Sealed Roof Deck

A sealed roof deck is a roof sheathing layer (usually plywood or oriented strand board) whose joints have been sealed so water cannot pass through the gaps between panels. It is one way of providing a secondary water barrier and is a core requirement of the FORTIFIED Roof standard developed by the Insurance Institute for Business & Home Safety (IBHS).

How it is built

Two methods are common:

1. Seam tape. Self-adhering flashing tape is rolled onto every horizontal and vertical joint in the deck. A standard underlayment is then installed over the whole roof and the covering goes on top.
2. Full membrane. A self-adhered underlayment is applied over the entire deck, sealing joints and field at the same time.

Deck panels must be clean and dry for the adhesive to bond. Edges, valleys, penetrations and transitions need careful detailing because water driven by wind will find any gap.

Why it works

When high winds strip shingles or tiles, the regular underlayment is often torn or lifted too, leaving deck joints exposed. Water entering through those joints soaks insulation and ceilings. IBHS testing found that one inch (25 mm) of rain on an unprotected deck can admit water equivalent to roughly nine bathtubs, and that sealing the deck can cut water intrusion by up to 95 percent.

Related upgrades

In the FORTIFIED Roof standard, a sealed deck is combined with:

• 8d ring-shank nails at closer spacing for the deck, which IBHS says can roughly double uplift resistance compared with smooth nails;
• a wider drip edge and a fully adhered starter strip at the edges;
• coverings tested for wind (for shingles, ASTM D3161 Class F or ASTM D7158 Class H).

Where it is used

Sealed decks are most common in the US Gulf and Atlantic coastal states, especially where state grant programs (for example in Alabama) or insurance incentives reward FORTIFIED construction. The concept applies anywhere wind-driven rain is a hazard, including typhoon and cyclone regions.