The Concrete Network
Flexible Drain Pipe
Site
Patterson Construction, Inc.
,

Flexible drain pipe laid into a trench with plastic below and surrounded by gravel. Patterson Construction, Inc.

Site
CertainTeed Corp
,

Strip drains, like this Form-A-Drain, double as stay-in-place formwork for the footing.CertainTeed Corp.

Drainage is a key element of any waterproofing or dampproofing system. Surface drainage is the first opportunity to mitigate water infiltration, but it is very difficult to keep all water away from foundation walls—both surface water and ground water can lead to problems. So it is very important not to use partial measures in draining water that ends up at the base of the foundation wall.

A typical sub-grade drainage system consists of:

  • A layer of 6-mil plastic forming a trough in a trench at the bottom of the footing to capture as much water as possible
  • A rigid or flexible 4-inch perforated drain pipe laid over the plastic and covered with clean well-graded gravel; some pipe is available with a sleeve to filter fines—this pipe should outlet to daylight, adequately drained soil, or a mechanically drained chamber (a sump)
  • A covering of geotechnical filter fabric, as a barrier between the gravel and the backfill
  • Backfill compacted in lifts to rough grade
  • Finish grade sloped away from the wall

Here are a few things to keep in mind:

  • Rigid drainage pipe, although more difficult to handle, may be preferable to flexible drain tile since it is easier to establish and maintain grade and avoid low spots where sediment can accumulate.
  • Lay rigid drain pipe with the slots to the sides, which creates a continuous channel for water to run in but still allows rising ground water or water from above to easily get into the drain.
  • Maintaining slope on a pipe may require it to be higher or lower than the footer in certain spots.
  • Even a level pipe will move water if the outlet is lower than the bottom of the pipe.
  • There are strip drains that lay against the outside edge of the footing. Some double as stay-in-place formwork for the footing. One example of this is Form-A-Drain which incorporates a double channel, one to use as the foundation drain and the other for radon mitigation or other ventilation, if needed. Brittleness in cold weather and deforming of the product in direct sunlight are potential problems.
  • Not all subsurface water trickles from above. A rainstorm or melting snow miles away may end up sending a surge of water underground into a basement or crawl space.
  • In very wet areas, an interior drainage collection pipe and trench and a sump pump can be employed to keep water under control.

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