Managing Roof & Yard Runoff
On most building sites, the main source of water around the house is runoff from the roof. Without proper management of rainwater and snowmelt, these may become the main source of water in your basement.
A well-designed system of gutters and downspouts that effectively collects roof water and discharges it away from the foundation is the first line of defense against basement water problems. Combined with a back-up system of footing drains around the perimeter of your foundation, this will keep most basements dry.
If you are building on site with serious water problems, however, you will need to take additional measures. If the water table is above the level of the basement floor part of the year, you should reconsider building a full basement. I your building site is at a low point that collects water or lies in the flow of water from surrounding properties, then you may need an engineered drainage strategy.
A house with a 1,600 sq. ft. footprint, including roof overhangs, will shed about 1,000 gallons of water for every inch of rainfall. In a downpour, that 1,000 gallons gets deposited close to the foundation unless you have an effective system to collect and discharge that water elsewhere. The water is likely to saturate the soil around the foundation and exert hydrostatic pressure against the foundation wall. Under pressure, the water will find it’s way through any cracks in the concrete or block wall, as well as the joints between the basement wall, footing, and floor.
Additional surface water may reach the foundation from yard areas and hardscapes, such as patios and walkways that do not slope away from the foundation.
Gutters and Downspouts
Some people live with wet basements for many years, only to discover that the solution was simple – start managing roof runoff properly instead of directing hundreds of gallons of water down their foundation walls.
This does not require anything exotic – just ordinary gutters with downspouts sized properly and kept clear of leaves and other debris. The more important component is what happens to the water when it reaches the ground.
If you have plenty of slope, and a dense layer of clay soil near the surface, then a simple splash block can do the trick of dispersing the water. Manufactured splash blocks that are typically about two feet long with a molded channel do a nice job of directing water away from the foundation. A couple of paver stones can work as well. Where the soil is likely to absorb the water an extended downspout to several feet from the foundation is often the best choice.
The extended downspout can be above grade if that is visually acceptable, or below grade using solid PVC pipe that drains to daylight. Consider upgrading to 6-in. PVC for long runs (see photos below).
Just make sure that the drainpipe does not deliver the water below grade right next to the foundation. Then you’ve engineered a perfect system to collect roof water and deliver it to your basement – an arrangement more common than you might think. And NEVER tie downspouts to the footing drains. This can overload the system, bring leaf debris from the gutters, and contribute to basement water problems.
Slope Around Foundation
To direct runoff away from the foundation, the ground should slope away from house foundation with a drop of at least 6 in. over the first 10 feet., as specified in the IRC (International Residential Code). This is also called a 5% slope, equal to a little over ½ in. of drop per foot.
When backfilling, place a layer of low-permeable soil, such as clay, near the finish grade to limit the movement of water into the soil around the foundation. On the surface, place a layer of topsoil for lawn or plantings (see illustration).
Where an adequate slope around the foundation is not possible due to close lot lines, a downslope lot, adjacent houses, or other physical barriers, the code recommends using swales or hardscapes to conduct water safely away from the building. Swales and impervious surfaces should slope away from the foundation at a minimum 2% grade, equal to about ¼” per foot, for a minimum of 10 ft. Another option for difficult lots is subsurface drainage to a storm sewer if allowed by code.
Flat building sites. On a flat site, it is very easy to create moderate drainage around a house at little cost, but this is often ignored. All that is required is to excavate a little less and, thereby, raise the foundation by 6 to 12 in. Then bring in a small amount of fill so the finish grade can slope away from the house at ½ in. per foot for 10 feet or more. Often the fill is already on site from excavation work. It often costs only a little more, or nothing at all to do things right. It just takes a little knowledge and forethought.
Swales are wide, shallow ditches that provide a low-cost, low-maintenance approach to managing storm water and snowmelt. They are often used at the lot line between two homes or along the front or back lot line.
In residential developments, swales are typically planted with grass and may also contain moisture-tolerant plants. They are often referred to as “grassy” or “vegetated” swales.
If properly designed, swales move surface water quickly away from houses and yards and prevent flooding around the house during heavy rains and snowmelt. The grasses and other plants slow the water flow, which reduces erosion and promotes infiltration back into the soil.
Depending on the local topography and codes, swales may drain the water to wooded areas, streams or ponds, man-made “retention” ponds, or into storm sewers by way of subsurface piping. If possible, the swale is sloped about ¼ inch per foot to promote drainage. On flat sites with porous soils, swales may be designed with level bottoms that hold water while it percolates back into the soil or evaporates. Where the slope is steep, stone or concrete “check dams” should be built across the bottom of the swale to reduce the water flow rate and prevent erosion.
Swale Maintenance. Grassy swales need maintenance like other landscaped areas. Typically they means mowing the grass, removing invasive plants, and removing leaf litter and other debris that could kill the grass over time. The grass is essential to slow the water flow, prevent erosion, and promote drainage back into the soil.
Where swales are not practical, gravel-filled trenches called curtain drains are sometimes used to interrupt the flow of water and direct it away from your home. Curtain drains can help with both surface and ground water and can be tied into storm sewers if permitted.
The trenches are typically 2-4 ft. deep and located several feet away from your foundation on the uphill side. They are generally open at the top, but can be disguised with paving stones or other landscaping features.
Both curtain drains and footing drains are sometimes referred to as “French drains,” which causes a lot of confusion. While both are similar in construction, the purpose of curtain drains is to intercept water before it reaches your foundation. Foundation drains are a secondary defense against water that has already reached the foundation. These drains are buried in the ground directly around the foundation footings and must be below the basement floor to be effective.
On a flat site or other site where you cannot safely drain the water away from the house by gravity, you may need to install a drywell. At its simplest, this is a hole in the ground at least 10 feet from your foundation filled with gravel or crushed stone. Its purpose is to collect storm runoff and allow to slowly percolate back into the ground.
The required size and distance from the house will vary depending on rainfall amounts and soil conditions, but a typical drywell is about 4 feet in diameter and 4 feet deep. It is important to line it with a heavy landscape fabric to prevent it from clogging with silt. Fill it with clean course gravel, crushed stone and cap the top with a layer of landscape fabric and topsoil.
You can boost the capacity and performance of a drywell for a modest cost by installing a plastic catch basin within the crushed stone. These are typically 50 gallons, although for smaller drywells a 20-gallon sump basin could work as well. The catch basin allows more water to collect more rapidly in the event of a very heavy rainfall or rapid snowmelt.
The basin should be set on about a foot of gravel or stone, with stone all around and on top. Use landscape fabric to line both the hole and the ground and the plastic basin.
How well a drywell works will depend on its loading, size, and the how rapidly the water can percolate into the soil. Well-drained soil that easily passes a perc test will be better than dense, slow-draining clay soil.
If you do get an inch of rain in an hour, and all 1,000 gallons of water are channeled into your 4-ft.-deep drywell (375 gallons if completely empty), it will be quickly overwhelmed and the gutters will back up – and that’s assuming it is in good shape and not silted up. So a large house might need two drywells, one on each side of the house.A well-designed drywell with adequate capacity, protected by filter fabric, and situated over well-drained soil can last for many years. On the other hand, if it’s allowed to fill with sediment and leaf debris, it may do little more than collect storm water until it overflows. A design with a lid for occasional inspection and maintenance is a good idea.
Bottom line: Drywells are a nice concept, and can work well for modest loads, but draining to “daylight” is the best choice when possible.