Footings & Foundation
Because the footings were so deep, we hired a contractor to dig a trench for the concrete footings and foundation walls. Our local building code called for a frost footing 42" deep so the garage could later be attached to the house. We actually dug 48" so the concrete block would stack to grade.
NOTE: Detached garages are built on shallower footings (sometimes 1'x1') that have 1/2" re-bar at the edges and tie into a floating slab with wire mesh.
The concrete footings are 8" thick by 16" wide. After the footings are poured and set about 24 hours, the concrete blocks are centered on top of the footing.
We laid six courses of 8" concrete block. Then we installed 11/2" rigid insulation around the outside walls.
The sixth course of block was right at grade. To make a shelf for the slab floor to rest on and to elevate the garage framing off the ground, 4" wide x 8" high blocks on the outer edge capped the foundation.
This top course of block has anchor bolts mortared in every 6' that will fasten the sill plate. To add extra strength to the block courses, some plans call for re-bar to be mortared in every couple of feet.
At the garage door opening, we left out 2 block courses to allow for the driveway apron.
Pouring the Slab
Fill the inside slab area with about 4" of gravel and screed it level up to the block shelf. At this time, you may opt to install a plastic vapor barrier (4-mil or 6-mil) under a couple inches of gravel to prevent moisture and gases from seeping in.
Position the concrete re-bar or reinforcement mesh.
Set an expansion joint between the slab and the block. It also serves as a screed mark for the concrete. Our slab sloped 1" every 12' (a total of 2") for drainage so the expansion joint was positioned accordingly.
We also hired a contractor to complete concrete work for the slab. He poured the concrete in quarter sections starting at the back. The slab took about a truckload (8 yards) of fiber glass reinforced concrete.
Our contractor finished the concrete sections by:
1) screeding each section, pulling then sawing back and forth, with a long 2x4 to spread the concrete fairly level,
2) floating the sections with a wood hand trowel to get the concrete surface even smoother,
3) troweling the edges completely smooth with a metal hand trowel and the rest with a bull trowel (a wide metal trowel w/ long handle),
4) finishing the surface by buffing with a metal hand trowel after the concrete has set awhile. Some people prefer to give the slab small traction grooves by brooming the surface. Brooming is also recommended for the apron.
After the slab sections have set, place an expansion joint across the apron opening. You may opt to let the slab cure a day or two before pouring the apron, or pour the apron after the garage is built.
Pour the apron with a slight slope. Finish the apron like the slab, except you may want to broom finish it deeper for added traction.
Cover the slab and apron with plastic and let them cure a day or two.