Flat Roof Decking: OSB vs Plywood and Your Other Options

The roof deck is the structural substrate on which your insulation and waterproofing system sits. Choosing the right decking material and ensuring it's in good condition is critical to the performance and longevity of the whole roof build-up.

OSB (Oriented Strand Board)

OSB3 is the most commonly specified flat roof decking for domestic projects. It's affordable, structurally sound, and available in large sheets (2.4m × 1.2m). It must be the correct grade (OSB3 = load-bearing in humid conditions). OSB is sensitive to moisture and must be kept dry during installation.

Plywood

WBP (weather and boil proof) plywood is an alternative to OSB that some contractors prefer. It has better moisture resistance than OSB and machines cleanly at edges. It's generally more expensive but more tolerant of delays between deck installation and roofing.

Concrete and Screed Decks

Commercial flat roofs are often on concrete structural decks. These require a different approach — typically a vapour barrier and insulation system above the concrete, with the waterproofing membrane on top. Existing concrete decks may also have falls formed in screed.

Checking and Replacing Decking

When re-roofing, the deck must be inspected carefully after stripping. Any wet, soft, or delaminated boards must be replaced before re-roofing. Rotten decking will compromise the bond of any adhesive membrane and will eventually sink, causing pooling. We always survey the deck and include any necessary replacements in our quotes.