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Foundation drainage: this article discusses building foundation drainage: footing drains, used to prevent foundation leaks and building water entry. This article series discusses types of drainage system s, including foundation drains or "french drains" for preventing wet basements and crawl spaces.
Our page top drawing of types of indoor foundation and basement drainage systems is provided courtesy of Carson Dunlop Associates.
[Click to enlarge any image]
Definitions of a Footing Drain, French Drain, & Perimeter Drain?
A footing drain, that is an exterior foundation drainage system placed outside the foundation wall near the wall footing, at the level we show, covered with gravel, and if the footing drain going to do anything, it is piped to daylight or to a catch basin that is in turn pumped to daylight or to a storm drain.
Well plenty of people do call interior foundation drains or perimeter drains a "french drain". We don't.
A "French Drain" is an outdoor buried drain line constructed to carry water away from the building, typically to a drywell or catch basin. Our sketch shows how we remove water from roof runoff that pours down a downspout.
So what is the difference between a french drain, a footing drain, and a perimeter drain. A French drain is shown above, and a footing drain is shown in our two sketches below.
A perimeter drain is an indoor drain cut into the floor around the perimeter of a basement or crawl space to intercept and remove water from the building interior. We illustrate perimeter drains above.
Find the end of the footing drain system that used to drain to daylight (see our photo below). The foundation drain system may have become buried with mud or covered by backfill. Clear any blockage at the end of the footing drain extension, open and check the end for water flow in wet weather.
Foundation drainage - drain tiles also called footing drains and by some folks "French drains" (a mistake - see FRENCH DRAINS for DOWNSPOUTS),: water flows in the path of least resistance. Perforated 4-inch PVC or flexible ABS are the least costly and most foolproof foundation drainage conduits. Foundation drains should pitch at least 4 inches in every 100 feet of length.
The top of the foundation drain, should be below the top of the finished basement or crawl space slab. From the low corner of the building, the foundation drain should continue to daylight or, if permitted by local codes, to a storm sewer, so that water will drain away from the building without relying on an electrical sump pump or other magic.
Footing drain hole perforations face down: if you use perforated footing drain tiles that include perforations only on one side, face the holes down. Water collects on the bottom.
Footing drains to a drywell? Some builders of homes on flat sites where drainage by gravity is not possible install a drywell to collect and store storm drainage from around a building. Watch out: in areas of wet soils, in wet weather drywells are often not dry at all and may themselves fill-up with water from nearby soils, making this scheme simply not workable.
Footing drains to a sump pit? One of our clients plagued with water entry at a flat site installed new foundation drains around her home, combined with an outdoor, frost-protected, duplexed, and battery backup sump pit system to pump foundation drainage to a nearby storm drain. Without the battery backup detail, this system might not have worked: guess when electrical power (for the sump pump) is most likely to fail?
Clogged Footing Drains
Even if foundation drainage was properly installed when a building was constructed, the system may no longer be working. Over time fine soil particles can enter and clog the foundation drainage system.
If you know that a foundation drain system was installed - perhaps you can find the end of it as we illustrate above - and if the building foundation is leaking water from low on the foundation walls, and if little or no water is coming out of the end of the footing drain in wet weather, it's a good bet that the drain system has clogged.
Our photo (below left) shows the footing drain that was excavated and removed at the home of a client whose house suffered recurrent flooding. The old footing drain was totally impacted with mud.
The photo at above right shows the ends of three new footing drains that were installed and carried to daylight. We remained a little nervous about just what the builder used for backfill - notice that silty mud coming out of the new drains? They may not have a long life.
Find and un-clog the footing drains: excavate at a building corner, find the footing drains, cut open the drain to see how full it has become with silt, and have the drains cleaned using high pressure water or other methods.
Reroute a non-working footing drain to a drywell if you can't get it to daylight
Add missing foundation footing drain sections that were omitted, such as around a chimney or building addition
Foundation Drainage Backfill Details to Prevent Basement Water Leaks
Bury the footing drains in gravel, both under and above the drain tiles. Extend the gravel backfill at least two-thirds of the way to the top of finished grade.
Gravel helps water flow easily into the drain system instead of seeking a way into the building, and the removal of water outside the foundation wall also avoids foundation collapse later. The gravel size needs to be larger than the holes in the drain tiles.
Our sketch (left) shows a less than optimum footing drain installation because the artist placed soil too close to the drain pipe.
Footing drain geotextile covers: some builders also install synthetic fabrics (geotextiles) over the footing drains before covering them to further slow the footing drain clogging by dirt and silt. Other builders place a layer of 15-pound building felt on top of the drainage bed to slow soil clogging of the gravel itself as well as to protect the footing drains.
Clay foundation drain cap: the top foot of backfill over the footing drains should be a low-permeance clay cap to keep surface water (those spilling-over gutters) away from the foundation. If plantings are intended, add 4 to 6" of loam on top of the clay cap.
Overloaded Footing Drains
We explain at GUTTERS & DOWNSPOUTS that it's a bad idea to connect the roof drainage system to the building footing drains. The added water volume may overload the drain system leading to foundation leaks.
And worse, when later in the life of the building the foundation drains clog, we suddenly begin directing 100% of the roof runoff into a lake of water trapped around the building foundation - virtually guaranteeing that the basement floods.
Our photos (below) show a home whose roof drainage system passed through the crawl space wall, across to the other end of the building, exited the crawl space to go back outside where it dove down into the building footing drain system. The result was a constant wet finished basement.
Notice in our second photo (below right) that a clue telling us the whole drainage system was clogged and backing up was that during rain water leaked out of the tee at the top of the vertical drain line that was connected to the footing drains.
At a recently constructed home we determined that the roof gutter system was connected to the foundation drains, and warned the buyer about the chances of basement flooding. The home was ten years old, and the basement, by every inside inspection indication, was "dry".
But two years later, following a period of heavy rain the client called to exclaim "Geez, our basement is full of water!" IT was like throwing a switch. The footing drain outlet clogged and the basement just filled right up.
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