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Old 02-23-2010, 09:25 PM   #1
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Progress on Storage building, more slab questions.


A month or so back, I posted about the idea of building a storage building in my backyard-


I have since gotten approval from my HOA architectural committee and gotten a building permit from my city building department. As I left it about a month ago, I want to concentrate on my slab first. I know it needs to be right before everything else can be.

What I have done here is put up my batter boards, pulled my lines, checked for squareness/level/plumb, and dug the required 12x12 footings. after that I put up the form boards and all the stakes to hold them in position. The tops of the form boards are level with what will be the surface of the floor.



Now I have to backfill the thing with sand. I have no idea how I'm gonna keep the sand on the island that is left in the middle. I guess I will keep it on there as good as I can and let the concrete take up the rest of the space. The footings were painstakingly dug to exactly 12x12-



There is 4" of fall over 12 feet. This is where the sand comes in, to fill the space. You can see the fall in this picture. This is one of the 12 ft sides. the 16 ft sides are the front and back, and the front has the small door step integrated in. What I did was take a 2x8, and cut it starting at 4" (3.5) all the way back to 8 (7.25) so the bottom would follow the contour of the yard.



Another problem I have is that I have a gap in some places up under my form boards where they run along the ground. I'm not sure what to do about this. It has to be filled in with something, because I know that the concrete will leak out when it's poured. I highlighted these areas in red.



Questions/comments/criticisms/concerns welcome.

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Old 02-23-2010, 09:38 PM   #2
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Progress on Storage building, more slab questions.


You need to remove the organic matter under the slab. This means cutting the sod out to a depth of 2-3 inches, then backfilling with road base or whatever they call it in your area. I assume there are no frost issues, since your footers are so shallow. You can use the removed soil from inside the slab outside the slab to fill the gap and to help hold the forms in place since it looks pretty light with no kickers. You will be surprised at how easy it is for the concrete to push your forms around.

How do you plan to screed it level?

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Old 02-23-2010, 09:42 PM   #3
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Progress on Storage building, more slab questions.


Sounds like a good idea. Frost heaves must not be an issue around here because I haven't heard the first person mention them. As for the grass, the building department just said to kill it. I will be putting plastic down to to keep moisture off the floor surface.
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Old 02-23-2010, 09:47 PM   #4
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Progress on Storage building, more slab questions.


Its pretty solid. every one of those studs on the outside of the forms go into the ground a foot or more. As far as screeding it, I figured I would use a straight edge across the 12 ft side. pull from end to end. Do you see an issue with something?
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Old 02-23-2010, 09:54 PM   #5
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Progress on Storage building, more slab questions.


Organic matter will decay and shrink. Is your slab designed to be strong enough to bridge from beam to beam?
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Old 02-23-2010, 10:03 PM   #6
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Progress on Storage building, more slab questions.


I can handle the grass removal. No big deal there. I'm not sure I know what you mean when you ask if the slab is strong enough to bridge from beam to beam..
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Old 02-24-2010, 12:34 AM   #7
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Progress on Storage building, more slab questions.


staple 6 mil poly to the form and let it hang down into the footing to stop concrete from running loose.
remove the grass and replace with gravel or sand as per above.
and cut a slope from the bottom of the footer to the inside of the gravel base. the square corners you have will be the weak spot in the slab.
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Old 02-24-2010, 03:13 AM   #8
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Progress on Storage building, more slab questions.


You will need to remove all the topsoil to a depth of at least 6-8 inches. This soil is soft and will compact when an applied load such as a slap is placed over it. When this happens the slab will crack unless it is designed to be self supporting rather than ground supporting. Secondly, even though you have dug your foundations to an exact depth there is no way to tell (unless you have experience in this area) whether this new exposed ground is of suitable load bearing capacity i.e. the soil is strong enough to take the load of the foundations walls and roof. If the soil is not strong enough, the foundations will crack and then so will your walls.
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Old 02-24-2010, 07:50 AM   #9
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Progress on Storage building, more slab questions.


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You will need to remove all the topsoil to a depth of at least 6-8 inches. This soil is soft and will compact when an applied load such as a slap is placed over it. When this happens the slab will crack unless it is designed to be self supporting rather than ground supporting. Secondly, even though you have dug your foundations to an exact depth there is no way to tell (unless you have experience in this area) whether this new exposed ground is of suitable load bearing capacity i.e. the soil is strong enough to take the load of the foundations walls and roof. If the soil is not strong enough, the foundations will crack and then so will your walls.
Should I call a contractor to come out and look at what I have done and get some advice on the depth of the footings?

This is what I have minus concrete and sand:



You can see from this illustration that the form in the back and th efooring are nyc shallower than the front.

Jlhaslip - Is this what you mean by taking a piece of plastic and stapling it to the inside of the form?
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Old 02-24-2010, 08:49 AM   #10
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Progress on Storage building, more slab questions.


yes.
and lower the 45 degree cut between the footing and sand.
I would slope from the bottom of the footing, but the angle you show is better than nothing.
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Old 02-24-2010, 08:56 AM   #11
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Progress on Storage building, more slab questions.


As far as putting plastic down, where does it need to be? On top of the sand or under the sand? The plastic I'm talking about will be to prevent moisture from coming up through the surface of the slab.
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Old 02-24-2010, 11:29 AM   #12
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Progress on Storage building, more slab questions.


When doing your rebar, installing a Ufer is easy and will keep you from having to install ground rods for your electrical service.
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Old 02-24-2010, 12:09 PM   #13
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When doing your rebar, installing a Ufer is easy and will keep you from having to install ground rods for your electrical service.
What is a ufer?

I bought some pvc pipe, to come up through the footings so I can pull a wire afterwards.
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Old 02-24-2010, 01:59 PM   #14
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Progress on Storage building, more slab questions.


It sounds like you have a very sloppy and unassisting "building department" where you are.

In most of the civilized world, your wood should be 6" minimum above grade.

Dick
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Old 02-24-2010, 02:30 PM   #15
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Progress on Storage building, more slab questions.


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It sounds like you have a very sloppy and unassisting "building department" where you are.
Why do you think I'm looking to other places for second opinions.

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In most of the civilized world, your wood should be 6" minimum above grade.

Dick
You mean the bottom plate or sill plate?

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