We need a French Drain around the foundation
9 September 2006
A. A French drain is simply a ditch with the means of transporting water from where you don’t want it to somewhere else. The most common design is to put a few inches of washed stone gravel, sometimes called “bank run” around here, into the trench then p lace a perforated drain pipe at about the bottom third of the trench, then fill the trench up with stone to near the surface then top off with grass or, in your case, the patio paver.
There are some tricks and laws of nature that must be followed for success. The drain and the pipe must be level or slightly sloped to wherever its outflow will be or else you will have merely constructed a moat and you’ll keep getting wet. I recommend t hat the trench be lined with filter cloth. That’s the black cloth with which you see construction job site silt fences built. I’d also place a strip of filter cloth to cover the perforated pipe before the rest of the stone is installed so that over tim e silt, called “fines”, won’t build up in the pipe creating a clog that only excavation can cure. Plus one more strip of cloth just under whatever you top the trench with; either soil and grass or paver bedding and pavers. The whole idea is to make the installation permeable and to keep it that way. The depth of the trench is usually a function of the calculus of drainage and site conditions. If the site has a topography that can work with the drainage away from the troublesome area then it can be use d to its advantage. When the trench is planned and dug I would expect that the bottom of the trench to be checked for slope with an optical level or builder’s transit. Width of the trench will end up being a function of how it’s dug, either by machine o r hand.
But before you go to all the mess and expense of installing the French drain, take a long hard look at what is going on outside where this water intrusion is taking place. Are the gutters overflowing and is water splashing up against the house? Is the l andscaping trapping surface water that otherwise might drain off on its own? Make whatever changes you have to in that regard and see what happens. The level of that patio should have been at least six inches lower than the floor level of the room gett ing wet. You may eventually have to resort to the drain system just to be sure you don’t flood when you are off on vacation someday only to come home to a mold disaster. As for the type of contractor who might undertake a French drain installation, don’ t overlook patio installers or well equipped landscapers. When I googled “French drain design” online to see what was out there I saw an ad for a company that specializes in such installation located in Northern VA but comes to Maryland. You might give them a try if you don’t have any luck locally.