# Should I seal the perimeter gap of my floating slab?



## toddmanqa

I have a two-story colonial with a full-basement in upstate NY. The basement has cinderblock walls and the basement floor is a poured concrete slab. The basement walls also have some sort of stucco/cement parging applied.

I have a 1/2" to 1" gap between the basement walls and the basement floor. 

Based on the question asked in this article:
http://www.stopmold.com/don_t_finish_your_basement.php

I'm fairly certain I have a floating slab vs. an interior French drain. I have a sump pit--no sump pump, but the sewer clean-outs and incoming water line are in the pit too. The previous owner had run the AC condensate into the pit too, but I've hooked the AC up to a condensate pump, and then to the outside.

In the three years that we've lived in the house, we've had a persistent mold odor in the basement. I've tried dehumidifiers to knock the moisture level down, and I've got it down the 20% range, but the odor remains.

A few months ago, I tried an interesting experiment. I went to Home Depot and got a garden sprayer and two gallons of silver-based anti-fungicide. I sprayed the antifungal into the perimeter and the results were amazing. For a few hours, the air was completely odor-free.

I've never had any water in the sump pit or flooding in the basement. 

I'm considering sealing the perimeter gap to stop the mold from entering the basement. 

Does anyone have any thoughts or experience with this?


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## jklingel

I see no advantage to a large crack, and I would sure seal it. I'd pour a healthy dose of 10% bleach down the crack, let it sit several days to dry, then seal it. You may have no vapor barrier under your slab, which would then be wicking moisture. THAT is a big problem. Seal the crack first, and see what goes.


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## Stillwerkin

Concrete is porous and allows moisture through it. Mabye some paint will slow this down.


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## concretemasonry

You undoubtedly have a floating slab.

The gap dimensions you mentioned are far to great to be due to the concrete shrinkage. Does the gap go the full depth of the slab thickness? Does is appear to have clean edges and is it formed?

At one time in much of the east, the gap was formed and even used as a sales gimmick to collect water that had already leaked into the basement envelope and collect it to keep the floor dry. Often it was directed to a sump or a floor drain. It was really ineffective and a poor substitute for building a dry basement, even though the the extra cost during construction to build a dry basement was extremely low. Some codes now require the fllor slab to have the 3 5/8" of the slab cast against the wall.

Filling the joint with a cement based material will result is a small crack later on that may not be a problem. If you feel you must fill the formed void, get a foam backer (rod foam in various diameters) and force it into the void so that the top of the rod is 1/2 the crack width below the slab surface. Then caulk with a quality flexible caulk designed for use in concrete control joints.

Dick


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## stadry

dick's right not that he needs any validation from me :no: it was called a ' slot drain ' & promoted by many bldrs ( correctly or not ) as dick posted,,, in theory, it worked fine til theory ran into practice :laughing: it filled up w/dirt, etc, & became a breeding place for insects, mold, mildew, etc.

i'd just stuff closed cell backer rod into it but forget the caulk unless appearance is a concern,,, apron stores won't necessarily have the diameter rod you need ( 25% larger than the space ) so find a const supply house.

filling that space w/cementitious mtl will result in a smaller shrinkage crk & many other crks as the filler cures - not my choice, either.


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## Dera_Mik

I just came across this post; I have the samething (1" wide and 4" deep/ht of slab - a Floor/Wall joint - they call it cove joint) in my basement that builder put it. It's allover the whole perimeter. 

I was going to finish my basement but by-chance I found that there was some dampness (spread across 12-14") at the base of joint - rest of the joints all around the perimeter of my basement is dry. I am not sure if it should be concern as it's only little (no standing water) bit of dampness at the bottom of the joint/gap - not touching any wood or framing but just concrete around it; 
Question - Is it just humidity in that corner (~1 foot) or water seeping through under the slab (possible high water table there). Can I just go ahead put drywall on the frame (already there - 4" away from basement wall) without worrying about any condensation in the particular small-area or should I pour some liquid hydraulic concrete of small layer (0.5 cm) to seal any small holes/porus or air gaps to joint to clay under the slab. I could put foam-backer and seal with caulking but will it stay in place and wouldn't water vapours/condensation will stop wicking any water from underneath the slab. I was reading about Xypex high’n dry concrete waterproofing crystallization product?? Any comments will be appreciated. Thanks.


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## Dera_Mik

Any advise to above questions are much appreciated in advance. Thanks


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## jmborsheim

toddmanqa said:


> I have a two-story colonial with a full-basement in upstate NY. The basement has cinderblock walls and the basement floor is a poured concrete slab. The basement walls also have some sort of stucco/cement parging applied.
> 
> I have a 1/2" to 1" gap between the basement walls and the basement floor.
> 
> Based on the question asked in this article:
> http://www.stopmold.com/don_t_finish_your_basement.php
> 
> I'm fairly certain I have a floating slab vs. an interior French drain. I have a sump pit--no sump pump, but the sewer clean-outs and incoming water line are in the pit too. The previous owner had run the AC condensate into the pit too, but I've hooked the AC up to a condensate pump, and then to the outside.
> 
> In the three years that we've lived in the house, we've had a persistent mold odor in the basement. I've tried dehumidifiers to knock the moisture level down, and I've got it down the 20% range, but the odor remains.
> 
> A few months ago, I tried an interesting experiment. I went to Home Depot and got a garden sprayer and two gallons of silver-based anti-fungicide. I sprayed the antifungal into the perimeter and the results were amazing. For a few hours, the air was completely odor-free.
> 
> I've never had any water in the sump pit or flooding in the basement.
> 
> I'm considering sealing the perimeter gap to stop the mold from entering the basement.
> 
> Does anyone have any thoughts or experience with this?


Hello, I know that you posted this awhile ago, but I am having a similar situation and need some help. Our basement floor is a floating slab. We also have a sump with a pump. Never have had any problems with visable moisture or flooding, but we just finished our basement. Installed new carpet and put up new baseboards. It went through the winter and spring last year and now we have a 1 to 2 inch gap between the bottom of the baseboard and the carpet. Also have that musty smell. Ahh.... Any suggestions on what we should do it now to fix it?


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## stadry

who is ' you ' ? :huh: zypex may well work

IF its a builder-installed sump system, chances are good its not filtered w/soil cloth,,, if i'm reading the ' gap ' thing right, could shrinkage be from green wood ? the only ' fix ' i know is rip it out & do it right but you probably already knew that :furious:


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## justincase123

It is common practice to leave a space between a floating slab and a block wall, Here (ky) it is called an expansion joint and concrete materials sells a mesh type expansion joint product especially for that purpose. It is spray glued to the block wall before the floor is poured and then the floor is poured to it. The purpose of the expansion joint is to allow a space between the concrete floor and block wall so that when the concrete floor expands and contracts, as concrete does, it does not expand and push your block off your footer. Over time the mesh will rot away and just leave the 1/2" or so gap and people are confused about why it is there.


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## concretemasonry

guest -

A concrete slab on grade does not expand, but has a long term shrinkage that essentially stops after a couple of months. In some situations, there MIGHT be a very small short term thermal expansion IF the slab and soil got heated to over 150F or so.

Some codes even require the slab to be poured with a 3-1/2" contact height with the floor to give additional support to the wall to resist the lateral soil loads in a basement. It is a method to use the slab as a diaphragm. Normally when the slab is poured against the basement wall there gets to be a 1/16" to 1/8" gap due to the initial curing shrinkage, but still can provide lateral resistance if there is any movement. A lazy contractor using a wet mix will be on the high end of that range and good, low slump concrete will have less shrinkage.

Dick


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## stadry

guest, this is a great answer & definition as many of us define 'expansion joint' differently,,, in 39yrs, i've never seen any bsmt slab 'expand and push your block off your footer.' conc does expand/contract in direct relation to temp changes( heat expands conc & cold contracts it [remember high school physics ?] however, most pro's agree expansion jnts aren't used in bsmts where temps remain fairly constant - its more likely a bsmt floor would contain contraction jnts while expansion jnts are to separate slabs from structures,,, then again, i've never placed conc in ky,,, the ONLY time we've seen cont expand to do any damage is on conc hgwys w/100' slabs - a method never used anymore :laughing:


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## justincase123

They have been doing it that way in ky for decades.


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## concretemasonry

KY is just one place and not necessarily applicable in other situations or places.

Dick


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## justincase123

True, we get all kinds of weather here and never know what we will get. I just know they do this here and I had to clean mine out with a putty knife and shop vac to get rid of the rotting expansion material which was feeding the mold. The expansion material had served its purpose to create the space between the floor and wall, after that it starts to rot over time and feeds the mold, I guess dirt and debri in the gap will also feed mold given the dampness of basements.


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## toddmanqa

*Update from original poster*

Using custom-ordered 2" backer rod and a couple of hundred dollars worth of crack sealer, I was able to seal the entire gap on the perimeter. As expected, the mold smell went away for the most part. 

One unexpected result was that I was actually getting some water off the walls, and which had been going into the gap. With the gap filled, a small amount of water does run onto the basement floor. Fortunately, the dehumidifiers were able to keep up with the tiny bit of water coming in.

Since the mold smell didn't go away completely, and we want to finish the basement, we will actually be taking out the concrete floor, digging down a foot, then putting a new slab in. The expectation is that a new slab with a vapor barrier and a properly draining aggregate will get rid of any remaining moisture problems (and hopefully mold smell).

Time will tell.


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## serfin control

*oh boy*

I know this is an old thread but I too have a floating slab. my basement flooded twice this year and now I'm ripping up the walls only to find 1x6s stuffed into the gaps. the wood above the gap is OK but the wood under the gap is completely dry rotted out. in some places the wood above the gap is so tightly compressed in the gap the only way to remove the dry rot underneath is to chisel up the foundation. not a great plan. it's a total nightmare because I can't find a way to remove the wood and my entire basement smells like a dry rotted piece of wood in a forest. It's labor intensive and time consuming to get it out of the gap. I've tried as much as I can. maybe a professional can help but that's thousands of dollars in labor to pick at the stuff by hand.


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## serfin control

McHugh2001 I don't have enough posts (15) to be able to PM you so I have to post my reply here, hopefully you see it!



mchugh2001 said:


> Hi- saw your post about the floating slab. I am now discovering this myself (Detroit area, 1939 house). I have all of the rotten wood removed from the joints (including interior walls) and am trying to decide whether I should fill them in, leave them empty, or install interior French drain. My current favorite option is do nothing and install Dricore subfloor. What solution did you end up with and do you have any regrets? Thanks!


I ended up removing as much as the rotted wood as I could by hand and vaccuum. What wood I couldn't remove, which wasn't that much, I treated with a liberal solution of anti-freeze and borax mixed together, which is supposed to kill any living organism. Then after it dried out, we poured, if I remember correctly, liberal amounts of latex over it to completely encase it. 

AS for what to do with the gaps in the floating slab, we basically left it alone. My basement is only half-grade and it never had a mositure or water problem, so I wasn't going to install french drains to bring more water in the house! especially in teh case of a power failure and my sump stopped. 

however, we ended nailing into the concrete slab treated 2x4's where the floor met the slab, essentially covering up the gap in the floating slab. To do that, my brother-in-law/GC removed the outer perimeter of nearly perfect 60 year old armstrong 12" VCT tiles and then placed them with newer, different colored tiles for a nice contrast. We insulted, put up fancier pre-finished brich plywood as paneling and then put larger baseboard to cover up whatever slab gap was still present. If the basement ever flooded again, I would just remove the baseboard, and let it all dry out. I purposely chose not to put drywall just in case for any reason it ever flooded again. The paneling would just dry out and the treated wood would not rot. 

As for the dri-core basement, my philosophy has been that I don't want to create water problems if there wasn't one. Since my basement was totally dry, and it was already covered up by perfectly good vct tile, I didn't want to spend more money and potentially create water problems that didn't exist before. 

Do I regret my decision to leave the VCT tiles installed over concrete? No and Yes. No because at the time I had just spend $17k to retrofit overhead sewers and then another $20k or so to *completely* gut the basement. I was looking to save money wherever I could and saving labor/supplies to rip up a perfectly good floor seemed like a waste.

But yes, I do regret it because the floor is cold in the winter. If I had to do it again I probably would have ripped up the VCT tiles, put in a water barrier and insulation, and then put in nicer engineered floor.

However, I installed tile in the basement bathroom, and for my little office in the basement, I had to tear out the tiles because they were 9" abestos tiles. I installed these way too expensive Karndean designflooring tiles because they are waterproof and removable. I like the tiles but they don't stay in place very well so they always move around and as my BIL says, look like crap if they're not flush. Someday I'll put in some other waterproof engineered flooring.

as for water issues, I've had none so far, there seems to be no issues with the floating slab to date based on what I've done. I still don't have any moisture problems, it's actuallly realy dry down here.


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