Chris d Craiker AIA/NCARB
The recent storms have reminded me of an old architects’ axiom: water and moisture are our nemesis. A friend called me the other day who checked under her house and discovered a large sinkhole. It was wet and mushy, and she asked for advice. On inspection, I saw a five-foot-wide pit in the middle of her underfloor area. However, I also noticed significant wetness around the foundation perimeter. The house had been recently renovated and had extensive concrete patios around the outside that did slope away from the house, however there were many roof downspouts piercing into the patio area.
Was there a separate drain for the downspouts? Did they connect to a French drain? Hard to tell. Naturally, all drains have to go someplace and at the very least, exit into garden or landscape areas depositing at least 3 feet away from a structure. Traditionally, they connect to a street curb to dump into municipal stormwater drains, but today we’re trying to keep as much water on-site and in the ground as possible.
What is a French drain? Sometimes called a filter drain, rock, drain trench, drain, or sub surface drain, it’s basically a trench dug adjacent to a structure and lined with filter- fabric, filled with gravel or rock with a perforated pipe that redirects surface and subsurface water to redirect away from the building.
It’s not uncommon for roof downspouts to be tied into French drains but this is wrong. Technically, the French drain is your foundation drain and not a roof drain. Downspouts from the roof should never be mixed with surface drainage. They should be run in separate pipes or lead away from the house into landscape areas at least 3 feet away. Just like your plumbing drain lines, there should be a clean-out to receive a periodic plumber’s snake. The lines do clog up on occasion. French drains are often designed to go around the foundation at 90° angles, therefore difficult to get a snake through. Thus, multiple cleanouts may be required.
If either the French drain or roof downspout drain are lower than the outlet, a sump pump may be required to evacuate the water. Installing a portable pump in a five-gallon container with a hose out and a water-resistant power source is a temporary solution but a permanent long-term system should be installed.
Basements are not common in California, but many homes are built on hillsides with living space below the adjacent ground. A French drain should be at or below the adjacent retaining wall to collect water over the entire length of the wall and exit out the sides to grade at least three feet away. Removing water behind a retaining wall will also reduce pressure on the wall which over time could cause the wall to fail.
How to tell if your French Drain is working
I’ll bet when one is buying a house, asking about the French drain function is a low priority. It should be at the top of the list of “To-Do” with any home inspection. A simple hose inserted into a clean-out or other orifice access and let it run until someone spots the water exiting. If it’s not exiting or backing up, call a plumber with a long snake.
Every site and foundation is different. Consult a professional soil or structural engineer for your site conditions. Interestingly, the French drain is not from France, but developed by Henry French, a lawyer and assistant US treasury secretary in Massachusetts. Go figure.
Chris d Craiker AIA/NCARB has his French drains cleaned every two years