French Drains and Water Tables

[Rob]

We just recently bought our first house (new construction), and while we feel we got an excellent value, we knew going in that it is in a wet area, as the lawn is soggy wet. This caused the sump pump to run a lot during our first week. So, I installed a check valve, which has greatly decreased the sump pump running time, and I will get a back-up generator soon. Our unfinished basement is dry, and before we purchased our neighbor said he has no water problems, but he installed a retaining wall. The builder insists our home has a French drain, consisting of "a layer of stone under the foundation", but I've read that french drains are trenches around the property to DIVERT water.

What is the difference between a "false" water table and a low water table? What exactly is a French drain? And ideally, how often should a sump pump run?

[stevea]

1) False water tables or perched tables are caused by the soils around and under the foundation. Certain types of soils cause water to get trapped in the upper levels of the substrate. This will cause the usual basement leak when hydrostatic pressure builds up against the foundation and will usually come in from the floor wall joint in the basement.

2) True water table is the level of the ground water in the soil.

3) Some sump pumps will run every 2-3 minutes at certain times of the year and once every few days at other times. Most sump pumps are more active a few days after heavy rains especially in the spring.

4) Many people call sub floor drainage systems and exterior footer drainage systems "french drains". French drains are named after Henry French, a farmer from Concord, MA . Henry French wrote a book called Farm Drainage around 1859. He is credited for designing a open trench with 2" pipes covered with tan bark with soil covering over it. These drains would allow ground water to drain out of a cellar so the farmer's potatoes, and can goods from getting wet and spoiling. Today's "french drains have improved with the use of crushed stone and various drainage systems.

More then likely your builder was referring to either an exterior drain or an interior perimeter drainage system which is either along the outside of the footer or along the inside of the footer (under the basement slab).

Hope this helps.

[stevea]

There has been this discussion in the basement waterproofing industry for some time. Some in our industry believe that a true french drain is done on the outside only. Some in our industry think that it is alright to install an interior french drain. Well as a basement drainage contractor for nearly 30 years I must voice my opinion on this subject.

When I read Henry French's book entitled Farm Drainage (1859) I found a chapter entitled Cellar Drainage. He even had a drawing showing the drains being installed on the interior.

I must say, I do agree with the exterior drainage guys, that it is best to keep the water from entering the basement at all. That being said, in new construction I believe one should install exterior and interior drainage systems. As far as retro, I believe that an interior system is the best choice. the reasons I say this is because an exterior system when it clogs cannot be maintained. It must be dug back up and replaced.

That being said, I do believe that an interior system MUST be a closed system. I say this because of 4 reasons, radon, ground humidity, bugs, and iron bacteria. I don't recommend any interior system be installed unless it is a closed system. I hope this helps consumers and contractors alike to understand why an interior system is better than an exterior system. I also recommend that an interior system have access ports in all the corners, this way a contractor will never have to re break the concrete basement floor in order to flush the system. I recommend the Grate Drain system, check it out at www.grateproducts.com