Seasoned 1" x 3" straight-grained soft woods such as pine, spruce or fir make ideal furring strips.
If the ceiling to which furring strips are to be applied has joists hidden by an existing ceiling, these joists should be located and marked before the job is started. Joists can be located by driving a nail into the ceiling, or by using a stud finder.
Joists are usually located every 16" or 24". After the first joist is found, measure across 16" and try again. After you have determined the spacing, locate and mark all joists with a chalk line so you can attach the furring strips without having to locate the joists again on each run.
Furring strips should be nailed across the joists (at right angles to the joists).