply a pipe that collects water from gutters and downspouts and transfers it to at least 10 feet away from the home or dispenses the water underground. Information about gutters and downspouts is specifically covered in "Gutters and Downspouts." However, related to drains, be sure to adhere to this overall design principle:
Install gutters and downspouts that terminate to an underground catchment system at least 10 feet away from foundations.
If a rainwater harvesting system is installed, properly design the drain to adequately manage the overflow and meet the discharge-distance requirement of 10 feet.
Installing A Drain
When installing drains, there are several important steps to follow:
Connect and seal the pipe directly to the downspout.
Install a non-perforated, corrugated, or smooth plastic pipe as the drain.
Bury the drain pipe at a slope consistent with the final grade around the home (i.e., sloped ≥ 0.5 inch per foot away from home for ≥ 10 feet).
Special Notes about Drains
Do not connect the gutter drain pipe to the perforated foundation drain pipe; this practice will soak the foundation.