Foundation wall thickness

[MWC104]

I'm talking to some contractors for bids on my garage and was curious what thickness I should go for on the foundation stem walls. The studs are going to be 2x6. I've heard 8" is the standard but I was concerned with flushing one of the walls with the existing house wall. The garage will be attached and bump out from the house (the current exterior wall of house will become an interior garage wall and be extended outward 4 1/2').

One guy told me I could have it poured 6" thick but I wasn't sure myself if that was a good or bad idea. Ideally I would like to have all the stud walls flush with stem walls on the interior (slab will be below the walls about 8"). It's mostly an aesthetic thing with me. I don't like seeing a lip between the stud wall and foundation wall.

 

[wssix99]

One guy told me I could have it poured 6" thick but I wasn't sure myself if that was a good or bad idea.

Properly reinforced, 6" is just fine. Parts of my concrete house are 6" two-story walls, supporting one floor and a 100 psf roof.

I would think that you would get more pressure to use whatever size wall matches up to the forms on your foundation company's truck. A 6" wall may also need more reinforcement, which could make it cost more than an 8" un-reinforced wall. (It would all depend on your design.) If your architect/engineer says you are good with a 6" foundation wall, I would feel totally comfortable with it.

 

[kbs2244]

The big difference between homes and garages is basements.
You need a thicker wall when there is a basement because it must resist the side loads.
But a garage is back filled so the are no side loads.
I have seen 4 inch for a garage but 6 inch is the norm for both and what most codes will want.

 

[larry4406]

If your wall is 8" above slab, and lot reasonably flat, I would dig footing below frost, pour a 2' tall concrete stem wall of 8" thickness with a 4" slab ledge on inside. This will give you a 4" concrete wall to frame off. Build a short wood pony wall off the slab to flush out your 2x6 construction.

In the houses my company builds we allow 4" stem walls for not more than 24" vertically.

 

[matt_i]

The big difference between homes and garages is basements.
You need a thicker wall when there is a basement because it must resist the side loads.
But a garage is back filled so the are no side loads.
I have seen 4 inch for a garage but 6 inch is the norm for both and what most codes will want.

This. Also referred to as "unbalanced fill". More or less the wall is also restraining the soil from tipping it over or blowing in the bottom.

Along those same lines in my thinking, its not too bad of an idea to eliminate the keyway in the footing for a garage due to same issue. Some regularly spaced rebar will keep the wall centered.

 

[larry4406]

This. Also referred to as "unbalanced fill". More or less the wall is also restraining the soil from tipping it over or blowing in the bottom.

Along those same lines in my thinking, its not too bad of an idea to eliminate the keyway in the footing for a garage due to same issue. Some regularly spaced rebar will keep the wall centered.

The houses and detached garages my company builds we use footing keyways and vertical bars to lock the wall to the footing. Doesn't matter if we have a 2' stem wall on a detached garage, or a 12' wall on a house. I build in VA-MD and we use aluminum concrete forms with snap ties and pins. Styrofoam is used to create the slab ledges and brick ledges.

Regarding unbalanced fill of buried basements, depending on the wall design, you can backfill against the wall prior to framing. Regardless of the engineering, some counties preclude this. One county in MD requires first deck be on the house prior to backfill which is dangerous; other counties don't care and our wall crack rate is no different between them.

Attached is a typical foundation detail for a detached garage. Bars at keyway are not shown but are installed.

 

[ernie2525]

24" deep and 12" wide. These are for the areas that "don't get earthquakes" because, if it's on this planet, it can/will get earthquakes. The people who argue that point are the same sort that build wood houses in "tornado country" and say"hurricanes don't hit here"
. 24x12. Lots of re-bar and wire too.

 

[Justanoldguy]

24" deep and 12" wide. These are for the areas that "don't get earthquakes" because, if it's on this planet, it can/will get earthquakes. The people who argue that point are the same sort that build wood houses in "tornado country" and say"hurricanes don't hit here"
. 24x12. Lots of re-bar and wire too.

24x12 ?? That is seriously crazy spending of money..

 

[matt_i]

You can pour a 6" wall, on top, set 1/2" OSB with outside surface in the same plane as the outside of the concrete wall, 2x6 framing, then have the drywall protrude inside from the concrete by 1/4". About as close as you're going to get without ripping every 2x6 down to 5.0" wide

You could also pour an 8" wall with a 6-1/4" step in it just like the drawing above but having the concrete guys hold that oddball dimension I think will be costly.

You could put the framing lumber directly on the floor and not have to worry about any of it, but I personally think a much better job is gained by the 8" elevation from finished floor, more or less to get the framing lumber away from wetness and splash.

Personally I think a 6" poured wall with a rebar grid is better than an 8" CBU block wall, unless the block wall gets the full treatment of bond beams requiring knocking out the top of the block webs, installing vertical bar, and grouting it all whole/solid at the end. Usually one of these steps gets skipped in the block wall, they hold up OK even empty with no rebar whatsoever in a garage as evidenced by construction I've heard about. But it will stairstep-crack and eventually can become a liability with frost heaving over many years.

 

[Cyberbear]

Pour the stem wall the same as the stud width and that should make everything work out right. You can finish off the drywall bottom edge in several ways, if you decide to go that route.

 

[volleyball]

Just make the garage entirely out of block and you won't have to deal with that devastating step.
Forms do not determine the width of the pour. They don't care other than too much pressure blowing them out.

 

[bczygan]

Here we use trench footings, and it's hard to dig a 6" trench in many soil conditions. So they get to be 8-12" or more.

And I would rather float my slab than hold it up with the footings.

In fact, with proper drainage, I prefer no footings at all, just a thickened and reinforced edge on the slab.

 

[nfk]

My garage stem wall is 8" wide.

 

[Justanoldguy]

Just make the garage entirely out of block and you won't have to deal with that devastating step.
Forms do not determine the width of the pour. They don't care other than too much pressure blowing them out.

This is typical NZ detail of garage slabs