Water in basement of new home
Hi, I’m a GC in central Virginia (mixed-humid climate) and we built a house a few years ago with a finished basement.
The background:
The house is on a relatively flat piece of land so the basement is fully underground. The foundation drain (with gravel backfill and filter fabric) runs about 100 yards away to daylight. I checked the fall on it recently, it’s about 15″ from house to daylight.
The foundation wall is concrete, the waterproof system is Tuff-N-Dri, we sprayed 1.5-2″ of closed cell foam on the interior and framed the wall inward of that. Downspout drains are draining well away from the house in 4″ and 6″ PVC. We have good positive drainage, no landscaping or hardscaping that would contribute to the problem. There are no inlets, pipes or other holes in the foundation that are causing the problem.
The issues:
The homeowners were seeing some warping in the basement flooring earlier this year. We investigated and found that 1) the foundation drain outlet had been buried by the farmer, and 2) the dehumidifier had been disconnected from the system. We remedied the issues and also put 3 layers of a full roll down waterproofing sealer (in lieu of the previous recommended paper underlayment) under the new flooring. No unusual cracking was visible in the slab. We didn’t uncover the walls.
Last month hurricane Debby hit our area and the basement took on about 1/4″ of water. It was clearly emanating from where the floors meet the walls (noticing pooling as we sop it up). We also noticed a little bit of moisture and a bit of bubbling coming up from the floor sealer.
What we know and questions:
– We did not put an interior subslab drain in- I’ll probably do that on all future houses. Or at least connect interior to exterior foundation drain – I’ve done this a number of times and seen it recommended on here plenty. Would it also be prudent to run a pipe from the interior drain to a future sump pit location if needed?
– We have opened up a little bit of wall down low and seen no evidence of moisture behind the insulation. I’m trying to avoid ripping into the walls and tearing out insulation if we can.
– I found that the foundation drain was located on top of the footing, not beside it. This was an oversight, but we backfilled with a lot of gravel and I would hope it would still drain fully. Would a high water table cause enough hydrostatic pressure, even with a fully functioning exterior foundation drain on the footing, to push water into the basement from the interior slab/footing joint?
– I was suspicious that a section of the foundation drain wasn’t connected to the drain to daylight. I put a hose on that to watch for water to the drain or into the basement and saw it draining to daylight about 4 hours later.
– I noticed another area where the moisture readings seem to stay higher (even a month later, with humidifiers running since the hurricane) and became suspicious that that area of foundation drain became disconnected. I put a hose on that area for a full day and didn’t see water inside or at the drain outlet. Drilled some holes under the slab and it was completely dry. Left the hose on all day the next day and same result. Is it possible the water wouldn’t reach the drain or is this an indication that it isn’t connected here? The slab is about 8-9′ below grade, it was backfilled with gravel and topped with common Virginia farm soil- sandy loam and clay- then topped with mulch. The sub who installed the drain is sure it’s connected- we’ve never had issues like this with him. Should we dig it up?
– What do we do? We’re willing to do what we need to in order to solve the problem for them, but we’re lacking confidence in the solutions. We would like to avoid tearing out the walls to put a French drain up against the foundation wall. Should we put a subslab drain in inward of the walls? Will that relieve the pressure enough?
Any help would be appreciated!
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Replies
You said your sight is mostly flat. That could be a problem the water will find the low spot and fill it, water is almost unstoppable.
You said “the foundation drain outlet had been buried by the farmer” Is the exit from the drain on property owned by the home’s owner? If not, this will be a big problem.
You said “The foundation drain runs about 100 yards away to daylight.” To drain correctly the pipe must slope toward the exit a ¼ of an inch per foot of run or 6.5 feet for 100 yards. Does the pipe have the required slope? The inlet to the drain must be about 10 inches below the basement floor level. Is there daylight reachable with slope from this level?
Does water exit the drain pipe after it rains?
I sounds like the drain may have worked when it was new. If so, consider having a camera run up the drain and any blockages cleared.
“connect interior to exterior foundation drain” is a very risky game in that this connection bypasses 100% of the exterior water proofing. If the drain should ever get blocked water will certainly back up into the house thru this connection.
Seems like any GC would know every thing I have said hopefully I said something helpful.
Walta
Thanks for your response.
The site is mostly flat, there is a slight grade away from the house, both natural and built up. We have appropriate drainage behind all yard retaining walls.
The foundation drain outlet is on their property, they had a farmer leasing some land for farming but he wasn’t supposed to disturb the area with the drain outlet. Unfortunately he had.
There was no way to run the drain 1/4” per foot to daylight on this property. I’ve seen foundation drain slope recommendations range anywhere from “you don’t need to slope it“ to 1/4”, with 1/16”-1/8” being common. This one is sloped about 1/16”. In the end we, with the owners, made the decision that we would utilize this slope rather than run the water to a sump pit that had to be discharged.
Yes, if it’s a heavy enough rain water does exit the foundation drain outlet. As noted above, I tested a few areas with a hose.
I’m toying with the idea of trying to run a camera all the way up to the potentially disconnected area noted above but haven’t done it yet. Maybe a task for next week.
Connecting interior to exterior drain is pretty common. We don’t do it on every house but we’ve done it on a handful. Everything we do is highly custom and very thoroughly and well designed. In this case we didn’t have an explicit foundation drain design and we didn’t do the interior drain. While yes it can allow water to the interior subslab, the idea is that it drains to a pump and discharges. The alternative would be not having that at all and the water having nowhere to go at all if the drain gets blocked.
Thanks again for your questions, it’s helpful to push me to think more about some things. I’ve been building very high-end custom homes for 20 years but I definitely recognize that I don’t know everything, and mistakes can still be made. I’m taking the responsibility here to be sure it is fixed correctly for my clients.
"Would a high water table cause enough hydrostatic pressure, even with a fully functioning exterior foundation drain on the footing, to push water into the basement from the interior slab/footing joint? "
"Drilled some holes under the slab and it was completely dry. "
If it's completely dry under the slab you don't have a high water table. In 99% of cases, "high water table" is mumbo jumbo from a basement "remediation specialist" trying to sell an expensive excavation problem when what you really need is to clean your gutters.
Foolishly I didn't drill the holes until a few weeks after the storm when I was testing the foundation drain. It's two separate incidences. If I had the presence of mind to drill the holes when the original flooding occurred I'm curious if water would have been coming up.
I agree that water will flow with less than slope but the flow is slow enough the pipe may not be self cleaning. The fact that it was blocked for some time and the low slope makes me think it is full of silt.
Was the basement installed slab on top of several inches of gravel?
A pit and a pump that would drain that gravel should keep the basement dry.
Walta
Yes, the slab is on 4" of gravel.
The slope is a little over 1/8" per foot, within standards for all plumbing-related drains that I'm aware of. I believe your main problem is locating the footing drain above the footing.
Crushed stone below the drain pipe should help but the pipe is there to provide a highway for higher volumes of stormwater. During heavy storms, the crushed stone can't handle the flow, so the water backs up until it reaches the drain pipe, and also gets through the joint between footing, wall and slab.
The only reasonable solutions I know of are to fully excavate the exterior and relocate the drains pipes to where they should be, or to cut a French drain around the interior perimeter, leading to a sump and pump that discharges the water away from the house. A sump and pump alone might be enough if the sub-slab zone drains freely, so you might be able to skip the French drain.
Thanks, seems reasonable. Do you think a French drain located ~ 6-8" inside of the concrete foundation face would have the same effect in this case? I hate the thought of removing all of the walls and closed cell.
Find the lowest point of the foundation. Drill 15” hole n foundation. Dig up about 2 feet down. Lay some gravel on bottom. Plop a sump pump and pipe it out.
… Otherwise you will go crazy trying to figure out the black magi… err drainage for a problematic basement. Good luck
With gravel under the slab, I see no reason to install interior collection pipes.
Dig a pit 12 inches deeper than the gravel install a pump anywhere in the gravel.
Any water in the gravel will find its way to this pit and the pump will remove it.
Walta