Can I make my entire patio into a drain?
June 9, 2010 7:34 PM   Subscribe

Insane in the membrane? Can I turn my entire concrete patio into a drain?

When we purchased our house last year, the previous owner mentioned that during prolonged periods of rain the below-grade slab beneath the house (garage, basement) becomes “damp.” We had just such a period this spring, and damp = about three inches of water.

Our house is surrounded on three sides by typical California concrete hardscaped patio, backed by concrete block retaining walls (house is set into a small hill). I believe rain water is entering through two locations: (1) at the concrete block foundation-concrete patio interface, and (2) through several hairline cracks in the patio slab. I do not believe that there is a significant amount of water coming into through the foundation from the hill at the back of the house, as there’s hardpan clay after a foot or so of friable soil: I am not a hydrologist, so I may be completely misguided about this.

When the house was built, the concrete patio was engineered so that water flows away from the house, with a half-step down between the back of the house and each side. Water is meant to run off the back of the patio to the sides, emptying off the side of the patio to the street, but after almost six decades of seismic activity some of the slab has receded an inch or so here and there, and now about an inch of water collects near the foundation when it rains, rather than pouring off the front of the patio (most water does pour of the front).

I want to keep the water out of our basement, but do not want to resort to water proofing the entire foundation (expensive), or sawing concrete and installing drains (less expensive). I want to know if the following idea makes sense as less expensive solution that leaves the existing concrete intact.

1. Pour a thin layer of pebble gravel on the patio and grade so that it slopes away from the house, using a level. This will repair the minor grading problem.
2. Lay a flexible, waterproof fabric over the gravel, coving the fabric up the side of the concrete block of the foundation.
3. Paint the fabric with impermeable membrane coating. At this point, the patio should be waterproof, and the foundation-patio interface protected.
4. For aesthetics, pour another thin layer of gravel over the membrane, and then add a low lying deck to protect and cover the gravel (we wanted to hide the concrete anyway).

I picture this as turning the entire patio into a drain, with the fabric/membrane coved up the sides of the concrete block on three sides, like a bath tub or shallow swimming pool. The gravel-covered concrete would angle toward the street, and will no longer leak.
posted by Izzy to Home & Garden (13 answers total) 1 user marked this as a favorite
It certainly sounds like something worth trying. I would want to see how it handles more heavy rain before starting the deck. The way these things go, something might try burrowing into it eventually, so it would be good to make the new deck removable.
posted by bonobothegreat at 7:48 PM on June 9, 2010

So you're putting the membrane between two layers of gravel? Assuming you want to be able to walk on the patio, I wonder whether that would perforate the waterproof membrane, defeating its purpose.
posted by exphysicist345 at 7:59 PM on June 9, 2010

Hmm. It's not crazy.... I'm not sure what I think about painted waterproof fabric though? What about sheets of semi-flexible plastics? (Slightly more expensive.)

One thing to keep in mind is: what's your ultimate goal with the area? I ended up taking out a deck, actually, and raising and regrading the area with new dirt, and building in a sump pump system and also burying a large pvc pipe, with drainage, to leach/guide away water from the house. So, if you eventually want to get rid of some of that concrete, and have actual yard and/or plants, don't execute this plan; instead, begin a little demolition project on part of it, and install an actual drain.

This is fascinating and please document every step of the way for us.
posted by RJ Reynolds at 8:16 PM on June 9, 2010

I don't know the shape or height of the deck, so this may not make sense but what about some of that corrugated fiberglass sheeting installed on an angle below the deck boards? It would be sort of like a suspended awning beneath the deck.
posted by bonobothegreat at 9:24 PM on June 9, 2010

I assume since you say your house is set into a hill, that the house is higher than the water table after significant rains? I used to have this problem in a previous house, which was built in a marshy area. Every time it rained the water table rose and our garage/basement area got water. We had a sump pump in there, and that took care of most of it. The walls would still be damp, and you couldn't store stuff directly on the floor. There was a second floor patio (atrium) which required a floor drain for any rain. That had a waterproof deck coating (a kind of plastic paint, with some kind of membrane underneath), but my memory is foggy on the details. I do remember that it was more trouble than it was worth though, as it got weakened pretty fast by harsh sun exposure.

My current house is set into a hill and had some drainage issues which we took care of by trenching, installing PVC pipe to carry everything to the street and a sump pump at the lowest level. I can't entirely follow your description, but I wanted to say that my experience with the impermeable membrane was not particularly good, whereas sump pumps and drains have been excellent.
posted by Joh at 11:30 PM on June 9, 2010

I do not believe that there is a significant amount of water coming into through the foundation from the hill at the back of the house, as there’s hardpan clay after a foot or so of friable soil: I am not a hydrologist, so I may be completely misguided about this.

Well, water still moves through clay, and clay can hold quite a bit of it- (one of the reasons clay soils on slopes are prone to slippage). When saturation point is reached the water has to go somewhere, and if there's no provision for drainage the water is going to come out into your below-grade basement. If it were my house, I'd be looking at what sort of drainage exists behind the retaining walls and make sure that's functioning properly. This is important not only to help with the water in your basement, but to keep your retaining walls intact.

1. Pour a thin layer of pebble gravel on the patio and grade so that it slopes away from the house, using a level. (etc)

This seems like a lot of work without knowing what really is going on with your site. Have an engineer come take a look before you build a deck that ends up getting ripped out next January.

The gravel-covered concrete would angle toward the street, and will no longer leak.

Where I live, it is no longer allowed to drain water off one's property; if you do any sort of work that requires a permit, you've got to re-engineer your drainage so that all water collected on site stays on site. I realize you're not talking about pulling a permit, but someone seeing you doing work might report you anyway, so double check your local codes so you know where you stand.
posted by oneirodynia at 12:23 AM on June 10, 2010

Could you install a dry well where the water collects (or right in front of it) and run any gutters toward it as well? We have some fairly shitty drainage issues here (in NH) and I've been looking at dry wells and drainage tiles. I'm not sure how translatable those solutions would be to your climate though.
posted by yerfatma at 5:27 AM on June 10, 2010

Your idea sounds like it might work, but only temporarily. I have trouble believing that will solve the problem for more than a year or two.

Also: as a gardener, I think the way clay works is the opposite of how you think it does: it doesn't drain well, so for significant amounts of rain the water is going to collect on top of that layer, not be absorbed by it.

You said you don't want to saw concrete, but the best solution is probably going to be to have the deck taken out. A landscaper with a crew and jackhammers could take it out pretty quickly. Then you could put in some kind of infiltration pit, or a patio like this one. Susan Carpenter at the LA Times had an article a while back about her 3000 gallon infiltration pit and installation etc, but can't find it online. May be worth an email her way.
posted by lemonade at 6:37 AM on June 10, 2010

I am a civil engineer, but I'm not your civil engineer, etc etc.

Intact concrete is actually excellent waterproofing. Once cracks start developing is where you run into problems. What you want to do first is to rout the cracks using a crack chaser blade in an angle grinder. Then fill the cracks with a good silicon or polyurethane based caulk. I believe we used to use Sikaflex 1A or 2C when I was doing concrete structural rehab.

If you need to re-slope the concrete, you can buy repair mortars that are suitable for thin coating applications (usually some sort of epoxy modified concrete). The type of mortar really depends on the application thickness, so it'd be tough to recommend one.

Also, when you say the slab has receded, does that mean it's settled or that voids have developed beneath it? Another possibility to reslope the slab is mudjacking, which pumps pressurized grout underneath the slab to bring it back to its original position.

The gravel and fabric solution is an interesting one, but I'm guessing it'll be only somewhat successful while looking like hell.
posted by electroboy at 7:17 AM on June 10, 2010 [2 favorites]

Also, there's a lot of stuff that can be going on under the surface. You're right that dense clay soils typically don't allow for a lot of movement of water through them, but it's certainly possible that the water is being funneled towards your basement through cracks or discontinuities in the clay or if the clay surface was sloped back during construction.
posted by electroboy at 11:17 AM on June 10, 2010

Thanks everyone for great comments.

My thoughts:

Here's a link to the sort of application I'm thinking of (LATICRETE® 9235 Waterproofing Membrane):

It's a waterproofing solution meant for swimming pools, water features, etc.

It's durable, and also, when covered with gravel as I plan to do, not going to get hit with UV rays like an exposed roof, so I imagine it'll last a long time. And if it starts leaking after a few years, I could just remove the top gravel layer and recoat (but I don't think that'll be necessary).

About the slope in back of the house: If there's water coming off the slope that's infiltrating under the patio slab, could that be be fixed by French drains, or even swales? I have a landscape architect who's coming to take a look, so perhaps she'll know what the best practice is for our situation.

What to do with the rainwater: It'd be great to capture it in a catch basin of some sort and use it for gardening. I'm not too worried about piping water to the street, as that's what's happening now (just not very efficiently).

Water table: Heh, heh, I don't think there's much of a water table in our southern California back yard to worry about.

electroboy: thanks for you thoughtful comments. My first thought, too, was to have a thinset layer applied to re-establish the slope, but I thought the fabric + gravel solution would be even better, as the fabric is meant to be a little bit flexible and will probably not crack (unlike concrete) during seismic activity. It seems like the interface between the slab and the house foundation is the most problematic, and probably the fist thing to develop hairline cracks. You're right about the aesthetics, it has the potential to look crappy, but what if the gravel entirely covers the membrane/fabric, like in a Japanese rock garden? We wanted to build a low lying deck to cover the concrete anyway.
posted by Izzy at 11:53 AM on June 10, 2010

I would definitely investigate the mudjacking, it's probably not as expensive as you think. (I'd guess somewhere between $1.50-$2 per sq ft.) and it provides a permanent solution, rather than a temporary kludge that will need to be removed if you ever sell the house. Given that the slab, and therefore the soil beneath the slab, is now sloping towards the house, filling that area with mortar is going to eliminate one of the paths that water could follow to your basement. The pool membrane and the gravel may not necessarily work if water is traveling under the slab to your basement.

Although you could probably build the deck on top of the slab, it's hard to say whether or not you'll get a building inspector that insists on dug footers. In that case, you'd have to punch holes in the slab and would probably be better served by demolishing it entirely and regrading the yard. So, I'd talk to a contractor about the deck and see what they think about whether you'll need to demo the slab or not.

If you're concerned about the joint between the slab and the foundation, you can put in an expansion joint by stuffing it with polyurethane backer rod and filling it with caulk. Tremco is a pretty common one for this sort of thing. You may need to visit a specialty store for it.

The big caveat to all this advice is that without seeing the layout of your yard and the condition of the slab, it's almost impossible to tell you what to do. Swales, french drains etc are all potential solutions, but it's difficult to say whether or not they'll be effective.
posted by electroboy at 1:11 PM on June 10, 2010

Water table: Heh, heh, I don't think there's much of a water table in our southern California back yard to worry about.
You're probably right, since you know where your house is, but don't assume SoCal = no water table. Both the houses I referred to are in Los Angeles! :)
posted by Joh at 10:33 PM on June 10, 2010

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