Attaching Patio to House

I'm planning on pouring a concrete patio up against my house in the back yard. A friend of mine has suggested that I drill holes into the side of the house foundation and plug rebar into them so when the patio is poured it will be "attached" to the house slab via the rebar. He says this is necessary to prevent the patio slab from sliding away from the house after it begins settling. I'm a total novice, but none of the books I've seen have mentioned this issue. Is he making up problems or is this something I should take seriously?
Thanks, Dave
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There's no need to attach the patio slab to the house. Slabs don't slide unless you did something seriously wrong, like build it on a hillside. The patio slab should not move appreciably, if at all, if you prepare the sub-grade soil correctly, add aggregate as required for drainage, and compact it.
R
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Rico, attachment is common "upstate" to prevent migration caused by front heaves -- esp. since most patios are floating and don't have footers to keep them in place.
Another way to do it, is to interlock to big eye bolts (no one said it was easy) and to put on into the wall and one into the slab (tacked onto the screen or the rebar) with the "joint" at the border of the two pieces (in the felt expansion pad) so that the slab can float up and down without drifting away.
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I know it's common, so is questionable construction. The dowels are silly.
Frost heave is only a concern if there's a chance that the sub-grade structure will be levered/ratcheted up with repeated freeze-thaw cycles. There is exactly zero chance of a slab being racheted upwards as there is nothing sub-grade for the freeze-thaw to work against. The slab floats on top.
The only thing you have to worry about is the tiny amount of differential movement between the house and slab, and you pointed out one way to deal with that.
R
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I just realized that I wasn't clear about something. The expansion joint material is all you need, not dowels or eye-bolts or anything else.
The same way that Bob Morrison (hope you're doing well, Bob!) sees little if any benefit in placing wire mesh in a concrete slab, I see no benefit in using dowels in an attempt to compensate for proper sub- grade preparation.
R
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We pour 500+ patios, driveways, etc. a year in Southern CA. Some soils engineers require 'dowels' to be installed on any concrete flatwork that adjoins to the foundation. In my experience this is a waste of time and money. 90% of problems with concrete flatwork occur when the sub-grade is not properly compacted or pre-saturated in highly expansive soil. The other 10% of problems come from incorrect joint placement,improper finishing and rarely bad concrete. Just my 2 cents.
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On Mon, 25 Jun 2007 08:01:35 -0700, RicodJour

Now I'm confused. Despite Bob's attitude, a lot of people do place wire mesh in concrete slabs. Maybe even you. By comparing** Bob's opinion there to your own here, it makes it seem like it would be prudent to put in the bars.
**Bob says little if any benefit. You say flatly no benefit, but still similar.

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You're right, it does seem that you're confused. ;) Bob believes that mesh in a slab is a waste of money and time - I agree with him. The effort and expense can be put to better use dealing with where the problems start, the sub-grade preparation. Same with the dowels. I'm not trying to put words in Bob's mouth. I don't remember him weighing in on this particular aspect but the approach is very similar.
R
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On Mon, 25 Jun 2007 14:55:34 -0700, RicodJour

Hey, I've been confused by less complicated things than this!

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What is a "front heave"?
I live in WI, where we have frost heaves, which is why you wouldn't try to pin the patio to the foundation. The foundation has footings and the patio doesn't, so it needs to be able to float in the event of a frost.
Is "floating away" apprciably a real concern in other areas?
JK
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determine soil type, drainage now and after, and whether the earth has been disturbed in recent years. in a climate that includes winter, and if needed by soil type, foundation footers of concrete may be used to a depth specified to below the frost line. if this is a warm climate beach house or pole house on sandy soil there may be a different local answer. make specific plans for rain drainage as needed by your climate. thickness of concrete and its future use as an enclosed part of the home may help determine its construction, such as an insulated slab requirement. your local building inspector may have plenty of experience with all your local factors and that helps you to do it right the first time. its size and placement and setbacks on your lot are best determined with him. see also: http://www.buildingscienceconsulting.com/resources/mold/Read_This_Before_You_Design_Build_or_Renovate.pdf
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wrote:

http://www.buildingscienceconsulting.com/resources/mold/Read_This_Before_You_Design_Build_or_Renovate.pdf
Down in Palm Beach County, Florida the rebar is a 'must'. The County would not approve the permit unless you drilled the rebar into the existing slab.
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Interesting. What's the reasoning?
If the patio base is prepared correctly dowel pins don't do any work at all. If the base is not prepared correctly the dowels will be called on to do all of the work, which will almost assuredly lead to the top of the slab spalling above the dowels.
Code requires three inches of concrete cover around rebar for ground contact slabs. So FL requires a 6" patio slab?
Makes no sense.
R
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wrote:

We just had a thread about stoops or front porches that sink, and one person, I don't remember who, said to put in rebar to keep the stoop from sinking.
This sounds similar, fwiw.

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headware wrote:

idea to me, especially if you live in an area with deep freezes. My reason is that with some heave from freezing you could end up with the patio slanted toward the house and have problems with water and/or ice. If not properly done, it might even expand enough to crack foundation or do something else weird.
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Horizontal movement isn't the problem- up and down movement is. 2nd big problem, especially if patio does frost heave, is water on patio ponding near house, draining in the crack where it meets foundation, and flooding basement. (I have a small dose of that here, with an abandoned patio buried under a 18" tall deck. No way to fix without tearing out deck.)
So, you want patio to have the classic 1/4" per foot slope away from house, you want yard past that to keep sloping, you want good flashing on house side where it meets the patio (and never pour concrete so it buries the siding), and maybe just for laughs a drain tile on house side of patio, going to daylight or a collector box. A good roof overhang or awning helps, too. Patio itself needs footings, either traditional or a monolithic pour based on shape of the hole and the fabric and rebar. Any good concrete flatwork company will understand, and do what is appropriate for local climate and soil conditions.
aem sends....
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So it sounds like this might be a really bad thing to do. I live in Austin, TX where we do occasionally have pretty serious (but brief) freezes, but I don't know how much they affect the soil.
My yard is on a slight slope with the back (where the patio is going) being higher than the front. I am planning on sloping the patio away from the house with the end running into a dry creek bed that will serve as drainage.
I guess I'll just have to see what the building inspector has to say about it.
Thanks for all the help.
Dave
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headware wrote:

Hey, if you are here in Central Texas, you should pay attention to this. I have only done 50 or so new slabs against old slabs, but they have all been engineered. In every instance, the engineer specified dowels into existing concrete, spacing, etc. I always do this here as I have had to remove 3 slabs (not mine) that have move away from the old foundation enough that it caused problems.
I know that many of the people in here are against dowels and some are against rebar at all. I have been working in this area for about 30 years and I have never seen anything but 2nd story patio slabs (lightweight concrete, 2" thick) poured without rebar. And I have never seen a slab connection that did not have dowels.
I have never had an engineer that is familiar with the soils in this area spec anything but dowels and rebar for new against existing. The soils in this area demand it.
--
Robert Allison
Rimshot, Inc.
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I'm

original post, nor did I know he was in Texas with their notorious funny soils and moisture control problems. What I said is true for 'up north' basement/crawl construction with block or poured walls, and frostlines that are actually below ground leverl. Slabs on clay, especially pretensioned ones, are pretty much outside my realm of experience. If the house is built on an itty-bitty runway, as it were, that is probably a whole different section of the how-to book, and one I haven't read.
aem sends...
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Well, I spoke to a contractor friend of mine here in town and he says that in Austin you definitely *do* want to use rebar dowels to attach the patio to the house. Frost heave is not a concern where I am because even though it does freeze over for brief periods of time, the soil itself never freezes.
He also said that since I have no experience working with concrete or building frames that I would be crazy to try this on my own. I had planned to build the frame myself and have the concrete delivered to my house. Since I would be putting natural stone tile on top of the concrete I wouldn't need to finish the surface, so I had planned on having my buddies (who have some experience with concrete) help me pour and screed it. However, given his prediction of total disaster (his actual words) I'm thinking of doing a soft-set patio instead and forgoing the concrete altogether. I was looking forward to a more challenging job but I don't want to get myself into a big expensive mess.
By the way, this is a 250 sq/ft patio we're talking about. At 4 inches thick, it comes out to a little over 3 yards of concrete.
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