Materials & Shingles · FAQ

What decking thickness does Illinois code require?

Illinois follows the IRC, which sets a minimum of 7/16 inch OSB or 1/2 inch plywood for rafters spaced at 24 inches on center. Most Madison County homes built since the 1990s use 7/16 OSB at 24-inch centers, which meets code but sits at the bottom of the strength envelope.

Trill’s standard upgrade is 5/8 inch plywood, which carries 40% more weight, holds nails better, and resists humidity-driven swelling. The upgrade adds roughly $1.20 per square foot of replaced area and is something we recommend explicitly when the existing deck is 30+ years old, when the rafters are at 24-inch centers, or when the home is in an exposed wind position.