Basement water proofing

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Old 01-15-13, 10:19 AM
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Basement water proofing

My house is located in a coastal area on the south shore of Long Island - within about a block from a canal used for boat access. I am not in a FEMA designated flood zone, though I am very close to it (part of the property next door is within the flood zone). In fact the FEMA map worked out pretty much as expected during Hurricane Sandy. I had water come all the way up my driveway and stopped a few inches from my front door and garage before receding. Many houses in my neighborhood were in fact flooded, some quite extensively. I was very lucky.

A few weeks after Sandy I noticed, for the first time, a small amount of water collecting along the northern wall of my basement. Less than an 1/8th of an inch, and confined to a small area. It occurred soon after a heavy rain. The basement is only about 250 square feet or so, taking up about a quarter of the home's footprint (NW corner). Thereafter, that condition repeated itself after heavy rains.


There appears to be quite a bit of washout underneath my concrete perimeter foundation. About 6 months ago I had some I-beams placed in the basement to take the load off of an interior foundation wall that had some lateral cracking. When they drilled through the concrete floor I was amazed to find a void of about 1 and a half feet underneath the basement floor. It almost seemed like the house was floating on air. I spoke to a structural engineer who told me that isn't unusual for my area. He suspects that there are piers underneath the foundation lending support.

Outside the northern wall of my basement is a concrete patio. The portion along the wall itself had quite obviously been raised above its original position - I'm guessing as the result of water getting underneath and freezing. I'm planning on redoing that patio anyway, so I purchased a cheap demolition hammer and tore out that section of concrete. Indeed, there was a void underneath that concrete of up to 2 feet along the home's foundation. I then dug a trench along the foundation on that affected northern wall, removing mostly sand with some soil on the top. I was hoping to find some sort of obvious cracking that could be repaired, but it actually seems pretty much pristine.

While digging the trench, at one point I decided to dig below the footer in one small spot to see what I might find. Sure enough, a few inches below the footer is standing water.

At that point my assumption was that I have a high water table, perhaps due to Sandy from a few months ago, and that the water was coming in from beneath the basement floor when it rained. I figured I could install an external French Drain system with a sump pump (can't use gravity here) - but that doing so would probably be overkill. My best amateur guess was that if it was really a groundwater problem, and I haven't had this issue in the past, probably over time the ground water would recede on its own without any major interventions.

However, now I am not so sure it is even a ground water problem. Why? Well the foundation is still exposed, and although it rained quite a bit yesterday and last night, no water got into the basement (as I would have expected it to). That leads me to believe that in fact the water is coming in via capillary action through the foundation wall. Since the dirt is removed, the water isn't going up against the wall, and therefore isn't coming through.

Now I'm thinking that I should just dig up the rest of the foundation (not very many linear feet left) and just coat the whole thing with an elastomeric membrane to keep the water from coming through the wall. I still think the exterior french drain with sump pump would be overkill, as my assumption is that if indeed it's a water table problem, which now seems less likely, it's probably temporary, as we never had such a problem before.

I would definitely appreciate any thoughts or advice, as I am a true amateur when it comes to this sort of stuff.

Thanks.
 
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  #2  
Old 01-15-13, 10:38 AM
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Welcome to the forums.

As long as you're willing to dig, I like your solution. The standard answer is diverting the water away from the foundation with grading, gutters and downspout extensions.
 

Last edited by stickshift; 01-15-13 at 10:50 AM. Reason: typo
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Old 01-15-13, 10:47 AM
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Thanks for the reply. It isn't too difficult digging out this foundation, as the 'basement' is really about half above ground. I only need to go down about 4 feet.

Should I do anything beside paint on the elastometric membrane? I know they have all sorts of plastics that people apply to the walls that further protect the wall and make a clear path for the water to run down to a french drain (which I don't have). Not sure if I should be looking into any of that or basically just stick to coating the wall several times with essentially liquid rubber.

Re-grading the surface once I'm done with the foundation tiself is a given. I definitely need to do that and will do that.
 
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Old 01-15-13, 10:53 AM
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Also. one other question. The elastomeric products need to be applied in warm weather so I would have to wait until the spring to use them. Is there any hazard in leaving the foundation exposed a few months until I can complete the process?
 
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Old 01-15-13, 01:51 PM
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leaving the foundation exposed is not a big deal for the concrete but it is not a good idea for the foundation due to potential freeze-thaw issues. If the ground under the footing freezes you will potenitally heave the building which could lead to multiple issues. (cracked foundation walls, binding windows & doors, drywall cracks, roof leaks, etc. )

There is a good chance none of this would happen but I would not take the risk. i would protect the footing to avoid freezing issues.
 
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