Bear with me – this will take a long explanation before I get to the actual question. I’ll try to be brief.
I have a damp, moldy basement. I had an interior French drain and 2 sump pumps installed several years ago (more than 10), but that did not completely eliminate the problem. I want water to be COMPLETELY eliminated from my basement, as I want to finish it and turn it into additional living space.
I am now installling an exterior French drain. The problem that I have run into is that, for some unknown reason, there are six courses of block below the level of the basement floor. I have no idea why the house was built this way, but the problem that it has caused is that the exterior French drain will now be significantly below the level of the interior sump pits.
My thought was to just bring the French drain through the block and have it spill into the sump pit and let the pump take it from there, but since I will be so much deeper than the sump pit, I have to come up with a different solution. Here are the ideas that I have had so far:
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Make the sump pits deeper and proceed with the idea of bringing the water inside the walls and into the sump pits. I wonder if this is a bad idea, because if I am keeping the water out of the house, am I shooting myself in the foot by intentionally bringing it inside? I plan to install new sump pumps with battery backups, but if they should happen to fail for some reason, my finished basement would flood.
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Keep all of the French drain outside. In order to do this, however, I would have to install additional sump pumps outside and UNDER GROUND. Can this even be done? Again, they would have battery backups, but would be completely underground, so cleaning and servicing would require significant excavation. Should I just put in a sump pit that is 10 feet deep and keep the top of it just a few inches below grade?
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Put the French drain just below the level of the basement floor and leave the additional depth below that level alone. Again, I have no idea why the foundation goes so far below floor level, but it does. Why would it matter if water is allowed to permeate the block below the level of the basement floor? I realize that moisture could still get into the concrete floor in this option, but I don’t think it would be enough to be an issue. If I go with this option, that will solve the issue of the drain being below the level of the sump pits, whether I go through the wall and into the existing sump pits or install new outdoor sump pumps.
Thank you in advance for your input and advise.
Best Answer
Water is either coming up from below (a rising water table) or its coming from surface water (the surrounding ground water in the saturated soil.) Either way, it needs to be collected and carried away.
IMHO, it’s preferred to collect and deal with the water outside of the basement.
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. 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 in drainrock and a perf pipe laid 6”-8” below the basement slab.
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 enclosure and pumped away.
IMHO, it’s preferred to collect and dispose of the moisture on the outside of the basement, rather than inside the basement. This will eliminate the possibility of mold inside the basement and eliminate a possible power failure causing the sump pump to fail (even with battery backups) and thus water damage. This may be the most expensive method of solving the problem, but it’s sure to work.
One reason your footings are so much lower than your basement slab is due to the frost line. If so, you may need heat tape on your drain and float switch.