French Drain FAQ

BULKHEAD a protective wall to block waves of water from damaging and washing away a shoreline, also called a seawall.

BUOYANCY the upward force of water upon a structure, not to be confused with upheaval, which is the swelling of soil because of an accumulation of moisture.

CATCH BASIN a surface drain that looks like a box. The discharge hole is in the side of it about 3 inches from the bottom of the box, allowing silt and trash to fall to the bottom, and as the water rises, it will drain down, leaving most of the silt and trash. SEE SURFACE DRAIN.

CHECK VALVE a valve that allows water to flow one way, but not the other, installed to prevent water from backing up into a drainage system.

CONDENSATION is the opposite of evaporation. When water vapor that is present in the air cools, it “condenses” and gets smaller, releasing the water from the air onto a surface area. This collection of water can be on the wood members of a foundation, causing them to rot and deteriorate, and the foundation floor will eventually collapse. This moisture can harbor and breed molds and mildew, which can be hazardous to one’s health. The cure is to provide plenty of cross ventilation, essentially ‘blow drying’ the wood members with air, taking with it the moisture.

CRAWL SPACE VENTILATION air that should be circulated under a structure, preferably through a cross-vent circulation system, or a forced air system. See CROSS-VENTS.

CLOSED DRAIN SYSTEM a drainage correction system whereas the water is collected in surface drains or gutters, then discharged through a non perforated pipe. When a perforated pipe is used to collect the moisture, it is called a French drain.

CROSS-VENTS openings through the perimeter of a structure that allow air under the structure. Without the proper number of cross-vents, the air will circulate under the structure, and there can be a buildup of moisture on the lumber and in the soil. With that moisture, the lumber can rot, the wet soil can cause foundation piers settlement or upheaval, and allow mold and fungi to grow. One square foot of cross vent space is required for every 100 square feet of downstairs living space. Ideally, one cross vent should always be installed within 3 feet of any corner of the structure.

CULVERT a drain for storm water placed under a road or berm of soil.

DRAINAGE CORRECTION a phrase intended to describe a situation whereas there is a problem with water, either surface or subsurface moisture, or even moisture in the air. Measure must be implemented in most cases to correct the drainage problem, by which many different methods may be employed to remedy the situation.

EROSION the removal of soil by wind or water from a surface area. If enough soil is eroded from under a foundation, it will settle and fall.

FORCED AIR SYSTEM a method by which air is sucked from under a structure and blown to the per meter of the structure, with fans. The system is usually activated with a control device that measure the moisture in the soil, and activates when it is too high. For a forced air system to work correctly, there must be enough cross vents or a backflow of air to draw from.

FRENCH DRAIN has been described simply as a ditch full of rocks. Many engineers call a French drain “lowering the water table”. French drains are intended most always for subsurface water problems only, whereas as the water table rises, it will fall into this “ditch full of rocks”, and then into a perforated pipe at the bottom of the rocks that will discharge the collected water downhill or to a pump. French drains typically run horizontally around the perimeter of the affected area. Any structure underground, as in a basement, garage, or room, must be accompanied by a perimeter French drain, and sometimes other means, to prevent certain and sure water problems. Caution: even though many call all drainage corrections a French drain, really subsurface moisture problems are the only place for a French drain. A surface drain system is usually employed, with proper grading to correct most ‘surface’ drainage problems.

GROUND COVER any cover over the soil. Ground cover can be vegetation of any kind, and some will cover the soil better than others. The best ground cover, of course, is concrete. The poorest ground cover are rocks, pea gravel, wood chips, sand, and the like. That is because those items will not shed water away from the foundation, but instead they direct water below, causing an accumulation of water beneath them, which damages the foundation. Good ground cover will prevent the underlying soil from drying up so quickly from the summer heat, but it also keeps excessive moisture from penetrating into the soil and damaging the structure. The soil must be placed to create a positive slope away from the foundation. Good ground cover will also prevent the erosion of that soil.

HYDROSTATIC PRESSURE simply the weight of water in a hillside, in the soil, pushing down from it’s own weight, which produces pressure for the water to push upwards in another area, such as an area downhill. With hydrostatic pressure, water can actually go uphill in some cases.

LEVEE an embankment of soil or concrete to hold back water from a creek, river, or watershed.

NEGATIVE SLOPE considered the immediate perimeter of a foundation that does not shed water. When the water ponds against the foundation, it eventually causes serious foundation problems with settlement or upheaval. see positive slope

PERFORATED PIPE pipe used in mostly subsurface drainage correction that has holes in about 80% of it’s sides. The remaining side without the perforations is installed on the bottom to discharge the water that percolates through the holes in the pipe.

PERMEABILITY a characteristic of rock or soil that allows water to pass through it, varying in different degrees according to the amount of separation between the soil or rock particles.

POP-UP DRAINS a drain at the end of a drain pipe that pops up when enough water pressure builds up behind it. This is considered a poor choice in drainage correction because water will pond inside the pipe for long periods of time, allowing a breeding pond for mosquitoes and bugs.

POSITIVE SLOPE sloping the soil around a foundation that it will shed water. see negative slope.

REVETMENT a pile of stones placed on a hillside to prevent erosion. Covering the stones with a small amount of concrete may help keep the stones together. Spraying gunite or shotcrete is a more expensive solution than building a revetment.

SCUTTLEHOLE BOX a box installed to provide access under a structure without allowing water to have access under the structure. A good scuttlehole box is like a storm cellar door, with a lid. Most scuttlehole boxes will also require a pump underneath to discharge collected moisture. A scuttlehole box can be designed to act as a vertical French drain as well as an access under the structure.

SEEPAGE moisture penetration that ‘seeps’ through mortar, brick, wood, etc., through direct contact with the material.

SKIM COAT applying a thin coat of high strength cement over a concrete or masonry surface, sometimes installed to cover exposed rebar or to repair crumbling concrete.

SURFACE DRAIN a drain that is like a shower drain, that collects water that flows into it.

SURFACE DRAINAGE also called runoff, includes all water at the surface, originating from rain or possibly from creeks and rivers. The other drainage is subsurface drainage, originating from hydrostatic pressure, a high water table, or water draining from cracks and fractures in the subsurface rock formations.

SWALE a dip in the surface soil or landscaping that collects water from two sides and channels it downhill.
a swale would be at least 4 feet wide, and sometimes 8 feet wide, but a 2 foot swale is really called a ditch, and a ditch can be a tripping hazard and can more easily fill up with silt.

VAPOR BARRIER a plastic material placed on the inside of a wall to deter moisture. A plastic vapor barrier should never be placed on the outside of the insulation of a wall because condensation will develop there and start to rot the wood members. On the inside of the insulation, there is a lesser degree of temperature change, and less likely for condensation to form.

VERTICAL FRENCH DRAIN is simply a French drain that does not run horizontally around the perimeter of a structure, but is installed vertically, and at only one location. At this one point, the ”ditch full of rocks”, in one hole, will collect moisture. The moisture is usually discharged through a pump assembly thereof, but it can also be discharged naturally with gravity if there is enough slope from the bottom of the vertical French drain, to drain away from the structure. The more area the outside walls of a vertical French drain, the more opportunity for the moisture to percolate into the drainage system to be discharged.

WATERPROOFING Installing a rubber product along a concrete or masonry wall to seal it off from moisture penetration. The incorrect waterproofing membrane can break down after contact with the sun’s rays, or soil particles, develop cracks, and allow water to enter the structure.

WATERSHED the area drained by a creek, river, or other region.