Drainage and Moisture "101"

Most of the water which enters a basements or crawl spaces comes from the outside, usually from roofs and surface water drainage. Yet, most of the "stories" about wet basements and crawl spaces blame "underground springs" and "that new construction up the hill". Even in those cases with changes in area drainage patterns, roof and surface water drainage is usually the primary source of the H20. Such roof and surface water drainage may also be the only solvable problem for most homeowners.

Poor Drainage Example Public Enemy #1! The intentions were good: a splash block, nice landscaping and very little soil touching the siding, but here is a perfect example of how to keep your basement wet. You collect all of the roof drainage water and dump it right next to your foundation wall.

Basements and crawl spaces will stay dryer if the are built and maintained in the following manner:

  • Yard Drainage - Slope soil around the house in such a way as to divert water away from the house.
  • Roof Drainage - Diverting all water from the roof and gutters away from the house. Don't allow roof drainage to saturate the soil around the foundation of the house. See: Down spout Drain Systems.
  • Gutter systems - Need regular cleaning and maintenance.
  • Tree limbs - Also need maintenance but this work needs to be done without damaging the trees. Tree limbs that overhang the roof and gutters make roof and gutter care much more difficult and contribute to drainage problems and wet basements.
  • Gravity drainage is preferable to pumps. Pumps require a lot of maintenance, a constant supply of power and more frequent repairs.
  • Drainage System Maintenance - Maintain the roof, gutters, down spouts, and drain systems to assure proper operation.
  • Window Wells - Maintain the window wells so that the soil level in the window well is six inches below the top of the concrete foundation wall.
  • Driveway and Stair Well Drains - Install, maintain, and repair any drain system that prevents water from draining into the basement, or when a driveway for a basement garage slopes toward the house.
  • In New Construction - Carefully seal the exterior of the foundation; install a quality perimeter drain system, and a separate drainage system for gutters and down spouts.
  • Splash Blocks - They usually don't work, don't bother getting any. The don't move the roof drainage water far enough away from the house. The same can be said for those "roll up" down spout drains! They look great in theory but they don't work.

In addition to proper maintenance on the outside:

Crawl Spaces will remain drier if the following precautions are observed:

  • "Head room" A minimum clearance under beams of 12 inches, and under floor joists of 18 inches. Larger crawl spaces tend to remain drier; they allow for more air circulation and easier maintenance and repair.
  • Vapor Barrier - A four mil black visquene (plastic) should cover the entire soil area.
  • Storage in Crawl Spaces - Most crawl spaces are too small to be used for storage. In addition, the storing of materials in crawl spaces often reduces vitally needed air circulation and creates conducive conditions for rodent and other pest infestation. Very large "crawl spaces", spaces with 5' or more of head room, may be fitted out with vented storage shelves.
  • Venting - There is a considerable amount of dispute regarding the role of exterior venting in keeping crawl spaces dry. In some climates such vents may add moisture to crawl spaces by allowing entry of warm and moist exterior air into the crawl space and increasing moisture through condensation. Most building codes do call for screened crawl space vents with 1-1/2 square feet of venting for every 25 linear feet of exterior crawl space wall.

My experience in the Pacific NW has convinced me that crawl spaces should be vented - the more the better!

Basements will remain even drier if the following precautions are observed:

  • Floor Drains - Floor drains must be kept in good repair.
  • Sump Pumps - If you have a sump pumps, it must be checked and maintained twice a year.
  • Cracks in Concrete Walls and Floors - Cracks in walls should be sealed with a concrete caulk.
  • Interior Drain Systems - One of the best ways to control basement and crawl space moisture is to build foundations with an interior and an exterior footing drainage system. Such drains are cost affective and become long lasting components of a crawl space and basement moisture control system. This is one more reason to build things right from the get-go!

    After a home is already built, interior perimeter drain systems maybe a necessary part of a drainage retrofitting project. But such projects are expensive and will not work without the exterior drainage considerations listed above. Interior drain systems should not be installed until after it is determined that properly installed and maintained exterior systems were found to be inadequate.

  • Sealing The Walls - Concrete walls can be painted on the inside with a moisture sealant. However, as with interior drain systems such solutions are relatively expensive and unlikely to work unless the more important and basic drainage and moisture control issues are addressed.

Homes in Areas of High Water Tables and Flood Planes: There is very little that can be done about water intrusion into basements and crawl spaces that are located in flood planes and/or areas with high water tables.