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Prevent Basement Flooding
The last thing any homeowner wants to do is waste money, especially when renovating and investing in a basement. However, if a homeowner doesn’t understand basement drainage, thousands of dollars could be lost because of water damage. On this page, learn how drainage works in relation to basements. You may also read through our Sump Pump Frequently Asked Questions page on the Engineering Division's website.
In a one-inch rain, 1,250 gallons of water fall on the roof of a 2,000-square-foot house. Without proper grading, gutters, and downspouts, some of this water flows into the basement. The below-grade water table can also rise due to flooding or seasonal site conditions. This is why drain tile systems are recommended around basement walls even in sandy or gravel soils.
Under normal conditions basements are designed to be dry (if somewhat damp/humid) spaces. In large part, humidity is unavoidable because the concrete used to construct basements is a porous material and will allow water/ moisture to pass through it at a very slow rate.
New construction is designed to avoid serious, free-flowing water problems in many ways, including: provision of sump pumps, exterior basement coatings and extensive tile/pipe drainage systems need to have a dehumidifier operating in these spaces to keep humidity levels within the recommend range.
Generally the recommended humidity level for basements is between 40-50 percent. If basement moisture is allowed to reach 60 percent, the basement will likely have a musty smell.
Many older homes were constructed without benefit of these modern systems. Under the right circumstances (e.g., high groundwater, poor exterior drainage, foundation cracking, etc this can result in serious moisture problems in the basement. In general, the following step-by-step approach to addressing basement water problems is recommended:
- Control exterior water sources by improving grading and gutter downspout systems
- Provide an interior or exterior drainage (sump pump) system
Approach No. 1: Install Proper Gutters, Downspouts, Correct Grading
A number of basement water problems can be solved by handling rainwater and surface drainage properly by using gutters and downspouts with extenders or splash blocks to carry the water away from the foundation. Sloping the grade away from the house, which may require hauling fill to the site, is also very important.
This should be done before any below-grade drainage system is installed, since the above-grade corrections may solve the problem. Even if a drainage system is required, it is necessary to remove water at the source as much as possible.
Approach No. 2: Exterior Drainage System
Installing an exterior drainage system at an existing building is the most costly, but also the most effective water control approach. This requires digging up the area around the foundation and rebuilding it similar to a new house installation. It also requires digging up shrubs and other obstacles around the house. Usually waterproofing and insulation are installed at the same time, in addition to making any repairs to the structure. The traditional exterior drainage systems use free-draining sand in the backfill. Drain tile can be placed beside or on top of the footing. Level drainpipe installations are satisfactory.
A minimum of 12 inches of coarse aggregate should be placed around the drain tile. Free-draining Membrane or Board It can be expensive to haul pea rock or sand to a site for back filling purposes. Instead, a drainage mat can be placed against the foundation wall and then back filled with any soil on site. The drainage must have a free-flowing path to the perforated drainpipe below. All exterior drainage systems must drain to a sump that can be pumped out.
The sump must have an airtight, childproof cover, and the discharge from the sump pump MUST be discharged to a location on the property that does not slope back to the home from which it is being pumped. Failure to do this will result in recycling of the pumped water back into the sump system.
Free-draining Membrane or Board
It can be expensive to haul pea rock or sand to a site for back filling purposes. Instead, a drainage mat can be placed against the foundation wall and then back filled with any soil on site. The drainage must have a free-flowing path to the perforated drainpipe below.
Draining to a Sump
All exterior drainage systems must drain to a sump that can be pumped out. The sump must have an airtight, childproof cover, and the discharge from the sump pump must be discharged to a location on the property that does not slope back to the home from which it is being pumped. Failure to do this will result in recycling of the pumped water back into the sump system.
Approach No. 3: Interior Drainage System Beneath the Slab
The most effective of the interior drainage systems is a perforated drainpipe installed inside the perimeter of the footing. This requires removing and replacing concrete at the slab edge. By placing the drainpipe beneath the slab drains the area to a lower level. Similar to an exterior system, the drainage connects to a sump.
The sump should have an airtight, childproof cover, and the discharge from the sump pump MUST be discharged to a location on the property that does not slope back to the home from which it is being pumped. Failure to do this will result in recycling of the pumped water back into the sump system. In cases where the basement walls are constructed from blocks (not a poured wall), a critical component of this approach is the dimpled plastic sheeting placed at the base of the wall and beneath the slab edge.
Dimpled sheeting is similar to a small egg crate and permits free drainage of the wall into the drainpipe. It is less expensive than many specialized drainage channel systems. In low permeability soils, this system cannot accept rising groundwater unless there is an aggregate layer under the slab.
Sump Pumps
If you have decided to have a sump pump installed there are a few basic things to need to keep in mind:
- A building permit is needed to complete this work.
- Prior to doing any digging work on a homeowner’s property, contact Diggers Hotline to have the yard marked for utilities. 800- 242-8511 or www.diggershotline.com
- A contractor should be willing to come out and review the situation. The contractor should give a free, written estimate. It is generally not good practice to agree to work without a written contract detailing what specific work is included with the price quote and what the warranty on the work is.
- Discharge pipe usually terminate on private property. In some cases, it may be desirable to discharge the water from the sump pump directly to the street curb or connect directly to a nearby storm sewer inlet. If this is the case, a permit to excavate in the Public Right of Way (ROW) is required. It is necessary that the work within the ROW be completed by a contractor prequalified to work in the ROW for Utilities by the City of Madison. This requirement is primarily an insurance requirement and protects the homeowner in the event anything should occur while installing this in the ROW. Prequalified contractors to work in the City ROW for Utilities are listed under Category No. 305 (Storm and Sanitary Laterals and Water Services).
- If the contractor discharges to the curb, homeowners will need to keep the discharge point free of snow during the winter months. Examples of when homeowners may want to connect to the curb or storm sewer include
- Sites where there is no location on the property to discharge the pumped water that will not allow the water to drain back toward the home
- Sites where the sump pump is running on a frequent basis (several times a day and often all year) and discharge to driveway/sidewalk is causing a safety (algae, mold/mildew, or slime to grow on the concrete surfaces) or discharge to the grass is so frequent as to cause the grass to die;
- Sites where the property owner prefers to have a buried discharge from the sump system for aesthetic reasons.
- The sump crock or pit should have an airtight, childproof cover. The discharge from the sump pump MUST be discharged to a location on the property that does not slope back to the home from which it is being pumped. Failure to do this will result in recycling of the pumped water back into the sump system.