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MichaelTP 04-25-2011 08:31 AM

How to create angled cement floor in crawlspace
My house is in an area with a very high water table, and my crawlspace (cement floor) regularly has water in it. Sealing cracks in the floor, running french drains etc are excercises in futility where we are, so life is about sump pumps and a dehumidifer.

I'm interested in reducing the amount of wet floor I have down there...I want to tilt the floor slightly so all the water runs down to one side (from where it will then find its way to the sump pit.) I'm trying to reduce standing pools of water to reduce the need to dehumidify. I'm thinking of putting down cement (imagine a square 10 feet across) that is elevated an inch or two at one side. How do I do that? I am a cement/concrete newbie.

All suggestions and thoughts appreciated, thanks!

CplDevilDog 04-25-2011 09:06 AM

Is there currently a vapor barrier down such as 6 mil poly plastic or other?

Cement might be overkill, if you just slope the floor to one side, add a couple of drains and a sump. Throw plastic over it all.

Sorry, I re-read your post. Looks like there is already cement down. Is there a barrier under it? Could pour resurfacing concrete on top of what's there but if no barrier was down originally, it's still going to let humidity through.

MichaelTP 04-25-2011 09:55 AM

I don't know what's under the cement floor, but it lets water through so humidity is certainly also there. What I hope to achieve with an angled floor is to eliminate a ten square foot pool of water that is surely a major source of humidity down there...instead the water, once through the floor, would run in a much narrower space to the sump pumps.

My concern (having never worked with concrete) is that when it's wet it will level itself. I need to create an incline.

CplDevilDog 04-26-2011 07:47 AM

If the original slab is bleeding water and moisture through it or cracks in it, any concrete poured on top is going to fail.

If you're going to pour concrete anyway, I would remove the original and start from scratch.

MichaelTP 04-26-2011 08:25 AM


Originally Posted by CplDevilDog (Post 636788)
If the original slab is bleeding water and moisture through it or cracks in it, any concrete poured on top is going to fail.

I'm not concerned with it letting water in (as you said, it's going to...this is a fact of life where I am) What I'm going for is reducing the size of the puddle. With the floor as it is now I have large sections that stay wet with half an inch of water on them until the water doesn't run to the sump pit. With an angled floor I intend to direct the water to the pump so it's in the crawl space for minutes instead of days. So my question is: How do I lay down cement so that one side is 3" higher than the other side (ten feet away)?

CplDevilDog 04-26-2011 08:45 AM

Clean the concrete surface and apply a bonding agent to allow the new concrete to adhere.

Stack (2) 2x6 3" high for the "high side". Rip OSB or plywood from 3" down to nothing for the sides of the form. After some setting has occurred, remove the side boards and feather sides out with extra.

Use a sand mix with very fine aggregates. Keep your mix on the dry side and it will stay where you put it. Use a screed board to "pull" the mix across your side boards.

In two years you can use a coal shovel to remove the broken pieces and assorted slurry. One year if you live in the North. :thumbup:

CplDevilDog 04-26-2011 08:49 AM

Sorry, didn't mean to come off so harsh, but it is a lot of work and effort that will not last long.

MichaelTP 04-26-2011 09:24 AM

I appreciate the humor and the information!:)

Got any alternative suggestions? Water coming through the floor is a fact...the hydrostatic pressure is enormous when the water table rises. I think having a stream in the crawlspace would mean less humidity than having a lake.

The location of the entry points for water are known. Could I glue guides (any raised waterproof material a couple of inches high) to the floor to channel the water to the sump pits?

CplDevilDog 04-26-2011 09:54 AM

Of course there are several things to consider.

1) Budget
2) Timeline
3) Working space
4) Access

If it were my place, I would

1. Drill a test hole in a the slab to see if there is a barrier present.
2. If no barrier and the budget is there, I would seriously consider removing the floor. If you are not using the area for storage, etc. I would not replace it. I would put in de-watering system, vapor barrier
3. If the budget is not there, could you put a sump pit at the center of the low spot? Run the discharge across the floor to the low side of the foundation.

Maybe even consider cutting in french drains through the existing slab and routing them to the sump. Cover with vapor barrier and pea gravel.

tpolk 04-26-2011 10:22 AM

if the slab is thick enough you could cut channels for the water to follow. if possible you need to get a laser level to see real slab elevations

MichaelTP 04-26-2011 12:42 PM


Originally Posted by tpolk (Post 636863)
if the slab is thick enough you could cut channels for the water to follow. if possible you need to get a laser level to see real slab elevations

Why didn't I think of that? Good suggestions (will an angle grinder with the right wheel be sufficient to cut a half inch channel?)

Red Squirrel 04-26-2011 04:59 PM

If you have that much water I would find out the actual spot it's coming out of (small hole in floor, or from wall?) and mitigate it from that point. Even if you were to slope it, that part of the floor would keep getting wet, and so would any items on there such as cardboard boxes and such. A source of mold right there.

MichaelTP 04-26-2011 05:16 PM

Red Squirrel, this is a crawlspace in a high water table area. Nothing is stored down there. Water is entering from 4-5 different points, and will not be stopped when the water table is high. My goal is to route the water to the sump pumps as effectively as possible.

CplDevilDog 04-27-2011 07:49 AM

Unless the water table is a pond, I still think you will have better luck with addressing the source.

Google search de-watering techniques or "lowering water table".

Depending on which solutions you choose, you may find the work is comparable or easier than just constantly fighting the symptoms. Better long-term results as well.

CplDevilDog 04-27-2011 07:51 AM

Here's the first result I found. I didn't bother to read it but it is a good example of how easy you can find good resources.

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