A well designed drainage system is an effective way of directing rainwater away from the foundation, and a veryt effective means for preventing many basement problems. But surface drainage is seldom sufficient, namely in rainy climates; under-ground drainage systems may also be indispensable...
The importance of drainage systems
A well designed drainage system is the first line of defense against water around the foundation walls.
Wall insulation can prevent cases of condensation damage in basements; ventilation can also fix some small problems with basements. But they can't solve structural problems; the most effective ways of keeping moisture out of your foundation are comprehensive sealing, waterproofing and drainage.
We can solve some moisture/condensation problems in basements through ventilation wall insulation. But sealing air leaks and a good drainage system are the most effective ways for keeping a basement dry.
The first step to solve moisture and water problems in basements is to install drain footings around the house and fix possible problems with gutters and downspouts.
Make sure your home's gutters and downspouts are working properly, and that the rainwater is being properly redirected.
If possible and practical, add extenders (minimum 4 ft/1.2 m) and splash-blocks to direct the water away from the foundation.
The ground next to your home should slope away from the walls on all sides of the foundation, for at least 10 feet/3 m.
Concrete walkways, patios or driveways sloping toward the walls should be rectified. Redirecting rain- and snow-water away, as quickly as possible, is essential for a good surface drainage.
A sloped ground with an impervious clay layer and a concrete sidewalk, as shown in the image above, is a good choice (image: University of Minnesota, Extension).
Surface drainage involves...
1) Sloped grounds
2) Gutters and downspouts
3) Drain footing and in some cases
4) Curtain drains, dry-wells, sumps and sump-pumps... Under-ground drainage involves...
1- several layers of gravel and
2- a proper pipe system at the footing of the house.
Surface drainage can prevent problems in basements and crawlspaces. But if the foundation walls aren't well sealed and the water-managed foundation system isn't properly designed, fixing the "surface problems" will not be enough.
Installing an under-ground drainage system is a must in rainy climates.
Typical under-ground drainage systems includes...
»» A 4-inches layer of washed stone under the footing and the slab, to provide a capillary break (the image above doesn't show it).
»» A gravel layer and a perforated PVC pipe (shown in the image above). The pipe is set below footing level, and empties into a sump pit or exits above grade nearby. The pipe should be covered with a minimum of 12 inches/30 cm of cleaned gravel, crushed gravel, or crushed rock, close to the walls. Gravel is very important, and cheap, and can avoid many water problems.
»» A filter fabric, covering the gravel layer.
»» Backfill material, capped with clay or other less-permeable soil.
For technical details, see: OEE Basement Insulation
Avoid sprinklers but also flower beds, bushes and trees near the foundation walls; they are a common source of unwanted water infiltration.
Curtain drains: curtain drains are trenches running alongside the house - about 1-foot deep - filled with gravel and sections of perforated PVC pipe; they can be very effective to reduce the amount of water around the foundation walls.
Dry wells: a dry-well is an underground structure - a tank or a deep hole filled with gravel covered with landscape fabrics - designed to receive the flow of rain water, or water from snow melting, and to dissipate it into the ground, away from the walls. Dry wells should be located at least 12 feet away from the home.
Sump basins and sump-pumps: sump basins and sump-pumps are relatively inexpensive and easy to install, and are very effective at removing water accumulated in the foundation.