Drill holes in slab to drain garage flr low spots?

jeffw_00December 10, 2005

When they built my 24x36 garage, the slab didn't come out quite level, there are some low points that collect water (going after the builder is a non-option). Currently I spread rubber mats (the ones with holes in them) over the low spots, but I got to thinking - what about drilling a 3/16" or 1/4" hole at each of the low spots (there are about 4 of them) and letting the water seep down into the dirt. My wife was concerned that critters might come up the holes, but what lives under the middle of a slab?




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I drilled holes through the slab in a garage in a former home and it worked fine for the ten years we lived there and suspect it is still working. I never noticed any critters coming up.

I think it would be a bad idea to try to put lots of water through holes like this, but a small amount didn't seem to cause any problem.

    Bookmark   December 10, 2005 at 4:53PM
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Can you patch in new cement to level the slab? It will look funny, but gosh this is a garage, right? I'd be concerned about cracking the cement by drilling. You could always paint the whole (level) floor.

    Bookmark   December 10, 2005 at 6:04PM
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I would not be concerned about cracking the concrete by drilling. I know someone who that for the same reason, water. I know of no problems. It does depend on how much water you have. I think the person I knew who did it used a 1/2 hole. You could alway enlarge your hole if you found it not working.

    Bookmark   December 10, 2005 at 6:58PM
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We tried to level the slab - BAD IDEA - the low spots got slightly higher and each big puddle turned into 3 or 4 smaller ones around it.

Thanks everyone else -
sycamore guy -how big a hole did you use?

and do I need a 'concrete' bit - or will titanium do?

    Bookmark   December 11, 2005 at 12:57AM
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What's underneath the slab? What type of soil is the slab sitting on? Was it properly prepared with a sand/gravel subbase? Or is it just sitting on clay? How low in relation to the surrounding topography is the garage slab?

Drilling holes in the slab could cause more problems than it solves if you don't have proper lot grading or clay soil conditions. It will also be a direct route for termite infestation into your home if the slab wasn't properly prepared with the right treated sub base. Know the answers to these questions, as well as how thick the slab is, and you'll have your answer on whether or not to drill into it. A less expensive option might be one of the new garage mats sold that channel water away. They're designed for cars that just bring a little rain or snow into the garage. If you've got other water problems beyond a small puddle of residual rain or snow running off of the cars, then you have a BIG problem, and one that won't be solved by drilling holes.

    Bookmark   December 11, 2005 at 9:16AM
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You must use a concrete bit. Titanium will not work for concrete.

    Bookmark   December 11, 2005 at 10:26AM
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I'm 99% sure what's under the slab is just dirt. I never saw any gravel or anything trucked in. They built the foundation and then re-filled the hole. (we have somewhat sandy soil).
Where would termites come from? the dirt below the concrete?
Slab is 4" thick.

Where would one get these garage mats? and how do they channel water against the tilt of the floor they're on?


    Bookmark   December 11, 2005 at 12:09PM
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I'm a neat freak so I actually drilled a series of holes in a circle, chiseled out the middle and put in a small floor drain. I think the drain was only about an inch and a half in diameter. I surrounded it with concrete to make it look almost like it had always been there. I could tell there was gravel underneath the slab.

If I was only going to drill a few holes I suppose half inch would be OK.

    Bookmark   December 11, 2005 at 1:38PM
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The gist of everyone's questions is two-fold:
1) If you drill through will the soil accept the water? or How permeable is the soil? sands and gravels are good for drainage, dense clays are not.
2) What is the liklihood that the groundwater table can be higher than the underside of your garage slab (even temporarily)? This will let water IN through your holes rather than drain it out.


    Bookmark   December 12, 2005 at 8:55AM
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I had a low spot right in front of the access door. Drilled the floor and the water will not drain.

    Bookmark   December 12, 2005 at 11:11AM
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Oh - i'm sorry - didn't mention water table - the backside of my properly drops off about 25 ft down to a stream. We have sandy soil and all water just drains down to the brook. I'm blessed (thank you, big guy) with a totally dry basement (knock wood), and what's under the slab is the same sandy soil, so I'm pretty confident of it draining. Question is - are there side effects? will mold or moss grow in the hole? will critters come up?


    Bookmark   December 12, 2005 at 11:44AM
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mold or moss grwoth is dependent on the actual soil and how wet it stays. I can't envision critters being a proble through these. You can always block it with a geotextile fabric or screening to keep them out.

Depending on the amount of water that is drained, you will get some local consolidation of the fill below the slab and start to form voids. Based on your description I'd say this is a moot concern also, as I believe that the settlement will be small and take a long time, if ever, to become an issue.

    Bookmark   December 12, 2005 at 12:59PM
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i would go with the small drain suggestion myself. the problem with smaller holes is tha tthey soon fill with dirt and debris and no longer function. at least with a drain you can scopp it out with a spoon if needed.

or else buy a wide push broom and sweep it out constantly.

    Bookmark   December 12, 2005 at 1:31PM
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problem is I have several low spots - would require several drains

    Bookmark   December 13, 2005 at 12:40AM
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Just drill a bunch of holes. You can easily mix up some mortor to patch those holes. Would think termites would be only possible gotcha,,, and I bet they'd make a mud tunnel so you'd probably notice it.

    Bookmark   December 13, 2005 at 10:26AM
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thanks - but I'm confused - termites live under the center of a 24' x 36' slab?


    Bookmark   December 13, 2005 at 11:24AM
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At least one variety of termite does, in fact, live in the dirt. They tunnel upwards (making mud tunnels as they go once reaching the surface) in search of cellulose to eat. It's the tunnels that they make that let you know they are there...


    Bookmark   December 13, 2005 at 2:36PM
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Hmmm, if the holes were near a wood wall, then I might be concerned about termites, but in the middle of a garage floor slab????? I doubt that they could make much headway toward any wall from there without being spotted, in which case one would have to drill holes in the concrete anyway to treat for them....

My concern would be freeze heave. Do you live in a cold climate and is it an unheated garage? Saturating the soil beneath the concrete with water will cause the concrete to heave when it freezes and crack.


    Bookmark   December 14, 2005 at 12:27PM
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I live in MA. It was 3DegF this morning. The garage is unheated. on one hand you make a good point. On the other hand, we're talking snow runoff, probably under 10gal per year. So I'm hoping/thinking that the soil can carry that much water away from the house.

    Bookmark   December 14, 2005 at 12:52PM
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Hard for me to imagine 3°F.
Must be why snow people have special winter junkers.
The good cars are all frozen in the garage to the floor.

    Bookmark   December 14, 2005 at 4:38PM
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I wouldn't worry about termites. If they are tunneling under the slab, they can find other ways to get to wood. If they come up through the holes you drill they will need to build a mud tube and that should be easily seen.

I also wouldn't worry about heaving. I drilled holes in ours with no ill effects despite going down below zero F almost every winter, often way below zero. I suppose if you had very little drainage below the slab and you put lots of water there it could cause trouble, but as you say you are only dealing with a few gallons spread out over a long time.

    Bookmark   December 17, 2005 at 12:02AM
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for no more water than that, what is wrong with a wide broom and sweep it out the door? then NO ugly holes to look at.

    Bookmark   December 19, 2005 at 6:02PM
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My garage floor is gravel and we do not have stem walls or anything. We currently have 5 inches of standing water in it. We have thought of trenching it and draining it to our downspouts and then pouring a slab. Does anyone have any thoughts? So little puddles of water isn't really a big deal.

    Bookmark   December 30, 2005 at 12:00PM
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The place we rent for a temp jobsite business office has over the past few years installed several drinking fountains in their workshop building areas, they simply drilled holes thru the concrete floor and let the drain run into the dirt/gravel or whatever is underneath there it has been like that for years with no problems. Besides if you have a problem just fill the holes back up with some mortar mix or something.

    Bookmark   December 30, 2005 at 4:36PM
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