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.
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..
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
Good luck with your big project!
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
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.
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.
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.
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.
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
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
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
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:
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
Keep the whole world singing . . .
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