Concrete deck


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Old 08-13-07, 01:11 PM
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Concrete deck

I'm in the process of bidding out a concrete deck at my house. So far I've been told three different things from the three contractors so far. Here is what I've been told that is basically the same on how the process is done.

1. We'll need a aggregate bed compacted on top of soil
2. Vapor barrier
3. Steel mesh
4. Concrete to be dowled with rebar into the existing footing of the house

Were the difference is the slope of the slab. One contractor is saying 2% of total length (its 20 feet) so approx. 5" of fall. That sounds like alot.

Second contractor is stating 1/4" to 1/2" per 8 feet. Approx 3/4"-1 1/2" total. So which is it?

Third conctractor states that he's sealing the concrete and water won't penetrate and a slope isn't necessary. ?????? Doesn't sound right.

I've searched the net and can't find the answer anywhere. Can anyone point me in the right direction?

Brent
 
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Old 08-13-07, 01:21 PM
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Unless there is structure on the slab, I wouldn't dowel it into the existing.

The slope depends upon climate.

1,2,3 Yes.
 
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Old 08-13-07, 01:24 PM
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I would go with the 1/4 to 1/2 inch guy. There needs to be a slope to allow water runoff. Depends how much water can be expected if you go 1/4 or 1/2. Naturally if a lot of water then 1/2. If it is protected and will only receive wind driven rain and snow then the 1/4 will do. All concrete should be sealed to prevent moisture absorption. If your contractor does not do it then I suggest you do it yourself. As you guessed it will not make up for slope.
 
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Old 08-13-07, 06:54 PM
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Don't let them connect the new slab to the foundation of the house. The two should be separated with an expansion joint.

The slope depends on what you like and how accurate the contractor can finish the surface. Typically a patio slab is as close to level as possible otherwise anything you place on it will lean. A well finished surface should not require more than about an inch over the total. However, if it's not finished properly, you could end up with shallow puddles (until the water evaporates), but personally I would even trade that for a more sloped patio.

You will need a good, well compacted, aggregate base for good drainage that will help in keeping the slab from cracking due to frost heaving. The thicker the better. I suggest at least 12 inches of that would be good. Coarser aggregate is better than fine aggregate. The wire mesh will further help prevent the concrete from cracking by reinforcing it; this should definitely be used.

The vapor barrier will serve no purpose except prevent water from getting at the concrete deck from underneath during the curing process (28 days after it's placed). Concrete cures best with water.

My impression of what these contractors are telling you is - call another contractor that knows a bit more about what you want to do.

Good luck.
 
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Old 08-13-07, 06:59 PM
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About 1/8 inch slope per foot is the rule. Don't believe the guy who says that sealer will make up for slope. Ordinary concrete sealer is water resistant, not waterproof. Also if this is outdoors and not an enclosed room addition, why would you need a vapor barrier? It would serve no purpose whatsoever.

Pecos
 
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Old 08-13-07, 07:17 PM
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Concrete deck

Brent -

Do not tie it to the house!

I assume you are in Georgia, so do not have appreciable frost heave potential. If you have proper drainange and a good base, any frost heaving is minimized.

If you are in the NW part of the state where there may be the potential, a little extra slope (but not over 2") is good insurance. Any frost heaving will tend to raise the outer portion of the patio slightly.

Make sure you saw control joints in the slab to prevent unsightly cracks. Concrete will always crack, but with proper joints, you will not notice. If you saw the joint, do it late the same day or one the next day. After a few weeks or so, clean out the saw cuts and caulk. You can always tool the joints if you don't mind the appearance.

Around here (Minnesota) a typical patio is 4" thick with some possibly 6" thick. A typical base is 6"-8", depending on the subgrade. We do use at least 4000 psi concrete and place welded wire mesh in the bottom half of the slab. We also use air entrained concrete for durability. My patio is 4" exposed aggregate, so I went with a steeper slope for faster drainage and to offset any possible heaving. It is holding up well in spite of the heavy clay soil, thanks to a 8" good base.
 
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Old 08-14-07, 05:11 AM
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Unlike from the contractors you have spoken to, you're getting good advice here.

One more option that you may want to consider is having the contractor install 6 inch diameter sono tubes vertically about 3-4 feet deep underneath the slab at uniform intervals to better support the concrete slab and thus minimize cracking due to settlement. If you go with 12 inches of aggregate, make sure the contractor compacts it in 6 inch layers. Thicker layers are very difficult to compact properly, especially with the light duty equipment they're likely to use around the house to prevent damage to the foundation. Some water added to the aggregate during compaction will also help achieve higher densities. The depth of the aggregate should be determined by the type of soil that you have at the site. If you tell us what you have (dig a small hole and measure to find out), I can advise further. I once had a house with 12 inches of topsoil all over the yard; obviously you don't want to build a concrete deck on topsoil or soft clay.

The more precautions you add the longer the slab will last without cracks, but the higher the cost will get. So you should balance the longevity and cost to suit your personal taste.
 
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Old 08-14-07, 07:08 AM
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Concrete deck

If you do not have concerns about the soil strength, do not over-engineer with sonotubes, that create a totally different structure and situation. - I am a registered engineer with 40 years experience and I rarely say that.

You are only building a patio with minimal applied loads that have no effect on settlement. The soil around your house was disturbed when it was built. If you are really concerned, have him try to compact the surface of the existing subgrade and see it there is excessive compaction - There will be some, but not measured in inches. Your only critical soil area will be close to the house.

Just find a concrete contractor that has a good track record of building in the area and reflect what you have heard here to your project. We are not there and you and the contractor are.
 
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Old 08-14-07, 02:17 PM
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alright. it's time for the totally inept to add his 2 cents.

the reason for a slope, in my mind, is not to drain the water off the patio per se but to prevent the water from running to the house and saturating the foundation area. This is the same reason you slope the ground away from the house.
 
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Old 08-14-07, 07:32 PM
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Originally Posted by Concretemasonry
If you do not have concerns about the soil strength, do not over-engineer with sonotubes, that create a totally different structure and situation. - I am a registered engineer with 40 years experience and I rarely say that.
Maybe you should have said it more often, lol.

I have a patio built that way, it's ten years old and looks better than some new ones!

It's a choice each owner has to make - build it to last at higher cost, or take your chances on a less costly job. Not mentioning all available options is worse than mentioning them and having them rejected.
 
 

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