How Your House Works
Residential Foundation Drainage System
DRAINAGE: Natural and artificial removal of surface and
 subsurface water from a particular area
The best time to solve drainage problems is before or during construction.
While drainage problems can be alleviated or eliminated after construction, it is almost always many times more expenisive - particularly if the foundation as been cracked or compromised.
Heavy local rains, water standing in your yard, or surface flooding, can cause the ground around your house to become saturated with water.
If enough water builds up around or under the foundation, the water pressure can cause the concrete to crack. Cracks in your foundation walls or floors will then allow seepage of water into your basement.
Dry Today ... Wet Tomorrow
Just because your new basement is dry, doesn't mean it will stay that way.
Statistically, more than 85% of all basements will experience a leak during the home's lifetime.
The water content in the soil surrounding your house can fluctuate seasonally. And the water table in many parts of the country can rise to within a few feet of the surface due to flooding or seasonal site and weather conditions.
A process called "Capillary Suction" (also referred to as "Capillary Draw") is what moves moisture through porous materials.
Simply put, this means anything that is dry will absorb the water around it.
After purchasing a house, it may be months or years before homeowners experience the effects of groundwater seepage or surface flooding because of drought or infrequent rainy seasons. New construction in the area will also affect ground and surface water.
House vs. Nature
Ground and storm water are natural elements of the environment. A house is not a "natural" part of the landscape.
Consequently, ground and storm water pose a threat not only to natural settings, but to health, property and finances as well.
- Health Risks
Wet basements and crawl spaces are sources of high humidity, which can produce surface condensation, mildew and fungi, musty odors, and toxic mold.
- Reduced Resale Value
Wet basements and crawl spaces reduce the value of the house ... at least by the amount that would be required to repair the damage and eliminate the cause of the problem.
Standing water around the footer and foundation wall can soften the soil, increasing the possibility of wall settlement and cracking.
Seepage under the foundation footer may erode soil away and cause the foundation wall and/or floor to drop or crack.
Crawl space or basement dampness always moves toward the drier upstairs areas, where excessive moisture can eventually penetrate the subflooring and buckle the flooring or cause warping, making doors and cabinets difficult to close or open.
- Your Homeowner's Insurance Might Not Cover the Damage
Most homeowner insurance will not cover water seepage from the ground into your basement, even if it damages your foundation and interior.
Insurance companies tend to consider seepage a maintenance problem, not "sudden and accidental" damage, therefore most exclude it from homeowner's insurance coverage.
"Seepage" includes sewage, water and/or moisture entering the basement from the foundation walls, or from the foundation floor.
Why a Foundation Drainage System
Building walls, waterproofing interiors, digging trenches, or building curbs after a storm are not only poor solutions to controlling where the water goes, they will not stop water that is going under the house.
The best way to deal with seepage is to relieve the groundwater through subsurface drain tiles and ensure that the walls are waterproofed.
Since it is impossible to eliminate the water, it needs to be diverted to someplace other than your home. A foundation drain tile system has proven very effective in dealing with such groundwater.
Perforated drain tile is placed around the foundation and, in some cases, under the floor. Since water will take the path of the least resistance, it flows down to the drain tiles instead of pushing against the foundation walls or floor.
How the System Works
A foundation drain tile system has four main components. The drain tile (pipe), the filter media (gravel), sediment barrier, and the water outlet.
All of these elements must be installed correctly for the system to function properly.
- Drain Tile
Drainage tile are pipes that are designed for subsurface drainage.
Originally drain tiles were made from 2' sections of orange or red clay pipe. They were laid in a gravel bed, with a 1/4" to 1/2" gap between each section to allow the water to enter.
Clay drain tiles were replaced with perforated plastic drain tiles in the 1970's.
Perforated plastic drain tiles are made of Polyethylene (flexible) or PVC (rigid), 4" to 6" in diameter, and have pre-drilled holes down the length of the pipe. The perforations are of varying size and spacing.
Both types are perforated on only one half (side) of the pipe. The perforated side is placed face down so that water enters from the bottom of the pipe. The tiles have sufficient structural strength that the soil around it will not collapse into the holes. Flexible drain tile is sold in rolls up to 250'' in length. PVC drain tile is sold in 10' lengths.
Drain tiles cannot lower the watertable any lower than the bottom of the tile. The tile will only run water when the watertable has risen to a height above the holes in the bootom of the tile.
Drain tiles work most effectively when placed along side of a foundation footer, instead of on top of the footer. This practice allows you to lower the effective water table an additional 6" - 8" below your foundation.
Shallow Foundations
(crawl space, no basement, attached garage, or slab foundation)
Shallow foundation systems are more susceptible to droughts than deep foundations because the footings are closer to the surface. During dry spells or droughts, dry soil can extend 6' - 8' below the surface.
Shallow (slab) foundations are very susceptible to movement caused by soils that can expand or contract due to changes in moisture. When dry they shrink and swell when wet.
This movement is frequently referred to as settling, which often causes structural cracks in foundations, brickwork, plaster, doors and windows that stick, etc. A large majority of this damage can be avoided by simply installing a drain tile system in areas that have this type of soil.
Water Injection System
The drain tile can used as a water injection system to help stabilize the soil, by keeping it moist during prolonged dry periods.
Instead of just draining water, the system includes one added pipe to add water during dry spells.
In dry weather, a garden hose is placed in a pipe at the surface, which connects to a 'T' fitting in the drain tile. With the faucet turned on low, the water is permitted to run over night, once a week, until the dry spell is over.
To maximize the benefits of this water injection system the draintile system should be installed so the piping rests against the footer as opposed to resting on top of the footer.
Deep (basement) Foundations
Drainage tile systems are designed to move the water away from the house. In most cases, a house with a basement will, and should have a clear water sump pump system installed.
Where underground streams and/or a high water table are present, sump pumps are usually required.
To prevent the build-up of water pressure, perforated drain tile is placed around the basement foundation and along the basement walls and, in some cases, under the floor.
The drainage tile slopes toward the sump (pit), and as the water level rises, the water seeps into the tile and is carried by gravity to the sump, where a pump removes the water.
- Filter Media (Gravel)
Water takes the path of least resistance. Heavy clay soils tend to resist water movement, which means the water finds it easier to travel sideways (into your basement) rather than force its way down through the clay.
Large diameter, rounded gravel (1" - 1 1/2") is used to cover the drain tile. The gravel permits the water to filter (flow) readily down to the drain tile.
The more gravel, the better. The gravel layer should be at least 3' thick, and preferably backfilled to within 18" of the finished grade. It will be extremely difficult, if not impossible, to dig up around the foundation later to add more gravel.
- Sediment Barrier
During excavation, dirt removed from the hole is disturbed and broken up. It is loosened further during back filling procedures. All of these small dirt particles (silt) can be easily carried through the gravel by the rain water or melting snow.
Without a barrier, these silt particles immediately clog the gravel and drain tile and render it useless.
The materials commonly used are a 4" layer of straw on top of the pipe and/or gravel, or 15# roofing (tar) paper placed around the gravel bed.
This sediment barrier prevents the silt from the backfill dirt from clogging the gravel and drain tile.
Unfortunately, some builders do not routinely install this barrier. If you are building a new house, ask your builder to make sure the sediment barrier is installed.
- Water Outlet
The water outlet is simply the place where the collected water flows to.
The water's final destination will depend on the terrain. It can be one of three places (determined by local buidling codes)
- Hillside
If the house is built on a hillside, the drain tile can simply come to the surface somewhere on the hillside.
Where permitted, this is considered the best situation, because the system depends entirely on gravity to work.
- Level Ground
If you build on level ground, you have basically two choices, install a sump pit or a large buried french drain.
Sump Pit
A sump pit is usually installed inside the basement of your home. The drain tile pipe runs beneath the footer and slopes to the sump.
The collected water is then mechanically pumped from the sump to someplace away from the house.
French Drain
A french drain is a large subterranean pit filled with gravel.
The drain tile pipe runs to this pit and the water fills this pit. French drains do not work well in areas where the water table rises above the level of the basement floor.
Foundation drain water should never be connected to, or allowed to enter into the sanitary sewer system, because it can overload the sewer mains and cause sanitary sewage backups into neighborhood basements.
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See Also:
How Your House Works
Get to Know Your House
One House ... Many Systems
Water Supply
Hot Water Supply
DWV (Drain, Waste, Vent)
Foundation Drainage
Sewage/Septic Systems
Natural Gas Distribution
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