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Old 07-08-2008, 11:48 AM   #1
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Pouring a slab in the crawl


Hi all,

I've been experiencing drainage / water issues in my crawl space. After a heavy enough rain (2-3 days straight) the crawl space will flood leaving about an inch or two of standing water in the low spots. Last winter I had a structural engineer take a look at the problem and he had two suggestions.

First, was to pull the grade back to direct drainage away from the home. This was pretty obvious as the grade sloped right into the back of the house where the crawl was flooding. I had this done.

The second suggestion he made was to pour a slab in my crawl space. He said that the level of the dirt in relation to the footing was too low.


|
| stem wall
|
-----
|
| footing
|
---------- dirt
He said first that this is a structural issue and that over time will cause problems and second that with a slab the water will not have a place to go -- i.e... it won't drain into the crawl.

I'm hoping to get some thoughts on this before I get into it. Does this sound like an appropriate action to take? Is it something I should even attempt myself (I have no experience with concrete)? Should a vapor barrier go down first? Any other thoughts or suggestions?

Many thanks

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Old 07-08-2008, 12:08 PM   #2
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Pouring a slab in the crawl


Well first I'll say you probably should heed the advice you got from a professional engineer that you paid for his expertise over anything anyone says on here for free.

I don't disagree with anything he said though depending on the circumstances the slab may or may not block water intrusion a whole lot. I'm assuming that he said you need a 4" slab on top of the footing so you'll end up with the new floor 4" above the current joint between the stemwall and footing. The water reduction will depend a lot on how it's coming in - but the engineer was there and saw it so I presume he's right.

Definitely put a vapor barrier under the slab. You'll probably want to put in 3/4" crushed rock up to the level of the top of the footing and then the vapor barrier, and then the slab. Will you have enough crawl space height after you do that?

Did he recommend a good way to pour the slab? I would think putting a slab in an existing crawlspace would be quite difficult unless you have a lot of crawlspace height (like 5 feet). Line pump I guess but screeding off will be difficult. At least you won't need to do any excavating under there. If you have no concrete experience I would definitely find a pro for this.

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Old 07-08-2008, 07:19 PM   #3
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Pouring a slab in the crawl


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Originally Posted by jogr View Post
Well first I'll say you probably should heed the advice you got from a professional engineer that you paid for his expertise over anything anyone says on here for free.
Agreed. Although, it wasn't an engineer I hired to come and inspect and give me a spec to fix the issues. It was a friend of a friend type of deal. That said, the guy that came is very well known for being a good, by-the-book, engineer.

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I don't disagree with anything he said though depending on the circumstances the slab may or may not block water intrusion a whole lot. I'm assuming that he said you need a 4" slab on top of the footing so you'll end up with the new floor 4" above the current joint between the stemwall and footing. The water reduction will depend a lot on how it's coming in - but the engineer was there and saw it so I presume he's right.
This is where things get fuzzy. I didn't actually get a chance for him to explain it all to me. I had to head off to work. I've now got second hand info from my wife and am trying to make sense of it. I probably should just pay the guy to come out again and have a chat but his rate is upwards of ridiculous (although probably worth it).

Anyway, I do know he said to use flow fill. Not sure what the advantage there is.

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Originally Posted by jogr View Post
Definitely put a vapor barrier under the slab. You'll probably want to put in 3/4" crushed rock up to the level of the top of the footing and then the vapor barrier, and then the slab. Will you have enough crawl space height after you do that?
It would be tight but, I think there's room. The only area I'm not sure about is where the furnace is. I'm a bit confused though. I imagine that if the slab sits on top of the footing, as you describe and as I've seen it in renderings, it wouldn't do a thing for the water issue. I'm almost positive the water is coming up underneath the foundation. If this is the case the water would still get in and pool up in the gravel (at least I think). Maybe that's ok (?) but I can't imagine it's ideal.

The only way I could see it blocking water entry is if it is poured pretty much directly onto the dirt. I don't know if that's crazy talk or not -- again, no experience here.

Quote:
Originally Posted by jogr View Post
Did he recommend a good way to pour the slab? I would think putting a slab in an existing crawlspace would be quite difficult unless you have a lot of crawlspace height (like 5 feet). Line pump I guess but screeding off will be difficult. At least you won't need to do any excavating under there. If you have no concrete experience I would definitely find a pro for this.
No recommendations for how it should be done. I assume it would be pumped in. Actually, I think it would have to be. It's either that or mixing it down there which I imagine would be an immeasurable pain the *.

Since I know it's going to come up.... There is a french drain. It's obviously worthless and could even, as far as I know, be a part of the problem. The engineer said that french drains are usually worthless because they are so often done incorrectly. He said we wouldn't need a french drain with the proper grade and concrete work and we should leave it alone.

Maybe that's crazy talk, I don't know.
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Old 07-08-2008, 10:07 PM   #4
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Pouring a slab in the crawl


What is the height of the crawl space now?? From what I see with your drawing and from what the engineer has stated, Is that your crawl space dirt floor is either even with or close to being even with the bottom of the footing. over time if the water still keeps coming in from under the footing. the wall and footing will sink or wash in. you need the slab to lock the wall and footing in place. I hope this helps BOB.

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Old 07-08-2008, 11:31 PM   #5
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Pouring a slab in the crawl


I (compliments of the wife) found some scribbles and notes from the engineer and after seeing these and talking to a concrete guy things are starting to make sense.

I'm not entirely sure what, if any, preparation needs to be done but according to the sketches the flow fill goes on top of the dirt and its depth raises the floor of the crawl space to about half way up the footer. This explains how it blocks the water, eliminating the path of least resistance.

The concrete contractor I talked to described flow fill as wearing down over time and getting "crumbly". After putting the pieces together it made sense that this would be a suitable material. The space should have been filled with compacted soils in the first place. The flow fill is now (or will be) providing that functionality.

At least that's what I've formulated in my head. Sound logical?
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Old 07-09-2008, 09:20 AM   #6
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Pouring a slab in the crawl


the 1st worry's water intrusion & best approached outside,,, anything after that's water management,,, water runs downhill & can be pumped - that's why we have hydrostatic pressure,,, i don't see how placing conc 1/2way UP a footer'll cause wtr to change its course permanently BUT having eyes on the problem's sure as hell lots better'n reading a post,,, common sense values don't always solve wtr issues.

lean cement fill's a decent mtl normally used for trench filling where the possibility exists it'll again have to be dug up, needs no backfill placed in lifts w/rqd compaction, faster, easier, cheaper,,, it needs to be well-jointed in your application to prevent random cracking leading to 'crumbling' (spalling),,, placing it over uncompacted soil won't mean anything other'n you've got lo-strength lcf over uncompacted soil & its cost a helluva lot to do it.

' functionality ' OR ' function ' ??? i didn't stay at a holiday inn last nite but did do this work for 8yrs in the n e

i didn

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