1) Size the slab: You should make
your slab 4” wider than the width of the building
and 6” longer that it's length. For instance,
on a 20' x 30' garage you would make your slab 20' 4"
x 30' 6". This is so you don’t break or crack
the edge of the slab when affixing the concrete anchor
expansion bolts into your slab.
2) Order the Correct Concrete Mix and Amount:
Your floor should be a minimum of 4” thick. Make
it 6” thick if you are intending large vehicles
such as trucks or larger RVs to be parked on it. The
concrete should be a minimum of 2500 psi concrete with
fibermesh reinforcement added at the batch plant OR
#3 rebar on 24” centers. You should also consider
going with a concrete mixture of 4000psi along with
the extra two inches of concrete if you are intending
more than a normal amount of weight.
Be sure to saw cut expansion joints within an appropriate
time after the pour is finished or trowel in the expansion
joints while the finishing is occurring.
As far as knowing how much concrete to order, you can
generally call the concrete company and give them the
specs of your slab. They can tell you how much to order.
Also, it's a good idea to have a place to pour any spare
concrete when your slab is poured. The driver will want
to empty his truck on-site. The cost of a concrete slab
to be formed and poured, including expansion joints,
would run approximately $2.75 per square foot in our
local area for a 4" slab on a level site.
3) Footing Requirements: (International
Building Code 2000 or “IBC 2000” requirements)
At the same time you pour the floor, you are going to
pour the perimeter footings. This is called a monolithic
pour (or monolithic slab). Perimeter footings need to
be 12” deep (you can include the 4” of floor
as part of the 12”) and 16” wide. The footings
will have to have two #4 rebars top and bottom, continuous
run around the complete perimeter. In our local area,
the footings would cost approximately $11.50 per lineal