The Constructor

Foundation Drain – Components, Working and Design

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Foundation drain is an exterior drainage system installed on the outer face of the foundation wall and near the wall footing, covered with a layer of gravel, serving the purpose of draining out excess water seeping into the foundation.

Components and Working of Foundation Drain

The main component of a foundation drain is a perforated pipe laid all around the exterior foundation, which captures the excess water and drains it either towards the storm sewer or a catch basin or soak-pit at some distance from the building.

Fig 1: Section of Foundation Drain.

The perforations are thousands of tiny holes or slits that allow the excess water to enter the pipe, and be drained away from the foundation of the structure. The pipe is normally kept covered with a mesh permeable “sock” which helps to prevent soil from getting into the pipe. It is then covered by layers of gravel of graduated sizes and finally backfilled with soil.

The gravel allows water to flow towards the drainage pipe, without allowing dirt and debris to clog it up. A weeping tile(porous pipe) is installed in such a way that it slopes away from the structure and carries the water towards the main sewer system.

Design Consideration of Foundation Drain

For the design of a foundation drain, various factors are to be considered to ensure an effective draining of water. Some of the factors are discussed below :

1. Code Requirement

The International Residential Code (IRC), in Section R405.1, requires drains around all concrete or masonry foundations that retain earth and enclose habitable or usable spaces below grade. However, very well-drained soils are an exception and mixtures such as sand and gravel do not require drainage.

2. Drainage Boards

In areas prone to heavy rains, drainage boards are used to drain off the water quickly to the perimeter drain by installing them on foundation wall. This prevents the buildup of hydrostatic pressure next to the wall.

3. Drain Material

Perimeter drains should be made from rigid drain tile or perforated pipe. Although flexible corrugated plastic pipe can be used, care must be taken to prevent it from being crushed during backfilling.

4. Moisture

The moisture content of the soil placed near the foundation has a great effect on the water drainage from footing as the moisture content is dependent on the type of soil placed.

Fig 2: Laying of perforated pipe of foundation drain.

5. Trees

According to the U.S. Department of Housing and Urban Development, trees should be planted no closer to the foundation than their eventual height. This prevents tree roots from filling perimeter drains and inhibits the tree from sucking all the water from the soil, which could lead to .

6. Placement of drain pipes

Drain pipes should be positioned alongside the footing—the best spot is near its base. Although tile doesn't need to be sloping, low spots (which can ed with silt) must be avoided. 

Types of Foundation Drain

1. French Drain

This type of foundation drain uses a perforated pipe that is laid beside the foundation wall structure. The slope of the pipe is kept away from the structure. The backfilling is done with porous materials such as gravel for easy flow of standing water from the surface to the drain pipe.

Fig 3: Foundation drain pipe covered with course material to avoid clogging.

2. Footing Drain

This is a highly effective water drainage system. A pipe is installed around the perimeter of the foundation walls on level with the footing. This pipe collects any water that would have leaked through the top of the footing into the basement and drains it away from the foundation walls. The drain is covered with gravel up to the soil’s surface.

Advantages of Foundation Drain

  1. It prevents water accumulation near the foundation and hence reduces the risk of settlement. Stagnant water may also encourage the breeding of mosquitoes and decay the plants.
  2. Soil erosion can be reduced by installing a foundation drain.
  3. Proper draining of water can help remove toxic and deleterious organisms from the stagnant water.

Disadvantages of Foundation Drain

  1. Installing a drainage system is a costly investment. It requires skilled and professional labours.
  2. Use of perforated pipe has a disadvantage of getting clogged regularly due to the accumulation of silt in the pipe. Hence the maintenance of foundation drains is hectic.
  3. Contamination of water bodies - According to the study, subsurface drainage systems can carry nitrate through the drain pipes, channelling it directly into the bodies of water such as streams, rivers and lakes.

Read More : Types of Plumbing and Drainage Systems in Buildings

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