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?
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.
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.
I know it's common, so is questionable construction. The dowels are
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.
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
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-
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.
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
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
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
Is "floating away" apprciably a real concern in other areas?
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
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
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.
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.
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.
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.
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
Thanks for all the help.
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.
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.
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
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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