Concrete Slab & Slab-on-Grade Foundation Guide

The slab foundation commonly used for sheds is called a slab-on-grade foundation.This combines a 3 1/2″-to 4″-thick floor slab with a 8″-to 12″-thick perimeter footing that provides extra support for the walls of the building.The whole foundation can be poured at one time using a simple wood form.

Because they sit above ground,slab-on-grade foundations are susceptible to frost heave and in cold-weather climates are suitable only for detached buildings.

Specific design requirements also vary by locality,so check with the local building department regarding the depth of the slab,the metal reinforcement required,the type and amount of gravel required for the sub base,and whether plastic or an other type of moisture barrier is needed under the slab.

The slab has a 3 1/2″-thick interior with a 8″-wide×8″-deep footing
along the perimeter.The top of the slab sits 4″above ground level,or
grade.There is a 4″-thick layer of compacted gravel underneath the slab
and the concrete is reinforced internally with a layer of 6×6″10/10
welded wire mesh(WWM).(In some areas,you may be required to add rebar in
the foundation perimeter check the local code.)After the concrete is
poured and finished,8″-long J-bolts are set into the slab along the
edges.These are used later to anchor the wall framing to the slab.
A slab for a shed requires a lot of concrete:an 8×10-ft.slab
designed like the one in this project calls for about 1.3 cubic yards of
concrete;a 12×12-ft.slab,about 2.3 cubic yards.Considering the amount
involved,you’ll probably want to order ready-mix concrete delivered by
truck to the site (most companies have a one-yard minimum).Order
air-entrained concrete,which will hold up best,and tell the mixing
company that you’re using it for an exterior slab.
An alternative for smaller slabs is to rent a concrete trailer from a
rental center or landscaping company;they fill the trailer with one
yard of mixed concrete and you tow it home with your own vehicle.
If you’re having your concrete delivered,be sure to have a few
helpers on hand when the truck arrives;neither the concrete nor the
driver will wait for you to get organized.Also,concrete trucks must be
unloaded completely,so designate a dumping spot for any excess.Once the
form is filled,load a couple of wheel barrows with concrete(in case you
need it)then have the driver dump the rest.Be sure to spread out and
hose down the excess concrete so you aren’t left with an immovable
boulder in your yard.

If you’ve never worked with concrete,finishing a large slab can be a
challenging introduction;you might want some experienced help with the

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