Garage floors and slabs are generally very easy to heat. The conductivity of concrete allows tubing to be spaced as far apart as 24 inches, although wider spacing could furnish enough heat, experience has shown that the output is too uneven.     

SOME RULES TO FOLLOW
Run tubing parallel to the wall with the greatest heat loss
Use Blue Board under slabs, Pink Board is for areas that do not have termite problems .
Hold tubing back 18 ins. from all exterior walls
Space tubing on 6 ins.centers, the first two loops next to greatest heat loss
Tie tubing every 3 feet or less, using plastic tiewraps
Do not tie tubing in end of loop
Always use a vapor barrier under the slab
If using insulation under the slab, put vapor barrier under the insulation
If ground water comes within 3 feet of slab, insulate under slab
Edge insulation is always required
Insulation on outside of basement and footing walls is recommended
Leave the tubing and wire in bottom of slab, do not pull it up. See page 31
Use protective elbows when exiting slab
To go from one slab to another, dodge down into dirt and back up or sleeve the tubing
Do not over tighten ties. Do not use wire to tie the tubing.
If wheeling or pumping  protect the tubing with plywood