New slab foundation

[cmullen]

Hello all,

I'm building a garage and doing most of the work myself. I've helped pour quite a few cement slabs, but never had to do it all by myself, and am running into a few questions.

The Plan: Dig a perimeter footing at least 12" deep by 12" wide, build forms to the high point in the yard, (very slight slope). Fill in with sand, till 6" from top of forms and then pour footings and slab all in one fell swoop.

The Questions:
1st. The vapor barrier... I've been reading conflicting advice on this, one set of people have told me that the vapor barrier goes between the sand and the cement. Another group has said that the vapor barrier needs to go down, then the sand, then the cement.

2nd. I'm building on a slightly slopped piece of land. I dug a 12"X12" perimeter and leveled the highest part of the ground. At the high end of the forms, the slab will be 6" above the ground level. On the low end, the slab will be 12" above the ground. My question is this, do you have to remove the topsoil accross the entire slab area? I'd rather just lay the vapor barrier (or sand and then vapor barrier) over the topsoil, then tap it down, and call it good. I'm concerned though that this might cause settling to occur later and cause cracks.

3rd. How do you calculate amount of cement needed?

Thanks!

[Rich]

1 - I typically put the sand and then the vapor barrier. Otherwise the vapor barrier really does no good because the sand will not be able to do it's job which is allow water to pass through it.

2 - I would suggest removing the grass and topsoil. If you don't the grass and roots will eventually decay and will leave a void under your slab which will cause cracking.. etc.

3 - Take the width times the length times the depth (in feet). That gives you cubic feet - divide that by 27 to get cubic yards. Get the volume of the footings the same way. Or you can use the calculator I have on the site to figure the cubic yards of the footings.

Hope that answers your questions. Let me know if you need any other help.