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Old 10-17-2008, 09:41 AM   #1
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Concrete Foundation Question


Opinions please - How thick should the concrete foundation be for a 4,000 sq. ft. BRICK house with 9' ceilings on basement, first and second floors. House is built on level ground. Also what psi should the concrete be? I'm getting differing opinions from the builders and architect.

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Teresa, nyc

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Old 10-17-2008, 10:14 AM   #2
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Concrete Foundation Question


Both your questions are answered prescriptively in the 2003 International Residential Code, chapter 4. You should check the code in your area.

8" is pretty much the norm, but it does depend a little bit on the soil conditions and how high the wall will be backfilled on the outside (unbalanced fill). The latter has more to do with reinforcement requirements than wall thickness. A wall with stone or brick may be thickened to accomodate the stone.

Concrete compressive strength also depends on a lot of factors, but for weather exposed concrete foundations in areas with severe weathering potential, the concrete must be 3000psi at 28 days and must be air-entrained between 5 and 7%.

Bear in mind that code is a MINIMUM requirement, but will give you a good basis for comparison with what you're being told. The foundation is not a place to try to save money when building a home.

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Old 10-17-2008, 10:17 AM   #3
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If you're in New York, be aware that they have some squirrely codes compared to other areas of the country. All the more reason to check with the building department to find out their requirements.
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Old 10-19-2008, 08:52 AM   #4
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Concrete Foundation Question


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Originally Posted by Teresa1127 View Post
Opinions please - How thick should the concrete foundation be for a 4,000 sq. ft. BRICK house with 9' ceilings on basement, first and second floors. House is built on level ground. Also what psi should the concrete be? I'm getting differing opinions from the builders and architect.

TIA,
Teresa, nyc
Concrete is relatively cheap, as far as how it impacts your total home costs....psi increases cost very little as well. Go with a conservative estimate, more is safer, and remember, if you go with an insufficient foundation, it will be really expensive to fix later.
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Old 10-19-2008, 10:47 AM   #5
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Concrete Foundation Question


The real question should be the footings. Footing dimensions are important based on your soil type. For 9' high basement walls and 8" thick wall will work but it needs to be properly reinforced.
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Old 10-19-2008, 11:59 AM   #6
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The real question should be the footings. Footing dimensions are important based on your soil type. For 9' high basement walls and 8" thick wall will work but it needs to be properly reinforced.
Yes, the footing are the important issue here. Around here the basement is 6" but footings have to be 8" or 10" depending on the area and below the frost line.

Thicker concrete with rebar is better as it will resist cracking when the house settles. The difference in cost is not that much. Course that is easy for me to say as I am not the one writing the check....

While you are at it think radiant floor heat if you intend to heat the basement.
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Old 10-19-2008, 12:23 PM   #7
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Concrete Foundation Question


For a footing the strength of the concrete is not that important and a 3000 psi or 4000 psi strength would be fine. Usually footing should extend 4" beyond the face of the wall and be 8" unless you have poor soil. Reinforcement is not always required, depending on the local code. The fact you have a "brick" home (probably brick veneer) is really immaterial since it does have little efect on the footing unless you have poor soil. Usually, a heavy home is beficial when it comes to footings and walls. You may need a thicker wall to provide support for the brick unless you use some of the advanced construction details.

The important thing is to follow the plans and designs. I assume you have soil boring and a professionally prepared plan. It is good if you have a permit since you then have inspector working to protect you from the contractor and yourself.

The wall of the foundation will depend on the height and type of floor system. Around here, 9' block walls are the normal construction and be easily adjusted if you use block. Both block and poured concrete will require about the same reinforcement.

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