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Need guidance on sealing exterior walls from moisture intrusion

Posted by vikingshelmut (My Page) on
Mon, Oct 18, 10 at 12:52

I need your help defining the next step in getting a moisture issue resolved on our first floor. Our house has a slab foundation, and when we bought the house we noticed a some signs of moisture intrusion around the exterior walls (stained carpet along walls, light mold on baseboard in limited areas). I called in a enviro guy who looked in the walls, etc and couldn't find any active mold infestations, but every year when it starts to rain we get a slight musty smell downstairs.

I believe part of the problem is that someone poured a solid concrete slab patio essentially all the way round the house, and it butts up against the exterior stucco right at the base of each wall. It seems to me that this pushes water that runs down the exterior walls right into the very very small gap between the exterior and the slab.

I plan on having someone re-do our backyard, and as part of that have them remove the slab, but I'd like to make sure there isn't anything else I should be doing.

For example:
1) Thinking of having a french drain put in around the house to facilitate drainage.
2) Since the exterior is stucco (50 year old stucco) should we be looking into having the stucco "sealed" or replaced with something more water resistant?
3) Once the exterior concrete is removed around the perimeter is there some sort of flashing one could install around the house slab to prevent any water from seeping under the wall's base plates?

Any other ideas? If you had to do this, who would you hire? I'd guess the landscaper could take care of the exterior concrete patio, but I doubt he would be able to do any work to seal the walls, so I'm confused about who I should be talking to to get this issue 100% resolved.


Follow-Up Postings:

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RE: Need guidance on sealing exterior walls from moisture intrusi

Where is the roof run off going. You make no mention of gutters or down spouts. Adding these or repairing what is there is an easy and cheap fix that I would first look at. Make sure the down spouts are shedding water away from the house.


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RE: Need guidance on sealing exterior walls from moisture intrusi

Good point, I forgot to mention that part. My down spouts all run onto the concrete patio, which itself does not drain correctly in some spots (one of the many reasons why I want to take it all out).

I do think that part of the problem is that the down spouts are pooling water on the concrete, but once we tie them into some managed draining system I wanted to be sure that there wasn't something else we should do to protect the wall bases from moisture intrusion, or who to talk to about that.


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RE: Need guidance on sealing exterior walls from moisture intrusi

This is a slab on grade?

That is when you enter the home all you can do is walk in the lower floor and walk upstairs?

If that is the case then fixing your gutters and rain leaders to drain away from the home and tearing the concrete out and grading the landscape so it runs down and away from the slab will fix that.

That said even a slab on grade home should have perimeter drainage, especially if it rains heavily at times in your area. Installing a perimeter drain on a slab foundation is a piece of cake, almost all labour. You could easily do that yourself or supervise some college kids.


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RE: Need guidance on sealing exterior walls from moisture intrusi

I'm guessing that it's slab on grade, but I don't honestly know anything more than it's just a slab. There is no crawlspace, etc. When you say perimeter drainage, do you mean something like a french drain where you dig out a trench around the perimeter, install a perforated pipe and back fill it with gravel? I just want to make sure I understand your suggestion.


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RE: Need guidance on sealing exterior walls from moisture intrusi

Keeping things simple. The patio should be sloped away from the house properly. It probably isn't. There should be flashing between the patio concrete and the house concrete (slab). Probably not. The yard should slope properly away from the house/patio combination - it may or may not.

Regardless of what you do, water will find a way through (anything)...the solution, as mentioned above, is to remove the water. It may very well be that a proper gutter system, with downspouts leading far away enough from the house to drain away (say 10 feet or more) and a bit of grading will do the job for you.

Take a really good look around with this in mind; think like a rain drop and ask yourself where you would go. Put a level on the patio and see if it slopes away from the house (1/4" per foot).


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RE: Need guidance on sealing exterior walls from moisture intrusi

When you say perimeter drainage, do you mean something like a french drain where you dig out a trench around the perimeter, install a perforated pipe and back fill it with gravel? I just want to make sure I understand your suggestion.

Yes, although there is more detail than that. As in all things there are degrees of doing as well. Code, best practices for the area and absolute best can be pretty different.

Retrofit on a slab on grade is easy to do absolute best.

Dig your trench, line with geotextile fabric leaving enough fabric for the last step.

Place your bed of drain rock such that the drain pipe will follow the actual foundation all around. So your trench will have to be deep enough and wide enough to accomplish all this. You probably can't make it too wide but there are material constraints and you want to leave some yard, eh?

Fit your perforated pipe. Do not use the black, corrugated stuff, use the white perforated drain pipe.

Before you glue, figure out where all this will drain to. Out the front, side or back? It has to go away, not just in a circle.
Use sewer pipe if there is any chance of vehicle traffic running over it.

Sock the perforated pipe.

Lay, don't throw, drain rock over the pipe once all fittings have been glued up, do not dislodge or break the pipe.

Wrap the geotextile fabric from step one over the drain rock/pipe install.

Top the geotextile with pit run, then with a decorative mulch or a stone garden or anything but plantings.
Place foundation beds/plantings out from the home, leave the area in which your drainage runs clear.

As your plantings mature the void behind them goes away. Most people cram the foundation plantings way to close to the house anyway.

OK, so there are technical terms for all this but I've tried to keep it simple, for me, lol.

Kaib has it...think like a rain drop or in this case like ground water coming up and/or surface water percolating down. You want to make it easy for it to go somewhere else.

Its all downhill from there.

And, if I may say, a fun and satisfying project given slab foundation. I've done too many basement foundation walls and that entail a lot of digging.


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RE: Need guidance on sealing exterior walls from moisture intrusi

Thanks for all the great info! Sounds like once we clean out the old patio and set proper drainage we should be OK. Now I just need to find the right landscaper...


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Need guidance on sealing exterior walls from moisture intrusion

Hope this techniques will come along and many people hoe are suffering form this problem will get the relax. thanks for the great information i will apply this in my home.

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