What is a French drain, a curtain drain, and an area drain?

I see many posts on bulletin boards and have spoken with many who are not familiar that there is a difference between a French drain, a curtain drain, and an area drain.  There is a difference so read on for more.

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In Texas with our black clay soil we tend to have issues with our foundations related to the shrinking of the soil during dry periods and the swelling during wet periods.  A large part of that problem comes with the swelling and these soils can literally lift a foundation when they swell.  Moisture control of the soils around the foundation is very important.  Therefore proper drainage around our foundations is critical to help prevent these issues!  However there appears to be a great deal of confusion with the types of drainage we see here and they are often mislabeled and misunderstood.  There are three types of drainage that we can not readily see as they are mostly buried and are frequently mislabeled.  They are the French drain, curtain drain, and the surface area drain.  So what are these three and which do we mostly use here?

One of the most misunderstood and used terms is the French drain.  These are subsurface drains that are used to collect and carry deeper subsurface water away from foundation footers.  I won’t repeat a very good Wiki post HERE that also has excellent pictures included.  Here in Texas the black clay soils are only capable of absorbing approximately 1/16″ of water per hour until they become saturated, unlike other better soils which can continually absorb water.  Once the black clay soils are saturated any further water becomes surface run-off.  More often than the grounds do saturate during heavier rains and it is the run-off that becomes a problem.  Because of the way a French drain operates it is not used heavily here in Texas although under some circumstances it is.  For instance a type of French drain arrangement can be used behind retaining walls to help relieve hydrostatic pressures (pressure from water build-up behind the wall) and prevent damage to the wall.

Another type of drain that we do sometimes see here in Texas is the curtain drain.  The curtain drain works on the same basic principle as a French drain but instead of being placed deeply in the ground it is placed near the surface of the ground.  An excellent article regarding curtain drains can be found on the Fine Home Building WEB site HERE.  As you can see a properly installed curtain drain will leave a visible indication of its presence.  Sometimes these are incorporated with landscaping to look like dry creek beds and other similar arrangements.  However many people do not like these arrangements, some can not have these arrangements potentially due to lack of yard space or Homeowner Association rules, etc.

The most common type of drainage we see here in Texas are swales between homes and the third misunderstood drainage system of surface area drains.  Swales are nothing more than drainage channels between homes, typically on the lot line between homes, that are intended to carry water to the front or back of the property line.  Using swales depend on there being enough slope downward from the home’s foundation to the swale, and sufficient slope of the swale to carry the water away.  Some times swales are not sufficient enough or the ground around the foundation, or in the yard, can not be properly sloped to the swales and that’s where surface area drains are used.  This site HERE has a very nice diagram displaying various surface area drains used around a home.  Surface drains can be an effective method of drainage if properly installed and terminated.

So when you do see a home advertised with “French drains” you should ask them to clarify exactly the type of drains that are actually in use.  Most likely the term is not being used properly and instead they intended to say area drains.

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