View Full Version : How wide does a foundation wall have to be?


CBR9Seadoo
10-26-2009, 12:54 PM
I am thinking about adding a second story to my garage and am curious on how wide the poured foundation walls should be?

I am planning to pour a 24"w x 12" deep footing and I was thinking the walls would be 8" thick, going to 5" thick to support the framed wall. Like in this pic?

Thanks.

bimmer1980
10-26-2009, 01:02 PM
sounds like it should work, but I would recommend calling your local code office.

You can also pick up a book called "Code CHeck" at your local library. They give the general building code for various situations....

in the copy that I have, it is calling out a minimum of 15" wide by 7" thick footing and 8" thick walls for 2 stories.

rburke65
10-26-2009, 01:10 PM
I know here, residential footers are 24" wide x 12" deep.......these are "healthy" footers.

pcmeiners
10-26-2009, 01:45 PM
Minimum wall thickness varies as to code. Check the 24" depth is to code. Also consider the PSI grade of concrete, unspecified good chance you will get a low load mix, don't let the contractor add extra water for workability; easy for him, but weakens your foundation. If possible once poured, keep the concrete wet for 28 days. Like the insulation envelope you plan

blkhonda1991
10-26-2009, 01:50 PM
if i remember correctly: the rule of thumb for footings is the width of your wall (8") plus 1/2 of the width on each side (4") x 2 so the total size of your footing should be 16" and the depth should be equal to the wall (8").

8" wide walls should be fine but will probably require additional rebar and from what my concret guy has told me to go with a 10" wall without the additional rebar its almost a wash

buening
10-26-2009, 02:10 PM
It will vary depending on your local code, but I can tell you that 8" thickness is the minimum according to ACI for plain concrete footings. This assumes you will not be reinforcing the footings.

Costner
10-26-2009, 02:14 PM
Since codes can and do vary so much I'd just check with your local building department. I'm guessing 8" will be sufficient but you never know if they have a local code requirement.

Also, as far as your step back to 5" you may find the extra cost of forming the wall with a "shelf" like that outweighs the cost of the extra concrete to just pour the whole thing 8" wide. The negative with a fixed width the entire way is that you will end up with a lip of concrete on the inside of your wall (and based upon your drawing some exposed foam as well).

I have such a lip in my garage and it isn't such a big deal, but it is something to be aware of in case you had any plans of a nice wall to floor transition.

twostory
10-26-2009, 02:19 PM
Your sketch is the right idea, but for a two story structure, you may need to maintain the 8 inch wide stem wall all the way to the bottom plate.

As everyone else said, talk to you local building permit office /or/ building inspector office. Make sure you get the same answer from more than one person. If you get it wrong, it will be a real pain in the a** to fix.

TRC51
10-26-2009, 03:22 PM
I can confirm that your building department will likely force you to maintain the 8". I was going to go 6" block knee wall over 8" poured foundation and the town denied it because the load centers did not match up. Made sense. So instead I just had them pour one wall 8" thick to my knee wall height. Turns out it was much cheaper than putting block in and it looks great IMO.

ddawg16
10-26-2009, 03:53 PM
Mine are 15"W x 24"H with an 8"W stem wall on top.....

The width determins how much load the wall can carry....12" is usually the min for a 1 story....but the height is what helps keep everything straight....I would want at least 24" so that you know it won't crack if the ground shifts a little....think of it as a 'beam'.....which in reality it is....

As for the stem wall...assuming you do 2x6 for the walls...that leaves 2 1/2" for drywall and shear wall....works out just about right if they are around 8" wide.....

BTW....it sure helps to have your location in the profile....then we would know if you need to worry about a frost line or not....

Kevin54
10-26-2009, 05:42 PM
Not knowing where you are at, but check this from San Diego's code http://www.sandiego.gov/development-services/industry/pdf/bnl18-04.pdf and from the other side of the dirt http://www.knoxcounty.org/codes/pdfs/irc_changes_overview.pdf

fordcragar
10-26-2009, 06:16 PM
Your building department will either approve or disapprove whatever plans you have; check with them first. If your plans deviate from simple construction, they might require an engineering designed set of plans.

Torque1st
10-26-2009, 06:31 PM
Use plenty of steel re-bars for reinforcement no matter how thick you make the concrete. Concrete has very little tensile strength.

uncleaud
10-26-2009, 07:49 PM
12" wide by 6" deep is minimum footer for a two story light frame house. Industry standard 16 x 8. If your stem wall is poured concrete then the width is more important than the depth of the footer. If your stem wall is buried both side then 6 inches is wide enough as all the loads against it are compressive and not lateral. All this is the minimum code requirements. Like I said earlier 16 x 8 with and 8 inch stem wall is the norm and little would be saved by doing it different. The overall depth of your footer depends on what the frost depth is in you part of the county.

Last edition of the code allows for less than six hundred square foot to be built on a slab with thickend edge.

CBR9Seadoo
10-26-2009, 09:39 PM
It will vary depending on your local code, but I can tell you that 8" thickness is the minimum according to ACI for plain concrete footings. This assumes you will not be reinforcing the footings.

Actually the footings will have rebar in them and that rebar will be tied into the walls.

krooser
10-26-2009, 10:40 PM
Built a two story house in 1975 on an 8" poured wall... still standing.

blkhonda1991
10-26-2009, 11:37 PM
Actually the footings will have rebar in them and that rebar will be tied into the walls.

unless you have extreme site conditions i dont see the need for a ton of rebar in the walls...the one place you absolutely need it is in the footings

Torque1st
10-27-2009, 12:06 AM
unless you have extreme site conditions i dont see the need for a ton of rebar in the walls...the one place you absolutely need it is in the footings

The walls need vertical rebar on 2' centers and at least one horizontal rebar about 6" down from the top. That is if you have rebar in the footings. Rebar is cheap compared to foundation problems.

blkhonda1991
10-27-2009, 12:17 PM
The walls need vertical rebar on 2' centers and at least one horizontal rebar about 6" down from the top. That is if you have rebar in the footings. Rebar is cheap compared to foundation problems.

we normally spec 2 bars in the footings and 2 bars at the top and bottom of the wall...never remember us putting any vertical rebar in any of our foundations

redsky49
10-27-2009, 12:47 PM
As for footers, if building on a spread footing, you will require reinforcement in both directions.

If dealing with a grade beam, an Engineer, Architect or other qualified individual will have to specify reinforcement which can be more complicated.

Typical footers require rebar when the distance from the edge of the supported wall to the outer edge of the footer is greater than .5 x footer depth. This is an old rule of thumb, not carved in stone.

Wall and footer reinforcement varies according to frost depth, anticipated construction weight, soil conditions, wind load, etc. Also local building customs may have other methods and requirements. There are a lot of possibilities, not just one correct answer to your question.

Additional reinforcement usually can't hurt (other than your wallet). If in doubt, consult an expert.

As always, offered only as opinion

redsky49
10-27-2009, 01:05 PM
By the way, here is the linky for a HUD document on wall and foundation construction:

http://www.cs.arizona.edu/people/jcropper/desguide.pdf

There is also a map for frost depths included with the HUD document, though the one I use (prepared by the Dept. of Commerce) I can't find online anymore.

This should be helpful. The Guv has tons of free info that is worth your review.

As always, offered only as opinion

buening
10-27-2009, 04:47 PM
To better answer the question, your foundation footing width is dependent on the load from the structure and the allowable soil bearing pressure. If you are building on really bad soil, the width of the footing will be larger otherwise you'll see many settlement cracks, among other issues. Yes you will need reinforcement the length of the wall (into the page looking at your drawing) in the footing. Common spacing of the longitudinal/shrinkage reinforcement is #4 at 12" centers. For example, a 3' wide footing would need 4 - #4 the length of the wall.

Depending on the width and depth of the footing you may need transverse reinforcement in it as well. If you are building a 12" wide by 8" deep footing for example, the 9" long rebar in the footing (1.5" concrete cover each side) would be useless unless you hook the bar ends. Rebar needs a certain length of concrete for the bond to develop (dependent on bar diamter), which is why you lap lengths of rebar in long construction pours. Hooked ends are common in footing details, FYI

By the way, I wouldn't recommend doing the "minimum" on this job since it involves two stories. A 24" wide by 12" thick footing would be the absolute minimum but I do not have enough details to give a correct answer. Consult your local building code department, which may or may not redirect you to a structural engineer. By 0.02

buening
10-27-2009, 04:52 PM
I would also suggest centering your sill plate on your foundation wall. The way you have it detailed, the load from the wall will be eccentric to the foundation wall. This will cause uneven loading in your foundation wall and footing, which will increase the concrete tensile stress in the interior face of the foundation wall and increase the tensile stress in the bottom of the footing near the exterior building side. Without having the reinforcement placed in the wall and footing for these stresses, you could have foundation issues in the near future.

Torque1st
10-27-2009, 05:38 PM
we normally spec 2 bars in the footings and 2 bars at the top and bottom of the wall...never remember us putting any vertical rebar in any of our foundations

The vertical bar is to prevent buckling (horizontal cracks) from soil pressure, frost action, or water pressure.

larry4406
10-28-2009, 06:38 AM
Your footing size will be determined based on your structure loading and the available soil bearing capacity. Poor soil - larger footing. Great soil - smaller footing.

Here in northern VA with average soils with 2500 psf bearing capacity, footings are typically 16 inches wide and 8 inches thick. In garages, an 8-inch wall is typically poured with an internal 4-inch slab ledge leaving a 4-inch stem wall for wood framing - this is similar to your pocket detail. The stem will support a 2nd floor. This information is for poured concrete walls using aluminum forms. The walls have horizontal and vertical rebar ber the civil engineer's detail.

CBR9Seadoo
10-30-2009, 10:58 AM
Thanks, for the info.

Doug B
10-30-2009, 06:53 PM
My local code enforcer allowed me to build a 1 1/2 story on top of a similar foundation detail (no rigid insulation). Not saying yours will, but he can give you the answer.