A while ago, a potential client asked me if I could come by and look at a problem that showed up in her garage almost every spring.

This is one of those very unfortunate problems that could have been easily prevented but is very difficult to fix. Obviously, in the springtime with the heavy snowmelt, ground water comes up between the garage footing and slab, soaking anything on the slab and wetting the bottom of the drywall. A functioning perimeter drain around the outside of the foundation may have prevented this but maybe not. The best way to prevent this is by removing the ground water from under the slab. The best way to accomplish this is with a drainage grid (some perforated pipe) under the slab. This drainage grid will gather any ground water before it reaches the bottom of the slab and direct it to either an outlet downhill or, if that is not available, to a sump pump. The drainage grid is neither difficult nor expensive to install. It just needs to be figured out during the excavation and foundation stages.

Site drainage is one of those things that nobody thinks about unless there is a problem. It can also be difficult to assess the probability of it being a problem. That is why, except for a couple of unique situations, I have put a drainage grid under every slab whether it be a garage or living space (especially living spaces) whether it is shown on the plans or not. I believe very strongly in Dr. Joe Lstiburek’s (buildingscience.com) statement that “You cannot build waterproof foundations; you can only build water managed foundations”.

A perimeter drain can certainly help reduce the amount of water potentially flowing under a slab, but it may not be low enough. As an example, there is one house in our neighborhood that I pass by on frequent walks. Obviously groundwater is coming up between the foundation and slab.

But- same day, same house with plenty of ground water present, there is no water coming out of the perimeter drain outlet. Hmmm… The perimeter drain is obviously not working in this example. OK, if it were working, would water still be percolating up in the garage? Maybe, maybe not. In my opinion, it’s a non-issue. Put the damn drain pipe under the slab, make sure it has a place to exit, and move on.

Underlying this whole drainage grid, perimeter drain discussion is the concept that these items be installed correctly, but that is a subject for another discussion.

As for the potential client with the wet garage? She decided that it was too expensive to retrofit the garage with proper drainage and she’s just going to have to live (unhappily) with it.