Inground Gutters Keep Water Away from the Basement
Underground barriers carry water away from the house
Inground gutters keep basements dry.
Whether they're called an underground roof, underground flashing or inground gutter, the idea is the same. Burying a waterproof membrane in a trench near the foundation wall diverts water away from the house and keeps the basement dry. The technique is especially useful in areas where gutters along the eaves are difficult to maintain.
ABOUT INGROUND GUTTERS
Inground gutters don't need maintenance.
When sized and installed properly, rain gutters collect water and move it away from the house efficiently. But gutters require periodic (and sometimes frequent) cleaning to remove leaves and other debris, and in snow country gutters can be torn completely off the eaves.
An alternative is an inground gutter, which diverts water away from the foundation wall. It's out of sight, doesn't require any maintenance, and is relatively simple to install, even as a retrofit.
Start by digging a 1-ft. deep trench around the perimeter of the house. It should be between 2 ft. and 4 ft. wide with the bottom sloped away from the house, 1/4 in. per foot. Lay the membrane in the bottom of the trench. Where it meets the foundation wall, the membrane can be held in place by a piece of pressure-treated furring and masonry fasteners. This prevents any water from seeping between the foundation wall and the membrane.
The width of the trench would depend in part on the roof overhang. If the membrane is to go all the way to the foundation wall, it should be at least wide enough to reach the drip line from the roof.
Add perforated pipe and rigid foam
When PVC drain lines are installed in the trench along with the waterproof membrane, water can be diverted to a rain garden, dry wellUnderground structure that captures, then slowly releases storm-water runoff so that it can be absorbed by the soil. or swale. Michael Maines described just such a system in a Building Science blog at GreenBuildingAdvisor.
Maines used a network of PVC drain lines, which could be serviced via distribution boxes with removable covers in the event of a clog. Water was piped to a dry well measuring 8 ft. by 12 ft. by 5 ft., big enough to handle an inch of rain before filling up. When it did, excess water went into a 6-in. outflow pipe.
After installing the EPDM membrane, Maines filled the 2-ft. wide trench with 3/4-in. washed crushed stone. A layer of permeable landscape fabric just below the surface kept silt out of the trench.
GBAGreenBuildingAdvisor.com technical director Peter Yost, in a separate GBA blog, makes the additional suggestion of adding a layer of rigid foam insulation beneath the membrane, which has the benefit of warming the soil slightly around the foundation.
While an underground gutter can be designed to cope with runoff from the roof, it won't do anything to control below-grade water. A number of factors can contribute to the problem: a lack of drain tiles at the base of the footing, clogged drain tiles, expansive soils used as backfill around the foundation, and subsurface runoff from adjoining uphill property.
ABOUT CURTAIN DRAINS
They capture and redirect ground water
When a layer of impermeable soil such as clay prevents water from percolating deeply into the ground, the water travels laterally instead, just below the surface. Eventually it can pool in low-lying areas or overwhelm the septic field.
Curtain drains (also called French drains) are built in open areas to capture and redirect that sub-surface water. They are often used to protect septic drain fields or dry out soggy lawns, but they are a great way to keep a basement dry in a house that has hills behind it.
Curtain drains consist of an excavated trench filled with crushed stone, perforated drainpipe, and filter fabric to prevent silt from clogging the pipe. The stone can be left uncovered or planted over. The excavated trench and the perforated pipe are both pitched slightly and run to daylight.
Curtain drains are not typically very deep, and in fact trenches deeper than four feet can pose safety risks for anyone working in them.
Give it enough slope
Landscapers might recommend a minimum slope of 2%, which is the equivalent of falling two feet in height over 100 feet in horizontal distance. This pitch will keep water moving briskly and help flush out any debris that gets into the line. Builders working on flatter lots, however, have reported success with pitches that are just enough to encourage water to move.
Drainpipe options include concrete, tile, rigid plastic, and rolls of corrugated plastic. Of them, corrugated plastic is the easiest to work with because there are no joints to glue together and it’s readily available in rolls of up to 250 feet long. High-density polyethylene is an extremely durable plastic, and some companies offer drain line made from recycled plastics — a plus. Pipe that is 4 inches in diameter should be big enough to handle gutter runoff. Curtain drains that handle runoff from a large area may need 6-inch pipe.
Finally, before you begin to plan your drainage system, think about the water you want to keep. Moving water off of and away from the building is only part of the solution. It may make sense to save some of it for irrigation or to simply recharge the aquifer.
Further reading
A GBAGreenBuildingAdvisor.com detail drawing can be found here: Underground Water Barrier Retrofit
OTHER CONSIDERATIONS
An inground gutter limits the types of plants that can be placed near the foundation. Anything with deep roots could eventually damage the
membrane and make the system less effective.
