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Old 01-22-2010, 08:21 AM   #1
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Concrete slab question


I'm going to build a shop in my backyard. The shop will be located on a slab. I initially said a 4" thick slab, but others have suggested different options. One option mentioned in this thread is to build the slab 4" thick and then provide a 2" riser all the way around to raise the bottom plates up to 6" above grade. I agree with this and I'm all for it, but I don't know exactly how to do it. I've poured a slab before, but only a flat one. How would I build the forms to do the 2" riser all the way around? Is it done it two steps, or all at once? Here is the thread with the building and the other options mentioned.

Building a storage building / small shop..please step in.

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Old 01-22-2010, 08:32 AM   #2
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ther is a thread/post here, not to old shows what you want

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Old 01-22-2010, 09:10 AM   #3
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ther is a thread/post here, not to old shows what you want
link?
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Old 01-22-2010, 09:51 AM   #4
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Concrete slab question


Concrete Footings for a Detached Garage???
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Old 01-24-2010, 07:23 AM   #5
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Concrete slab question


I do want to do the slab correctly. I didn't realize that it had to be so complicated for a storage building. However, I don't want to cut corners, so I guess I better do it right from the get-go. I have a couple of questions.

1. What is the difference between fiber reinforced concrete and regular concrete with rebar added? I had planned already to use fiber reinforced so that I wouldn't have to hassle with rebar. It's my unbderstanding that rebar isn't needed when using fiber reinforced. Of course, I'm no expert though

2. What is the importance of footings? I had it planned to just pour a regular 4" thick slab, like the one pictured below. This is the way I always thought would be ok, but from reading the thread above, I guess footings are a major integral part of the whole foundation. What exactly do footings do?

My idea:



3. I would like to do the forms for the curbs, but I'm not really sure this is necessary. I would like to have the bottom plates up off of the ground more, but I guess it is ok if the aren't. I'm trying to decide weather or not to do this.
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Old 01-24-2010, 07:35 AM   #6
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Much easier to install a couple courses of block around the perimeter than concrete stub walls.
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Old 01-24-2010, 07:54 AM   #7
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The footings are to support the building not the slab. The depth needed (below frost line) gives support that is not affected by the frost heaves. Adding block walls (2 rows) will bring the wall materials away from the ground. Using the fiber re-enforced cement means you do not need rebar or wire mesh for this slab. Adequate drainage (4" 3/4" gravel) below the slab is needed.
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Old 01-24-2010, 08:20 AM   #8
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The footings are to support the building not the slab. The depth needed (below frost line) gives support that is not affected by the frost heaves. Adding block walls (2 rows) will bring the wall materials away from the ground...Adequate drainage (4" 3/4" gravel) below the slab is needed.
So am I understanding you correctly by saying that basicly the weight of my building would probably be too much for the edges of the 4" thick slab in the illustration above? Does the added concrete or block "stem wall" in that area give the added strength needed?

What is the purpose for the gravel underneath the slab? It was my understanding that plastic needs to be placed down under the concrete before it is poured to prevent moisture from coming up through. Are these subjects two different concerns, or am I on the right track?

Thanks for replies folks.

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Old 01-24-2010, 09:14 AM   #9
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Originally Posted by J S Machine View Post
So am I understanding you correctly by saying that basicly the weight of my building would probably be too much for the edges of the 4" thick slab in the illustration above? Does the added concrete or block "stem wall" in that area give the added strength needed?

What is the purpose for the gravel underneath the slab? It was my understanding that plastic needs to be placed down under the concrete before it is poured to prevent moisture from coming up through. Are these subjects two different concerns, or am I on the right track?

Thanks for replies folks.
Bob already answered the support and gravel questions. You quoted them in your reply. The plastic/gravel installs are part of the same issue, to keep moisture away from the slab. If you plan to heat this building, I would put board insulation under the slab.
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Old 01-24-2010, 09:32 AM   #10
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Bob already answered the support and gravel questions. You quoted them in your reply. The plastic/gravel installs are part of the same issue, to keep moisture away from the slab. If you plan to heat this building, I would put board insulation under the slab.
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After a little more reading, I understand more about footings now. For the building I'm doing, What would be an acceptable dimension for the footings? I was thinking maybe 10" deep and about 8" wide? Would that be enough?
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Old 01-24-2010, 09:43 AM   #11
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Concrete slab question


I can't tell what your location is, so it is impossible to answer how large your footings need to be. A few general rules.

1. Assuming you need a permit and are planning to get one, your local building inspector will certainly be able to tell you the minimum footing depth and width in your location. Usually if you go with code requirements you are OK, unless you have unusual conditions.

2. If you are not planning on getting a permit, or don't need one, the required width and depth of footing is determined by the strength of the soil, the loads to be exerted by the building, the depth of frost, the amount of water in the soil, and the geometry of the building. Foortings can be sized by an engineer, architect, or knowledgeable contractor.

The slab is really not a structural element, since it is only 4 inches thick. The main function of the slab is to provide a convenient walking surface, and to provide adequate support for light loads such as shelving. If you need significant support, say for a column or a heavy jack, you need to install footings where necessary. The footings can be a deeper part of the slab (i.e. integral with the slab). In general, footings need to be placed on suitable soil, ideally sand or gravel, to provide drainage and support.

The construction of the slab has been previously discussed. The footings typically are constructed with rebar, however this is not always necessary, check with your local code officer to see if it is required for your particular type of building.
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Old 01-24-2010, 09:43 AM   #12
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You need to at least post your location for anyone to really help you out and state what kind of soil conditions you have.
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Old 01-24-2010, 09:55 AM   #13
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Its right under my username folks..

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Wetumpka, AL. We don't get much frost or frost heaving or anything like that. There may be two or three weeks during the winter season when it gets really cold or below freezing, and by that I mean 25 degrees. We actually had some teen temps this year a few weeks ago.

The building does require a permit because I'm in the city limits, and everything has to be inspected, even the slab. I will give them a call back tomorrow and see what they say about the footing requirements.
I'm trying to do most of this job myself, so I'm trying to figure out exactly what needs to be done. I hate to have to call someone and get them to come over and look, because I'm trying to avoid having to pay any contractors. If I do everything on my own I can save a good bit of money. I understand that time is valubale to contractors, thus the reason I don't want to inconvenience any of them.

I don't mind putting forth the effort, I just want to go forward in the right direction. Looks like I will at least be digging footings with a shovel.

As far as the soil is concerend, How would I find out if the soil is adequate or not?
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Old 01-24-2010, 11:14 AM   #14
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Concrete slab question


Ayuh,... JS,.. You're getting Good info from some of the Best,...
But,...
Just a thought,...
With the name JS Machine, I see the possibility of Huge, Heavy loading, which a 4" slab might not support...
What's the intended purpose of this shed anyways,..??
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Old 01-24-2010, 11:32 AM   #15
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I'm just a Toolmaker / Machinist man. I'm building this "shop" behind my house to use as a storage building. ,mainly just to put the lawn care stuff in and maybe a small work bench. We're not talking about any heavy machinery...

My main objective with the project is to gain a better understanding of house/ dwelling construction. I plan to eventually move to a new home site and build a house. I'd like to do alot of the work there myself as well. This build will not only benefit me in the experience area, but also hopefully, if done correctly, increase the value of my current home.

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