Pouring your Garage Foundation

by Murray Anderson

A solid garage needs to be built on a solid foundation and while preparing and pouring a garage foundation is usually a job home owners would prefer to leave to the pros, with hard work, patience and attention to detail, a competent DIYer can build a solid garage foundation. Here's how

Understand the basics

A garage foundation is typically built in a number of stages - site preparation, laying out and pouring the footings, foundation walls and finally, installing the concrete slab/floor.

Site preparation

First, contact your local municipal services and ask them to mark any gas, power or telephone lines that might be near your building site so you will know where you can safely dig.
Use a chalk line to lay out the perimeter of your garage. Use string and wooden stakes driven into the ground at the corners to mark the footprint.

Footings

Dig the trenches for your footings around the perimeter of your garage. Local building codes will specify the minimum depth and width of your trenches but in general the trenches should be at least 12” - 18” wide and minimum18” deep. (in areas where the ground freezes, the code may require footings at least 48” below ground level to get below the 'frost line' and prevent the concrete from shifting).
While digging the footings, also remove 3” to 4” of soil from the floor area to accommodate the concrete pad.
Install wooden forming boards (usually 2 X 10 or 2 X 12) in the footing trenches , holding them in place with wooden stakes. (these boards will be removed after the footings have set up).
Next, install 1/2” rebar horizontally on all the footings, using rebar chairs to hold the rebar 3” off the bottom and sides off the trench.
Pour the concrete for the footings and after the concrete has been poured but before it has set, install rebar every 12” to 16” extending vertically 8” to 10” above the surface This will tie the foundation walls into the footings.

Foundation walls

Typically foundation wall are 8” to 10” high and centered on the footings. Here again installing rebar horizontally in the foundation walls before pouring concrete and after the concrete has been poured, insert anchor bolts into the moist concrete extending 2” to 3” inches above the surface to hold the sill plates of the wooden garage walls.

Concrete Pad/floor

Spread a 2” to 3” thick layer of sand or fine gravel then compact it down..
Next, install a vapor barrier over the sand layer to help keep moisture out of your finished garage, and once again install rebar (this time in a grid pattern 16” on center) over the floor to provide stability to your concrete pad.
After the concrete is poured into the floor area use a long 2” X 4” to level the surface and provide a slightly rough surface that won't be too slippery to walk on.