French Drains & Protecting Basements from Flooding and Water Intrusion

Another atmospheric river is bringing above average rainfall to Portland this week.  According to the USDA, precipitation trends are are changing throughout the Pacific Northwest Region of Idaho, Washington and Oregon. In recent years we have seen an increase in winter rainfall and a decrease in the summer months. During the previous two atmospheric rivers in December and November we received dozens of calls for help from homeowners and businesses throughout the region. We frequently hear “my basement has never flooded before, why now? The answer is that the methods for draining roof, ground and surface water that used to be sufficient simply no longer are.  In fact the way a lot of downspout drains are set up makes the problem much worse. We often find downspouts draining to single-wall flexible perforated pipe buried near basement foundations. Roof structures drain an enormous amount of water quickly in these heavier rainfalls. Pushing that water into the ground near the foundation increases the hydrostatic pressure in the soil dramatically and increases the hazard of that water finding its way into your structure. Improperly designed or installed french drains are something we see every day.  It’s very important to use the best practices and installation methods and materials in the design of a system to make sure that it has the capacity to drain the area it serves.  done improperly, they will fail quickly, or flood your basement.  Here is a photo of a failed “french drain”  . . . it’s actually a water infiltration system, which is the opposite of a french drain but the term french drain or drain tile is very vague in common usage.  This perforated pipe in the photo was installed to take water from an entry doorway and channel it to a trench filled with drain rock. There was a filter sock around the pipe to prevent dirt from getting into it but no filter fabric to keep soil and silt from clogging up all of the void space between the stones.  Without a separation fabric the system will get clogged with soil and have very little capacity. The pipe was good, much better than single-wall flexible perforated pipe we usually see, which has tiny perforations that re easily clogged by dirt and roots. especially without filter fabric. A proper french drain must be embedded in a generous amount of drain rock or clean crushed gravel and wrapped in non-woven geo-textile separation fabric to function well and this kind of assembly. It can be used to leech water into or out of the ground but there are better systems and methods of draining water into the soil than a typical French drain assembly.  More on that later . . .