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I'm in the process of building a new house with basement. The deep end and approximately 3/4 around of the basement wall is approximately 8 ft. deep. do I need a French drain this deep basically at the footer. I have waterproofed the block wall with Damtite water proofing. Looking for other suggestions to help prevent problems years later.

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So moisture can either come in from below (a rising water table) or come in from surrounding ground water (rain) in the saturated soil.

If it comes up from a rising water table, it will enter the living space from below through cracks between the foundation wall and slab.

If it comes from the surrounding ground water, it COULD enter the living space through the wall, especially if the exterior side of the foundation wall is not sealed properly or through the crack between the foundation wall and slab. Either way the moisture enters the living space, it will need to be collected and disposed.

I think the best way to collect it is on the exterior side of the foundation wall by installing a 6” perf pipe in drainrock and laid 6”-8” BELOW the interior basement slab. (Some use “filter fabric” around the drainrock to keep “fine” soil particles out of the rock and pipe.)

To keep the subsurface water from seeping through the wall, I recommend installing a moisture barrier on the exterior side of the foundation wall and install a 2” thick plastic mesh on the wall to allow water to flow down to the perf pipe. If dirt is allowed to be backfilled against the wall, the dirt could hold the moisture giving it a chance to seep through the wall.

Once collected it needs to be disposed by extending a solid pipe over an embankment or in a collection well and pumped away.

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  • Is there a risk that the perf pipe will at some point collapse, or does the pipe have as long a service life as the structure? Obviously a collapse would leave a void and an unsupported foundation if the pipe were under the foundation. To understand your answer: You are suggesting that the perf pipe is placed below the level of the foundation, but outside the footprint, i.e., not under the foundation, right? Oct 20 '18 at 11:09
  • So ideally there would be say a 1 ft wide layer of drainrock next to the foundation from the surface all the way down to the perforated pipe? Sounds great to me, but are foundations actually built this way--crushed rock and no soil in contact with the below grade sides of the foundation? Oct 20 '18 at 16:48
  • @JimStewart Where I live it’s very wet. When the ground gets soaked, the basement wall acts like a dam. It holds back water...and it doesn’t matter how much : 4” or 30’...it creates the same pressure on the wall. (That could be a trick question to ask a first year engineering student, if you want.) That pressure will find a crack and cause a leak. Pressure is greatest at the bottom...right where the slab meets the wall. The perf pipe is just to let the water drain away. (Water drains in the direction of least resistance.)
    – Lee Sam
    Oct 20 '18 at 18:56

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