Expert: Bruce E. Johnson - 9/6/2011

Question
I have built several homes in New Jersey.  The local building inspectors have always required a french drain around the outside of the footing, which is then piped under the footing into the sump pit.  The rationale appears to be that if the water table were to rise to the depth of the foundation, then the french drain would relieve the pressure from under the slab or from the outside of the foundation walls.

This makes no sense to me.  I can understand the usefulness of a french drain, if it were to take the water away from the house, to the road, or to a storm sewer of some sort, but to send the water into the basement seems wrong.  If the water level were to rise to the point at which the drain became necessary, a sump pump would hardly be able to handle such a large quantity.  It almost appears to be akin to drilling a hole in the bottom of a boat in order to relieve the water pressure on the outside.

What am I missing?  I would like to understand the usefulness of the drain the next time I install one.

Thanks.

Answer
Hi William, footing or perimeter drains around basement foundations are important and necessary to not only keep water from building up and coming through your floor but they also serve to keep hydrostatic pressure from building up and pushing against your foundation walls which could then conceivably fail or rupture.  If you have ever seen a retaining wall leaning over, it is the hydrostatic pressure of the ground water that pushed the wall over. The reason for draining the foundation water into a sump is generally due to the building lot not having enough slope to allow for the water to be moved via gravity to a drainage basin or other storm sewer facilities.  Because of this the water now drains into a sump and is pumped out on a regular basis as needed and to use your analogy it is much like a boat bottom with an automatic bilge pump to keep the boat from sinking when too much water enters the hold.  You would be amazed by the damage ground water pressure can exert on a foundation or a basement floor.  Even a swimming pool, when drained has to have it's floor drain left uncapped or the ground water can actually push the pool up from it's hole.  So for all of the above reasons, most building codes require foundation drains around basement foundations.. I hope this information helps, please feel free to write again regarding this or other matters, sincerely, Bruce Johnson ..bejohnsonconsulting.com