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Old 03-26-2013, 12:08 PM   #1
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Floating Slab Questions


Have plans to build a 20x20 garage in my backyard in KS. Local code allows slabs up to and including 400 sq. ft. in size to be constructed as per the attached pic. Before I contract this out, I wanted to get a bit more familiar with my options.

After reading a number of threads, I still have a few more questions:

1. Stem Wall

a. If a stem wall is formed when the slab is poured, what is the minimum recommended height?
b. Is there any advantage to using treated wood on the bottom plate in lieu of a stem wall?
c. Can a stem wall be installed after the slab has already been poured? If so, how?

2. Recommended slope for a 20x20 slab? I assume the slab should slope down, towards the garage door (16x7), correct?

3. Local code requires 2-4" of sand. Can this be improved by adding gravel or other material? Mixed? Above/below?

4. Local code calls for remesh 6" x 6" - 10 x 10. Can this be strengthened by layering, or using different material?

5. Local code requires a single 1/2" rebar be run continously throughout the entire perimeter of the footer area. Can this be strengthened by using larger diameter rebar? Or, run two 1/2" rebar parallel to each other?

6. So far I've gotten two estimates, $2700 and $3300 for a 20x20 slab done as per local code (see attached pic) using 4000 PSI concrete. I was expecting quotes closer to $2000. Thoughts?

Thanks in advance for everybody's input!
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Old 03-26-2013, 12:20 PM   #2
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Floating Slab Questions


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Originally Posted by HomeGuy2 View Post
Have plans to build a 20x20 garage in my backyard in KS. Local code allows slabs up to and including 400 sq. ft. in size to be constructed as per the attached pic. Before I contract this out, I wanted to get a bit more familiar with my options.

After reading a number of threads, I still have a few more questions:
Before I answer your questions.....#1 rule of garages....it's never big enough.....I would give some serious consideration to trying to go bigger....min is 20x25 (2-story) and it's not big enough....

1. Stem Wall

a. If a stem wall is formed when the slab is poured, what is the minimum recommended height? It's easy to use a 2x8 on the inside...outside is what ever is needed to bring the form down to ground level.
b. Is there any advantage to using treated wood on the bottom plate in lieu of a stem wall? You have to use PT regardless...anything that comes in contact with the concrete.
c. Can a stem wall be installed after the slab has already been poured? If so, how? Yes, but I don't suggest it...just makes it harder and cost more.

2. Recommended slope for a 20x20 slab? I assume the slab should slope down, towards the garage door (16x7), correct? Unless you plan to use your garage as a wash bay, you don't need much slope. If you had a 1" from front to back, that would be more than enough....

3. Local code requires 2-4" of sand. Can this be improved by adding gravel or other material? Mixed? Above/below?

4. Local code calls for remesh 6" x 6" - 10 x 10. Can this be strengthened by layering, or using different material? If you mean wire mesh....yes...personally, I'd do #4 rebar instead of mesh.

5. Local code requires a single 1/2" rebar be run continously throughout the entire perimeter of the footer area. Can this be strengthened by using larger diameter rebar? Or, run two 1/2" rebar parallel to each other? In my area...it's 2 #4 rebars at the bottom and 2 #4's at the top...in your case, with the stem wall, you would have the rebar in the stem wall.

6. So far I've gotten two estimates, $2700 and $3300 for a 20x20 slab done as per local code (see attached pic) using 4000 PSI concrete. I was expecting quotes closer to $2000. Thoughts? As usual, everyone thinks it will cost less than it really does....your estimates are pretty cheap....concrete alone is going to almost be that much....wood for the forms...another 200 or so...rebar...labor....and that is assuming no excavating is needed....

Thanks in advance for everybody's input!

If you want an idea of what to expect...click on the garage build link in my signature...

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Old 03-26-2013, 01:39 PM   #3
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Floating Slab Questions


Quote:
Originally Posted by HomeGuy2 View Post
Have plans to build a 20x20 garage in my backyard in KS. Local code allows slabs up to and including 400 sq. ft. in size to be constructed as per the attached pic. Before I contract this out, I wanted to get a bit more familiar with my options.

After reading a number of threads, I still have a few more questions:

1. Stem Wall

a. If a stem wall is formed when the slab is poured, what is the minimum recommended height? I'd recommend having the slab and top of stem wall at least 8" above the adjacent grade (on 3 sides). you can slope your driveway up to meet the slab height. The higher you can set your wood framing above the adjacent grade the better you'll be.
b. Is there any advantage to using treated wood on the bottom plate in lieu of a stem wall? Wood that is in contact with concrete should be preservative treated.
c. Can a stem wall be installed after the slab has already been poured? If so, how?
As dawg said it can be, just costs lot more $$$
2. Recommended slope for a 20x20 slab? I assume the slab should slope down, towards the garage door (16x7), correct? recommended slope on concrete to direct water is 1%, which is 1/8" per foot. for 20' I'd recommend 2-2/12"

I agree with dawg, 20'x20' is kind of small. Locally where I am a two-car garage is at least 24'x24'.

3. Local code requires 2-4" of sand. Can this be improved by adding gravel or other material? Mixed? Above/below?

4. Local code calls for remesh 6" x 6" - 10 x 10. Can this be strengthened by layering, or using different material? if the base is properly compacted the concrete is strong in compression (weight being placed on it). if not, then you can get into some tension loads and this is where reinforcing comes into play. welded wire fabric mesh is a minimum. if the garage is attached to the house it typically requires a class I vapor barrier (minimum 6 mil polyethylene vapor barrier). I always specify one even if the garage is detatched from the dwelling to retain the moisture in the concrete.

Look up slow curing concrete on the internet for the various ways of doing so. Slow curred concrete will develop more of its compressive strength than uncured concrete.

5. Local code requires a single 1/2" rebar be run continously throughout the entire perimeter of the footer area. Can this be strengthened by using larger diameter rebar? Or, run two 1/2" rebar parallel to each other? I prefer #5, but that's only a personal preference, that's what my dad always used, #4 is ok

6. So far I've gotten two estimates, $2700 and $3300 for a 20x20 slab done as per local code (see attached pic) using 4000 PSI concrete. I was expecting quotes closer to $2000. Thoughts?

in my area I'd expect the stem wall to run $5000 (of course our footings must be 48" into the ground, not 12" like yours, we use a frost wall) and about $2500 for the slab. This is for a typical 24'x24' garage.

I take it this will be a one story garage?

The reason I ask is I notice the stem wall is 12" thick (or wide) without a footing. The width of the wall when bearing upon undisturbed virgin ground is what supports the weight of the structure. If the weight of the structure per foot of foundation is greater than the soil bearing capacity of the virgin material then you could have some settling. May want to check with the building official and see what their experience has been with this.

For a one story garage I doubt you'd have a problem, but something worth asking about.
ok, I've rambled on long enough ...... good luck!
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Old 03-26-2013, 04:10 PM   #4
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Thanks for the replies! I definitely feel more confident about the process.

Am I correct in assuming a "stem wall" is the small, raised portion of concrete (approx. 4-6 in. wide; varying heights) that raises all the framing above the garage interior floor/slab? If water pools up on the interior floor the stem wall prevents water from soaking into the 2x4's that are bolted directly to the slab? Is this correct?

Yes, this will be a single story garage, and yes, I'd love to have a garage larger than my proposed 20x20 (400 sq.ft.). However, according to local code, any slab 401 sq. ft. or larger must have 30" footers. I've gotten quotes on 22x22 and 24x24 slabs (with 30" footers) and the costs go up dramatically (approx. $2000++). Plus, the sewer line for the house is in close proximity to where the garage will be. Didn't want to take a chance of someone hitting it with the deeper execavation necessary for the 30" footers.
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Old 03-26-2013, 04:25 PM   #5
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Quote:
Originally Posted by HomeGuy2 View Post
Thanks for the replies! I definitely feel more confident about the process.

Am I correct in assuming a "stem wall" is the small, raised portion of concrete (approx. 4-6 in. wide; varying heights) that raises all the framing above the garage interior floor/slab? If water pools up on the interior floor the stem wall prevents water from soaking into the 2x4's that are bolted directly to the slab? Is this correct?

the detail you showed does not indicate any portion of the concrete stem wall being above the top of the concrete slab. most builders and designers would recommend (in my opinion) having the stem wall above the top of the slab.

A stem wall is usually a section of foundation wall that extends from the footing and up to the sill plate. A stem wall is typically 8" thick, your sketch indicates 12" wide.

Yes, this will be a single story garage, and yes, I'd love to have a garage larger than my proposed 20x20 (400 sq.ft.). However, according to local code, any slab 401 sq. ft. or larger must have 30" footers. I've gotten quotes on 22x22 and 24x24 slabs (with 30" footers) and the costs go up dramatically (approx. $2000++). Plus, the sewer line for the house is in close proximity to where the garage will be. Didn't want to take a chance of someone hitting it with the deeper execavation necessary for the 30" footers.
my only suggestion is to measure the vehicles you are planning on storing in your garage. If your stem wall goes above the top of slab as is recommended, and the stem wall is 12" in width (as shown in your sketch) this leaves 18' for a vehicle (and no way to walk around the front or rear unless the garage door is open.

Point I'm trying to make is be sure it will work for your needs.

Good luck!
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Old 03-26-2013, 04:49 PM   #6
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Floating Slab Questions


Quote:
Originally Posted by HomeGuy2 View Post
Thanks for the replies! I definitely feel more confident about the process.

Am I correct in assuming a "stem wall" is the small, raised portion of concrete (approx. 4-6 in. wide; varying heights) that raises all the framing above the garage interior floor/slab? If water pools up on the interior floor the stem wall prevents water from soaking into the 2x4's that are bolted directly to the slab? Is this correct?

Yes, this will be a single story garage, and yes, I'd love to have a garage larger than my proposed 20x20 (400 sq.ft.). However, according to local code, any slab 401 sq. ft. or larger must have 30" footers. I've gotten quotes on 22x22 and 24x24 slabs (with 30" footers) and the costs go up dramatically (approx. $2000++). Plus, the sewer line for the house is in close proximity to where the garage will be. Didn't want to take a chance of someone hitting it with the deeper execavation necessary for the 30" footers.

No thats not a stem wall,here's a link to one ,what your describing is the way you should pour it though


https://www.google.com/search?q=stem...w=1146&bih=494
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Old 03-26-2013, 05:27 PM   #7
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Floating Slab Questions


You say code requires sand so I assume that means they want you to fill it in. Since you are pouring a monolithic slab and footing, both should be on the same material. Sand does not compact like other soils so usually they flood it with water then compact when damp and pour before it dries. Gravel compacts more than sand because it usually has some clay and stone content, (here in New England anyway). You would get better bearing if you pour the entire slab / footing on undisturbed soil, as long as it is uniform in type, than if you add 4" of sand under the slab only. I have always been told if the soil type varies there should be at least 1' of compacted gravel to compensate for unequal bearing capacity.
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Old 03-26-2013, 10:48 PM   #8
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Floating Slab Questions


Do you need a ufer in that slab (ground for electrification of the garage)?
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Old 03-27-2013, 09:39 AM   #9
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I don't think you can accomplish a poured in place stem wall while doing a monolithic slab. I'd leave rebar sticking out vertically around the perimeter of your monolithic slab and then lay a course of CMU as your stem wall. Fill in the CMU with concrete so that it become and integral part of your foundation.
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Old 03-27-2013, 10:01 AM   #10
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I don't think you can accomplish a poured in place stem wall while doing a monolithic slab. I'd leave rebar sticking out vertically around the perimeter of your monolithic slab and then lay a course of CMU as your stem wall. Fill in the CMU with concrete so that it become and integral part of your foundation.
Really?



So what would this method be called?

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Old 03-27-2013, 10:08 AM   #11
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Really?



So what would this method be called?

I stand corrected.
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Old 03-27-2013, 10:12 AM   #12
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Brock....just so you know....I had my reservations too when I was first going out for quotes on the slab. I assumed that the footing and stem wall would be poured 1st and then they would come back and do the slab inside.

Learn something every day....
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Old 03-27-2013, 10:13 AM   #13
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One note to the OP....

I can not stress the importance enough of making sure your anchor bolts are in the right place when you do the pour....especially if you want your studs 16" OC.

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Old 03-27-2013, 10:26 AM   #14
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in my locale we install the footings then the foundation wall. Typically slabs are not installed until the building is weather tight. we do get a bit of rain and snow here. thing I love about the forum is you learn so much about how things are done in other parts of the country, after all knowledge is power.

and dawg is right about the anchor bolts, nothing like having to either notch studs or retrofit anchor bolts.
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Old 03-27-2013, 10:28 AM   #15
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Brock....just so you know....I had my reservations too when I was first going out for quotes on the slab. I assumed that the footing and stem wall would be poured 1st and then they would come back and do the slab inside.

Learn something every day....
No worries. I literally learn something every day on this site!

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