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Old 04-26-2017, 09:51 PM   #1
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Help with low foundation height


Hello,

We bought a detached house 4 years ago. The previous owners built a two-story extension to the house (without a basement). Unfortunately the foundation height above grade line for the extension is extremely small, maybe something like 4'' on average, with places where it is even lower. I am afraid that the wood laid out on the foundations (walls, windows) may get wet and rot. There is a french drain system around the house, but the grading is not really well done. At some places the slope goes slightly towards the house. With a foundation that low I am afraid some water may get over the top and into the actual wall before it gets absorbed by the soil and drainage system. The wall is finished with brick but the masonery may not be 100% waterproof.

I should also mention that I live in Canada and we obviously get our share of snow.

I tried digging along the foundation maybe 1 foot deep and laying down some stones to prevent problems with humidity under one of the windows. I am wondering if I should do this all around the extension or if it is a waste of time or even harmful. Maybe lowering the soil and regrading to divert water to swanes or towards the street would be the real fix?

Thank you for your help,
Olivier
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Old 04-29-2017, 09:47 AM   #2
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Re: Help with low foundation height


I like your last sentence the best. Around here grade has to slope away from the building for a minimum of 3' horizontal. Instead of focusing so much on the wall, focus on the grade away from the wall so the water never pools in the first place, then see how it performs and take it from there.
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Old 05-01-2017, 12:53 PM   #3
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Re: Help with low foundation height


Thank you Mike for the reply. For now, I have the three following ideas:

1. Lower soil and regrade (your suggestion). One problem though is that the house sits on really flat land overall, so if I lower soil and grade away from house, I will not have much choice but to form a kind of swane system around the house which will accumulate water. Not really nice to walk into.

2. Remove soil around the perimeter of the extension and replace with rocks. I am aware this will create more water accumulation along the foundation, but the water will also be further away from the wall. I still prefer water in my foundations than in my wall, especially given the fact that there is no basement in this section.

3. A variation on fix #2, add PVC pipes to connect the drainage rocks to the French drain system.

While browsing through the papers of the house, I notice the landscaping was done 2 years after the drain. My best guess is that the drain guys probably knew their business and left proper grading and soil height around the house, but the landscapers screwed it up.

One difficulty I have with this issue is that it is hard to assess whether there is an actual problem or not. I'd need to open up the walls or windows to see if there's any humidity in there. Rain generally does not result in pooling of water around the walls, but who knows what happens once every 10 years when there is a major storm. It is also hard to assess what's going on under the snow cover.

Anyhow, I decided to have the house first checked by the company who did the French drain installation 10 years ago. They may be able to suggest something. Will post the result.

Thank you,
Olivier
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Old 05-31-2017, 09:24 AM   #4
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Re: Help with low foundation height


Well this thread failed to gather much attention but I will still post the updates to my problem. Hopefully somebody will jump in and comment.

So I had a bunch of people come at the house to look at the low foundation height around the extension and suggest fixes.

- One firm suggested to dig down the entire yard. We looked at the relative height of the curb vs the top of the foundation and it seems that it would be theoreticaly feasible to get the minimal 8'' clearance and still get a small positive slope to drain the water away to the street. However, this solution really requires to lower the grade everywhere. My concern is that there are three huge maple trees in the backyard and their root system will be significantly damaged by such operation. I am not sure they will recover. I would hate to lose these trees, not mentioning the risk of damage to the house if they die.

- One landscaper proposed to lower the grade locally around the extension (let's say over a distance of 3 ft) and build a retaining wall around this soil. First problem here is that I am not really fond of having a retaining wall in the middle of my backyard that adults or kids may trip over. Second issue is that the water on the low portion around the house has no escape. There is a French drain underneath but this is slow acting.

- Another landscaper proposed to lay 8'' high landscaping blocks along the perimeter of the extension at a couple of inches from the house. He would fill the space between the blocks and house with small rocks. This will allow to raise the soil (and a paved stone patio) by a couple of inches and improve the slope. He says it does not matter if the small filling rocks lie against the brick or aluminum wall above the foundation since they will be dry. He also does not seem to be overly concerned about water getting trapped between the blocks and the foundation. He says very little water will get in there and whatever is there will drain down to the French drain. It is true that there are roof overhangs with good gutter system above much of this space but still I am concerned especially in spring when the ground is still frozen and the snow melts.

Despite its issues this last option seems like the most sensible. My idea is to retain this suggestion but to add vertical pipes in the gap to drain the water down to the French drain system.

The landscaper was not fond of this suggestion and said it may overload the sump pump. Plus they are not technically allowed to perform this type of work. Note that I have two sump pumps, one draining to the city sewers and a backup pump pushing the water outside the building in case of failure of first pump or sewer overload. I feel this should give me enough pumping capacity for the largest rainfalls. The landscaper is not consistent with himself when he says that not much water will get in the gap but still he is afraid of overloading the pump.

What do you guys think? I can include pictures if it helps understanding the issue better.

Thanks,
Olivier
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