Restoration Guide: Foundation Drainage
Editor's Note: This is article 4 of 9 in Chapter 1: Foundations of the Old House Web Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.
Section 1--Surface and Subsurface Drainage
Poor drainage around your home can cause problems like ground water leaking into your basement or structural damage to your foundation. Proper surface and subsurface drainage leads water away from your foundation through grading and/or drains. Factors like how your soil absorbs water, your ground water table height, and housing density can all affect your drainage. If you live in an area with high housing density, non-porous soil, and/or a high water table, you'll need to develop a drainage system to protect your foundation.
1.1: Techniques for Better Drainage
Old House Web has eight tips for better drainage around foundations:
- Provide sufficient grading away from the house: Although many housing codes have less stringent requirements, the ground should slope away from a foundation at a 10 percent grade for a distance of 8 to 12 feet.
- Create a protective barrier around the house perimeter: A layer of clay or bentonite laid under the topsoil next to the foundation directs water away from the foundation. Sod in roll form is a good alternative.
- Make furrows to channel water away: Cut pathways into the existing slope to guide surface water away from your home.
- Terrace your slope: If a steep slope leads down to your foundation wall, terracing may slow the flow of water to the foundation.
- Install and maintain roof gutters: Rain gutters that are kept free of debris should direct water to downspouts and away from the foundation. Downspout extensions or underground drain lines enhance this drainage.
- Install trench or soil strip drains: Where there is a steep slope toward the foundation, trench or strip drains cut between the slope and the foundation and direct water away. These drains require regular maintenance.
- Provide adequate drainage around basement windows: Areas around basement windows designed to allow light in can fill with debris and water. Make sure they have adequate drainage to the foundation drain or via a pipe.
- Surround your home with free-draining material: If the soil or shrubbery against the foundation is heavily compacted or non-porous, replace it with porous material.
Section 2--Foundation Drainage
Wet basements indicate a foundation drainage problem. This may be caused by blocked or broken drainage system components. The first step in dealing with a potential drainage problem is locating and correcting any site and roof drainage problems. If that doesn't solve your problem, here are four other options to consider.
- Install an interior drainage system: Such a system can be placed on top of the basement floor slab or cut into the angle between floor and wall. The drain channels water to a sump pump which then removes the water. This system can help with the results of leaks, but doesn't address the underlying issues.
- Install a sump pump: The pump sucks up excess ground water and directs it away from your basement. If exterior drainage systems are not practical, a sump pump is the best solution. If there is danger of methane or radon gas in the basement, the sump pump should be vented outside the building. Sump pumps are best for minor or intermittent amounts of ground water.
- Install foundation drains: This is a fairly major repair which entails removal of all or portions of your existing basement slab. New drains must be installed to direct water to a sump pump, and then a new slab must be installed.
- Repair or replace your drainage system: This is the most drastic and expensive measure to eliminate water in the basement. You need to excavate soil away from your foundation and install a new drainage system.
2.1: Materials for Drainage Systems
New materials are now in use for drainage systems. Concrete and clay pipe have been largely replaced by flexible, corrugated polyethylene pipe. Filter fabrics are also used in some applications, depending on local codes.
Sheet or geocomposite drains are wall-drain systems that are gaining in popularity and can be found in four basic types:
- Dimpled polyethylene or polystyrene sheets, combined with filter fabric, are used to form a continuous drainage channel. They can be used on foundation walls and also for under-slab drainage.
- Formed polyethylene sheets without filter fabric can be used with dimples facing the wall. This provides a continuous drainage space one-quarter of an inch deep.
- Matting of various types can also be used. Fiberglass sprayed with modified asphalt; a mineral/wool combination blanket; and matting made of geotextile fabric and a three-dimensional polyethylene core are a few of the new-technology drainage systems now coming into use.
- Grooved extruded polystyrene boards with a network of drainage channels cut into one side, allowing water to drain away from the foundation, are made by at least two major manufacturers.
Sheet drainage systems first became widely used in Canada, but their low cost is making them increasingly popular in the U.S.