5.2 High-Wind Underlayment Rules (IBC Table 1507.1.1) & Sealed Roof Deck Options (Taped Seams, Two Layers, Full Self-Adhered)

Why the Deck Needs a Second Line of Defense

Shingles, tiles, and metal panels shed water. They do not seal it out. In a hurricane, the first failures are often at the edges: a few shingles or a strip of underlayment peel off. Once the covering is gone, wind-driven rain pours through the joints between sheathing panels into the attic. Insulation, ceilings, and contents can be ruined even though the framing survives.

Two sets of rules deal with this:

1. The code, 2021 IBC Section 1507.1.1 and its three tables, which tightens underlayment where the basic wind speed is 140 mph or more.
2. FORTIFIED Roof from IBHS, which requires a sealed roof deck on every FORTIFIED roof regardless of wind speed.

Know which rule you are quoting. The PSI exam tests the IBC, and FORTIFIED is a voluntary program (Section 5.3).

The IBC Underlayment Tables

IBC 1507.1.1 makes underlayment follow three tables. For asphalt shingles:

The high-wind attachment details in Table 1507.1.1(3):

• Caps at least 1 inch nominal diameter. Metal caps at least 32-gauge (0.0134 inch); power-driven metal caps at least 0.010 inch; plastic caps at least 0.035 inch thick at the outside edge.
• Cap nail shank at least 0.083 inch for ring-shank or 0.091 inch for smooth-shank nails.
• Cap staples at least 21 gauge.
• Long enough to penetrate through the sheathing, or at least 3/4 inch into it.

Two points trip people up. First, cap staples meeting these specs are allowed. Second, smooth-shank cap nails are allowed if the shank is at least 0.091 inch. It is bare staples and uncapped fasteners that don't meet the high-wind table.

Tile, metal, slate, wood, and the other coverings have their own rows in the same tables. Tile, for example, uses ASTM D226 Type II, ASTM D2626 Type I, or D6380 Class M roll roofing.

The Two IBC Alternatives

IBC 1507.1.1 lists two exceptions that may be used instead of the table requirements:

Exception 1: Taped deck joints. Apply a strip of self-adhering polymer-modified bitumen membrane (ASTM D1970) at least 4 inches wide over all joints in the roof deck, installed per the membrane manufacturer's instructions for that deck material. Then apply an underlayment that would be approved for the covering at wind speeds below 120 mph. Taping the joints is what the industry calls a sealed roof deck.

Exception 2: Two layers of heavy felt. Use two layers of ASTM D226 Type II or ASTM D4869 Type IV:

• a 19-inch strip along the eave, then 36-inch-wide sheets overlapping successive sheets 19 inches;
• end laps 4 inches, offset 6 feet;
• a fastening grid of 12 inches between side laps and 6 inches at side and end laps;
• cap nails with caps at least 1 inch and the same shank rules as above.

The 19-inch overlap on a 36-inch sheet leaves a 17-inch exposure, so every point on the deck has two plies.

FORTIFIED Roof's Sealed Roof Deck

FORTIFIED requires the deck to be sealed on every designated roof. Its checklists and standard details offer three routes:

1. Tape plus underlayment: 4-inch nominal ASTM D1970 tape, or 3-3/4-inch AAMA 711 Level 3 tape, over all deck seams. Then a D226 Type II or D4869 Type III or IV felt, or an equivalent synthetic with an evaluation report, fastened in a close grid of cap nails.
2. Two layers of underlayment, felt or approved synthetic, installed with half-lap coverage and the program's close cap-nail grid.
3. Full self-adhered membrane: ASTM D1970 over the entire deck. FORTIFIED recommends a bond-break layer, such as #15 felt, between the membrane and shingles.

FORTIFIED's standard details set the exact lap widths and fastener spacing for each roll width and product. Follow the current detail sheet for the product you use.

Material Notes

• ASTM D1970 covers self-adhering polymer-modified bitumen sheets. It sets a minimum thickness of 40 mils and tests the sheet's ability to seal around nails. For hot locations, such as under dark metal or tile, use a product rated for high temperatures by its manufacturer.
• Synthetic underlayments are tested to ASTM D8257 and accepted through evaluation reports. The 2021 IBC tables list D226, D4869, and D6757 by name, so a synthetic used where a listed type is required needs an evaluation report showing equivalence. FORTIFIED requires one showing equivalence to D226 Type II and passing the D4869 shower test.
• Prep: tape and self-adhered membranes need a clean, dry deck. Prime where the manufacturer requires, such as some OSB or cold conditions, and roll the tape down firmly.

Attic Moisture Caution With Full Self-Adhered Membranes

A full layer of self-adhered membrane is a strong water barrier and also a vapor retarder on the outside of the deck. In a hot, humid climate, moisture that reaches the attic must leave through ventilation, because it can no longer dry outward through the deck. Manufacturers and FORTIFIED stress adequate attic ventilation with full-coverage membranes (Section 9.4). An unvented conditioned attic designed under IRC R806.5, with insulation at the roof deck, is the other accepted approach.

Choosing an Approach