Concrete floor over dirt basement?


Just bought an 1890 two-story brick house with a stone foundation. The basement is dirt right now, and I'd like to pour a concrete slab down there. There's sufficient cieling height for a 4" slab without excavating, which I'd definitely prefer not to do. Do I need a vapor barrier? Can I just pour right over the dirt? I'm just starting to do my research, so any thoughts or info would be appreciated.
JP
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Much depends on local conditions. The ideal is 4 to 6" of washed gravel between the footings (the house does have footings doesn't it?) topped with a vapor barrier then 4"+ of concrete. If there is ground water that makes the dirt wet, even if it is only seasonal, the washed gravel helps prevent it from wicking into the concrete along with the vapor barrier resulting in a dryer basement. If you have moisture down there you may want to install drainage pipes in the gravel to collect the water and channel it to a sump pump, as a house that old will not have any exterior drainage around the outside of the footings. Do it right you never will be sorry, do a cheap inadequate job and you may have to do it over correctly to be happy..
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The ideal would be crushed stone, a vapor barrier, and insulation under the slab, but I can understand why you'd want to avoid the effort involved in doing that.
For one thing, a house that old may not have the greatest footers. You certainly don't want to undermine them excavating for the slab.
If' it's bone dry down there all the time you can probably get away with putting down a good vapor barrier and pouring on top. But I'd consult local experts familiar with the area and conditions.
One thing to keep in mind. Wet concrete throws off a *lot* of water vapor while it is setting and drying out. It has no place to go but up. In an unfinished house it's not a problem, but you're going to want to provide positive ventilation to get rid of all the moisture or you'll be asking for mold issues.
Have you checked for radon? Now would be the time to plan for remediation if you have a radon problem, and that will almost certainly required gravel under the slab so the space can be depressurized and exhausted externally. That's usually the first step in remediation.
Good luck with your big project!
Paul F.
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JP
Before you pour, or have poured, your new floor consider installing an interior ground ring for your electrical system. While you are putting in your drainage system you dig the trench around the periphery a foot deeper and put a bare number two copper conductor around the edge. You make sure that it is long enough to come up out of the pour through a nonmetallic conduit at the point immediately beneath the service disconnecting means / customer service unit of your electrical system. It needs to be long enough to go all the way to the neutral buss bar of the service equipment. Were the end overlaps the tail to the service equipment you install a split bolt connector to bond the two runs of the ground ring to itself. You then back fill the trench to the installation depth of your drainage piping. Connect that ground ring to the same buss bar were the neutral conductor from the utility's transformer is terminated. This will provide your electrical system with a far better grounding electrode system than anything else you are likely to have on the site unless you are served by an entirely metallic underground water utility supply that is free of plastic pipe or exterior coatings. Such a grounding electrode system will make your homes electrical system much more resistant to surge and spike damage. It is especially worth doing if your electrical system has only driven rod electrodes. If you install a surge arrester a good earth ground will make it much more effective. It will never be as easy to install a good grounding electrode system as it is during this work. It's your home so it may be worth doing it right while it is easy to do. -- Tom Horne, Electrician
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You really ought to have some gravel and a vapor barrier. But exactly what you need to do and perhaps can get away without depends on the historical conditions in the basement. If you're in arizona and the basement ground has been desicated hard earth for the past 100 years you can just pour some concrete on top. If you're in the SE and the basement soil is still suitable for crops if you added some lights then you need the gravel and vapor barrier.
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Thanks for all the replies. I'm continuing to research and will likely post back with more questions. Given that this is a real fixer-upper I'm definitely going to be hanging out here a bit I'm sure.
JP
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Regarding the use of expansion joints in the basement, I priced out the "actual" stuff at around 40 cents a lineal foot for the 1/2"x4" stuff. Then I got to thinking that Celotex is only $8.40 per SHEET which will get me 88 feet, so only about a dime a foot. My question is, will this do the same job as the real stuff? Thanks. JP
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May be OK indoors, but outdoors it would deteriorate fast from the weather. Anything that allows movement will work. Indoors, there is little thermal variation over the course of a year so anything may do the job, including nothing. Variation indoors may be 30 degrees over the year while outdoors it can be 100+ along with freezing water in joints.
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What I'm planning to do is to excavate down around the perimeter of the basement a few inches and put in a drain pipe ("tile"...why the heck do they still call it that?) that leads to a sump pit I'm going to dig. Then I'm going to put down a few inches of washed concrete and tamp and level it. On top of that I'm going to put a double layer of 6 mil poly with overlapping, taped seams. This I will run up the wall 6" or so and temporarily tape it. Then I'm going to run a perimeter of 4" expansion joint material against the wall. Not sure how I'm going to hold that in place - maybe tape it to the poly in a few spots? Ooops! - forgot about my new interior ground ring for my electric system. That goes first. So now I think I'm ready to pour my slab, right? Rather than mess with reinfocing steel mesh I'm thinking of paying the extra 8 bucks a yard for the fiber reinforcement material. I've got a couple guys to help that have done a lot of concrete work, and I'm going to rent a power trowel. Am I missing anything?
JP
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2" of insulation under the slab if you are in a cold climate.
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replying to Ed Pawlowski, Tim NS wrote:

Does the insulation go on top of the poly or under the poly?
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On Mon, 04 Jan 2016 14:44:01 +0000, Tim NS

Generaly poly on the warm side - which would be on top
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replying to clare , Tim NS wrote:

That's what I thought but wasn't sure because of the moisture issue
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.
Insulating the floor is a good idea if it gets cold in winter and heat bills are big, where I am Zone 5 it gets to -20f and the dirt at 5 ft down is 50f most of the year. I dug out a basement, put in a drain tile system, 2" polyisocyanurate foam board and ran radiant tubing for any future heat system. Heat rises and having a warm floor is great if you might use the basement as living area. The foam keeps it warmer in winter even without the radiant tubes hooked up yet. The surrounding dirt is cooling you all winter. costing you money to heat it so insulation does help. Have a center drain pitched to the sump pump. the sump must go outside not in city drain in my area. A sink or condensate or washer can go into the city drain. I dont think an expansion joint is necessary, All the high quality 80-90 yr old houses I see dont have any and are still sealed tight and dont leak, a joint just is an opening for water to come in, we just had 7-8.5" rain friday, I didnt leak anywhere, my neighbors didnt leak, concrete is to wall with no joint. A vapor barrier is a good idea ad added protection even with foam board. I have 2 pits and pumps, one for the tile system goes outside, and the sink, furnace and dehumidifier condensate and washer go to the city drain. A ground system someone mentioned sounds interesting.
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...
That's probably a good idea, but now we're talking some excavation too. I'm gonna have to think about it. It's not really going to be used much except for storage and the clothes washer/dryer. Thanks for the info.
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Tar impregnated sheathing ( black Celotex) will be just fine for expansion joint. Many places sell it pre cut.
As to your project:
It sounds like you do not have a floor now.
I'm not sure what you mean by washed concrete, unless you mean wash out. Wash out can be very difficult to work with if you do not have machinery. Perhaps you meant washed gravel? Any coarse, anti capillary stone will do - 6" preferred.
You would be much better off using one layer of 10 or 15 mil underslab vapor barrier. Here is one brand from W R Meadows: http://www.wrmeadows.com/wrm00068.htm called Perminator that I use. Here's a movie:
http://www.youtube.com/watch?v=NPQHxemJyy0
watch out for that red tape - stickiest stuff I know of.
It would be normal to nail the expansion joint to the concrete walls with a PAT
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DanG
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Me neither - I meant washed gravel. I'm reading "Renovating Old Houses: Bringing New Life to Vintage Homes" and that's what was recommended under the concrete and vapor barrier.
JP
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