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toby 12-01-2011 10:44 AM

installing french drains in finished basement
I'm curious to know what the end result looks like when installing french drains in a finished basement. Is there a gap between the wall and flooring? If so, how large is it, i.e. inches from wall? Foundation wall or drywall? When reinstalling tile or carpeting is there a space around the perimeter of outside wall? If so, how much? Does cement or gravel show? Please let me know as soon as possible. If you can send me pictures that would be much appreciated. Thank you in advance

BridgeMan 12-01-2011 12:40 PM

By your referring to "french drains", do you mean an open grate system to remove water that's already in the basement? Or are you trying to intercept water that's outside your foundation walls and under the floor, using buried conduit leading to a sump pit? I've seen commercially-installed systems of the latter, typically with an inch gap immediately adjacent to the wall, when water is infiltrating through wall cracks. If that's what you're dealing with, then you need to leave a similar space between the foundation wall and back of finished wall. And don't forget some access ports for flushing the system every year or so.

Perry525 12-04-2011 10:45 AM

When you open up the floor you create the possibility of water coming in through the hole at times of high rainfall.

French drains are best made outside, where you can leave the trench/ditch open while you find out how much rain it can cope with. The problem with a French drain is that while it is open you can see where the water is coming from, where it is going and if the slope and down hill, or pumped sump can cope, but once it is filled in, you loose perhaps two thirds of it capacity and that may cause a problem with back up.
An open ditch on the outside, will mean that the only moisture in the wall is rising up from the foundation, once you fill the ditch flooding may cause the water to seep through the walls.
Water will rise four feet up a wall from the highest point in contact with the soil.

mudmixer 12-05-2011 02:43 PM

A proper french drain inside the area of a basement should be at the level of the bottom of the footing, just as external french drains are installed.

The interior french drains have a layer of properly draining sand installed after a layer filter fabric in laid between the existing soil and the backfill.

The properly draining backfill is placed and left a couple of inches (2" - 4") below the finished floor level and concrete is poured over the excavated are and over the tops of the footings.

This assumed the wall construction is the preferred strip footing with a concrete block or poured concrete wall used to support the basement walls. Some codes require 4" of concrete on the top of the footing to provide additional lateral stability for the wall.


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