Strapping (Framing)
Strapping is the practice of fastening thin lumber (typically 1x3 or 1x4) across the bottom of ceiling joists or roof trusses to create a flat, level plane for drywall installation. Strapping is essentially furring applied to ceilings. It is standard practice in many regions to correct truss irregularities and provide a better nailing surface.
In-Depth Explanation
Ceiling strapping is particularly important when roof trusses are used because manufactured trusses can vary slightly in depth and straightness, and their bottom chords may not lie perfectly in the same plane. Strapping across the bottom of the trusses, shimmed as needed, creates the flat, consistent surface that drywall requires for a quality ceiling finish. Strapping is typically installed perpendicular to the trusses at 16 inches on center (for 1/2-inch drywall) or 12 inches on center (for 3/8-inch drywall). In cold climates, strapping provides an additional benefit: it creates a service cavity between the drywall and the insulation, allowing electrical wiring and junction boxes to be installed without penetrating the vapor barrier or air barrier at the ceiling plane. Strapping is standard practice in New England, eastern Canada, and other northern regions, while in other areas builders may hang drywall directly to the truss bottom chords. The added labor and material cost of strapping is generally considered worthwhile for the improved ceiling flatness.
Usage Example
The drywall crew appreciated that the framer had strapped the entire ceiling at 12 inches on center, giving them a perfectly flat and well-nailed substrate to work with.