Materials & Shingles · FAQ

Does Illinois humidity affect the OSB versus plywood decision?

Yes, and it’s the strongest argument for plywood in our climate. OSB swells about 15 to 20% in thickness when it hits high humidity, and unlike plywood it doesn’t fully return to its original dimension as it dries. Over five to ten years, repeatedly soaked OSB develops a wavy surface that telegraphs through the shingles.

In Madison County, where summer dew points sit above 70 for weeks at a time, Trill recommends plywood (typically 5/8 inch CDX) on any home built before 2000 or any home with marginal attic ventilation. New homes with proper ridge venting and synthetic underlayment do fine on 7/16 OSB. The price difference is roughly $1.20 per sheet, or $300 to $400 on a typical decking job.