Do you need to put a French Drain in the whole basement, or just where the actual leak is happening, such as on one wall? If the basement is only leaking on one wall then it makes sense to only do waterproofing AKA French Drain on the one wall, right? First we need to understand why the basement is leaking then we can decide about a full or partial fix.
There are mainly two types of concrete foundations on homes here in New Jersey. The first is a poured solid concrete 8" thick concrete wall. The second type is a masonry wall made with concrete block. Concrete block foundations are made from pre-formed hollow blocks.
During construction a type of waterproofing is usually applied to the foundation before the backfill is done. A tar like substance is applied to the exterior of the concrete walls, both poured and block alike. Tar is a very good waterproofing substance and has been used forever. The tar applied to most homes is good for 30-40 years or so. They don’t last forever and neither does the tar on the foundation.
Once the tar is gone the foundation is no longer sealed. The porous concrete is laid bare. Water creeping through underground soil crevices creates hydrostatic pressure up from beneath the floors and against the sides of your home. Hydrostatic pressure is when a body of water surrounds something that does not float. When this water pressure forms around the foundation the un-sealed concrete starts to absorb the water.
So, now we can get back to the question: full or partial fix? It is a two part answer; the first part has to do with the tar sealer. Either your foundation is sealed or it isn’t. We already know it isn’t because the basement has taken on water at least once. If the tar has washed away from one part of your foundation already it is already starting to wear out in other parts as well.
The second part has to do with the water table. It is important to understand how the water rises around your basement and can cause basement flooding. Most people know that the water table has something to do with ground water. The word table provides an image of a flat surface, like the surface of a tabletop. Water tables can become elevated when they receive more water than they drain off. This is often from unusually high amounts of rain or snow melt. High water tables are often above the level of basement floors or crawlspaces. This almost always causes flooding in these areas.
Because the French Drain is actually a Bonded Deep Channel Pressure Relief System, the drainage pipe is in a deep channel below the basement floor. This deep channel system is designed to pump away the water table as it rises under your house. This works because the pipe is below the floor under the perimeter of your basement. When the water table rises, it rises levelly under your basement, not just under part of it. So, in order for the deep channel system to work properly, it should be under the whole basement.