Concrete foundation for shed

UpToNoGood · Jul 19, 2011

I want to do some foundation work myself and have never worked with concrete other than mixing a bag and dumping it in a hole for a fence post.
Any way I picked up 40 acres and want to build a vacation/retirement house over time, but figured I would start with a shed first.
I want the shed to have a concrete floor and maybe cinder blocks for 2 layers followed by wood and a Metal roof.
I am not going to work the concrete when it is poured that would be hired out for sure.
I do plan to set the form myself before the concrete is poured. About 15x20. So my question is would re-bar be needed or could I just use the wire mesh?
I will also be setting a form for a 500gal propane tank to sit on for the same concrete pour.
Think 5 inch thick will be fine for both?

mchildress · Jul 19, 2011

I would use rebar and might think about a perimeter beam around the outside edge.

UpToNoGood · Jul 19, 2011

What is "a perimeter beam"?

mjncad · Jul 19, 2011

If you're going to form it yourself; then I assume you'll be doing the reinforcement yourself.

In any case, I'd go with rebar on chairs regardless of who installs the reinforcement. WWF (Welded Wire Fabric) is always thrown on the ground and the concrete contractor says his guys will pull it up to the middle during the pour. Yeah right as the guys doing the pulling are also walking on it. Besides, do you want to wrestle with a roll of fabric that doesn't want to lay flat without persuasion?

5" thick should be adequate, 6" is better. Just make sure you have 2-1/2" and preferably 3" of concrete cover over your reinforcement. #4 (1/2") rebar 24" O.C. each way should give you a pretty strong slab. Put a control joint in each direction and you should be good to go. Sawcut control joints are better than tooled joints, especially if you're going to have anything on casters in the shed. A 3/4" - 1" deep sawcut is adequate. You can even rent a walk behind concrete saw and do the joints yourself. You can always caulk the joints to keep crud out of them.

crash325 · Jul 19, 2011

Just my way of thinking.

It would be best if you got someone that has done concrete work to help you. Reading about how and what is not the same as having done it.

Yes you should use Re-bar, but is a waste if not done correctly.

mjncad · Jul 19, 2011

UpToNoGood said:
What is "a perimeter beam"?

Also known as a grade beam if separate from the slab or a thickened edge if part of the slab. A thickened edge will beef up the slab. Say 2 - 3 times the slab thickness, about 8" - 12" wide with a 45-degree transition from edge bottom to the slab bottom.

What's you frost line?

UpToNoGood · Jul 19, 2011

I am fine with using Rebar.

The frost line is about 2 feet.

TedLaRue · Jul 19, 2011

Sorry for this long post. I got carried away.

I also recommend rebar on chairs instead of mesh. The savings from using mesh would be insignificant compared to the peace of mind from using rebar on your small slab.

You can buy plastic chairs to hold the rebar up to about 1/2 the thickness of your slab, or you can use rocks/stones/bricks that are about the right thickness. Don't use wood; it will wick moisture up to the rebar and cause it to rust.

-----

Around here, 4" is a standard slab thickness, and should be sufficient for a shed floor. While typing this, I see others have recommended 5" or 6", but I think that is overkill for a reinforced shed floor. I also think control joints are unnecessary for an indoor floor unless you expect the underlying soil to be very moist and also subject to serious freezing.

Also, around here under the slab we lay down some sand or gravel covered by a layer of builder's plastic to control moisture. That may be overkill for a shed, especially in your area.

You didn't say anything about a footer (others are calling this a "perimeter beam") around the perimeter, and since I'm not familiar with Texas soil structures, I'll leave that to someone else. If you don't need a full size footer, you might consider digging a trench around the perimeter a little deeper than your slab to give you some extra thickness there.

A full size footer/perimeter beam should be twice as wide as it is thick, and twice as wide as the blocks you are using as your foundation wall. For example, if you are using 8" wide blocks, then the footer should be 16" wide, and 8" thick. If you have special soil conditions, then those numbers might be different for you. There should also be two bars of rebar running down the center of the footer's thickness and spaced about 9 to 12 inches apart with pieces wired together where they overlap.

That is probably overkill for your short 16" tall block wall, but you should probably have a little more thickness around the perimeter, and make sure there is at least one stick of rebar in it.

-----

For our propane tank, I poured two separate mini-slabs, one for each "leg" of the propane tank. I chose two thick mini-slabs rather than one long thinner slab. Less chance of a crack developing in the middle. Just make sure the distance between the centers of the slabs matches the distance between the legs on the tank. Your propane supplier can tell you the distance between the legs.

TedLaRue · Jul 19, 2011

Something else to consider:

In some locations you will be taxed for "permanent" buildings, but not for temporary buildings. If you build a shed having 4x6 timbers serving as "skids" as the foundation, you can move it, so it isn't permanent and not taxable. If you pour a slab, it is permanent and taxable.