Column #816 10/09/10
On The Level
Q. Our house is about 20 years old and has two places in the basement where small amounts of water sometimes come in after heavy rains. In one place I can see that the water is coming through the bottom two rows of blocks. In the other place I have dug a trench next to the foundation and see that there is sometimes a lot of water standing there. There is no tar waterproofing left on the outside of the walls. There is a sump pit with a pump connected to a black flexible pipe around the inside of the foundation but very little water comes out of that pipe even during that big rain we just had.
Contractors in the past have offered different solutions. Most want to install a drain pipe around the perimeter of the basement floor. That doesn't seem to address the real problem which I feel is water pressure from outside the foundation wall. One suggested installing a French drain with gravel fill. There is a drain in some areas around the exterior, but it’s become full of sand and gravel. Another suggested using a membrane to waterproof the exterior surface of the foundation. The one I'm most interested in uses a sheet drain and strip drain that is applied to the outside of the wall and drains down to a pipe drain but we’d have to dig up the yard around the foundation to put that in. What do you suggest?
A. Leaking basement walls are probably the most common problem homeowners wrestle with around here and especially during multi-inch soakings like we had last week. When your house was built builders placed a perimeter trench drain around the inside of the basement walls down near the footers and then approximately four inches of gravel was placed over the whole floor, sheet plastic over that and then concrete, also about 4 inches thick, was poured on top. That was the code then and the County or City code inspectors had to come out and look before the builder could pour the basement floor.
What appears to have happened in your case is the concrete pour of the floor ended sitting right on top of the concrete footing and tight against the bottom of the lowest course of foundation block. That has effectively created a seal at the bottom of the wall and water that enters the hollow cores of the block can’t work its way down to filter out at the bottom block course into the gravel then into the perimeter pipe and off to the sump. That’s why you don’t see any or much water flowing through that black plastic pipe but you do see it flowing out onto your basement floor. That’s frustrating.
Don’t try this unless you have a plan in place but I have taken a hammer and driven a hole into the block just above the floor level only to have water shoot out from the inside of the foundation wall like a hose to prove the point that water was standing inside of the block wall. The contractors who want to cut in a new interior perimeter drain would perform a variation on this theme.
Your thinking about intercepting the water before it gets inside the wall is to my thinking the smartest approach. The old asphalt coating that you say is gone was intended as damp-proofing and it never really worked but was required. But adding a sheet drain system at this stage of the game would require excavation down to the footers all around the foundation. That can and will be both messy and expensive and it’ll work. That technique was developed to de-water commercial buildings that went many feet into the ground and the hydrostatic pressure of the ground water had to be intercepted.
The approach I’d suggest you take begins very elementally and starts with making sure your gutters and downspouts are in fact doing their jobs and any rainwater that hits your roof is directed away from the house. That goes too for the grade around the foundation. We like to see a drop in elevation of the dirt against the foundation wall at a rate of an inch or two per foot for about six feet away from the wall. 99% of basement water problems start from the roof and the home’s exterior stormwater drainage system.
The take a hard look at your landscaping. A 20 year old house has had time to grow some pretty monstrous bushes near the house that prevent good drainage. Get rid of them if you must. Piling up mulch sometimes hides poor drainage so check that if you’re a mulcher. Don’t give up on those old “French” drains. Re-excavate and clean them out. This time when you fill them back with gravel be sure you place a layer of filter cloth over top to prevent sand clogging. Try all this before you go throwing tons of money at contractors, sheet drains and messy excavation.
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