Fight Water Intrusion With A Sealed Roof Deck

Fight against leaks and water intrusion by sealing your roof deck. There are several options including peel-and-stick membranes, tape, two layers of high quality felt, taped synthetic roof membranes and polyurethane foam spray. Watch the IBHS video above to see what happens when you don’t have a sealed roof deck.

Benefits Of Sealing The Roof Deck

Within minutes of losing your umbrella in a rainstorm, you and your clothes are soaked. The same will be true of your home and its contents if the roof deck is not sealed and the roof covering blows off in a hurricane or other severe rain storm. Once the roof covering and felt are gone, the wood decking will be soaked by rain, and water will pour through cracks between the wood panels that make up the deck. The water will soak the insulation and damage ceilings and walls.

Keeping Water Out

IBHS field investigations and tests at the IBHS Research Center found many typical roofing felts quickly blow off the roof in hurricane-force winds, even when the felt is new and has not been degraded by heat and exposure. Historically, the roofing industry has referred to the felt paper as a way to help water drain away once it gets behind the roof cover.

IBHS believes a better solution is to seal the roof deck against water intrusion. A roof deck should be sealed using a membrane that is strong enough, when properly attached, to stay in place if the primary roof cover blows off, and watertight enough to keep water out of the attic after the primary roof cover is lost.

Evaluate Your Options

New Roof / Re-Roofing

The following are qualified methods for sealing the roof deck. Some details have been summarized. There are five material options for sealing the roof deck.

Option 1: Two (2) Layers of Underlayment

Install two (2) layers of ASTM D 226 Type II (#30) or ASTM D 4869 Type IV (#30) underlayment in a shingle-fashion, lapped 19 inches on horizontal seams (36-inch roll), and 6 inches on vertical seams. Fasten underlayment at approximately 6 inches on center along the laps and at approximately 12 inches on center in the field of the sheet between the side laps. Secure underlayment using annular ring or deformed shank nails with 1-inch-diameter caps (button cap nails).

Notes:

  • Photographs of product labels and installation required.
  • If ASTM 4869 felt underlayment label does not specifically state it is Type IV, the product must weigh 26 lb/100 sq ft to meet this requirement.

Option 2: Self-Adhering Polymer-Modified Bitumen Flashing Tape

Self-adhering polymer-modified bitumen flashing tape at least 4-inches wide, meeting ATSM D1970, applied directly to the roof deck (or primer if required by manufacturer) to all horizontal and vertical joints in the roof deck; then a #30 ASTM D226 Type II felt or #30 ASTM 4869 Type IV felt underlayment or a reinforced synthetic underlayment which has an ICC approval as an alternate to ASTM D226 Type II felt paper is installed over the entire roof deck and secured with button cap nails.

Notes:

  • Photographs of product labels and installation required.
  • If ASTM 4869 felt underlayment label does not specifically state it is Type IV, the product must weigh 26 lb/100 sq ft to meet this requirement.

Option 3: Self-Adhering Flexible Flashing Tape

Self-adhering AAMA 711-13, Level 3 (for exposure up to 80°C/176°F) compliant flexible flashing tape, at least 3 ¾-inches wide, applied directly to the roof deck (or primer if required by manufacturer) to all horizontal and vertical joints in the roof deck; then a #30 ASTM D226 Type II felt or #30 ASTM 4869 Type IV felt underlayment or a reinforced synthetic underlayment which has an ICC approval as an alternate to ASTM D226 Type II felt paper installed over the entire roof deck and secured with button cap nails.

Notes:

  • Photographs of product labels and installation required.
  • If ASTM 4869 felt underlayment label does not specifically state it is Type IV, the product must weigh 26 lb/100 sq ft to meet this requirement.

Option 4: A Full Layer of Self-Adhering Polymer-Modified Bitumen Membrane

A full layer of self-adhering polymer-modified bitumen membrane (peel-and-stick) meeting ATSM D1970 is installed over the entire roof deck.

Note: Some local building departments (e.g., Miami-Dade and Broward counties in Florida) prohibit the use of a self-adhered membrane applied directly to the roof sheathing. Check with the local building department for any restrictions in your area.

Option 5: Reinforced Synthetic Underlayment

Reinforced synthetic underlayment which has an ICC approval as an alternate to ASTM D226 Type II felt paper installed with 1-inch-diameter cap nails, horizontal and vertical underlayment laps are sealed/taped. The synthetic underlayment must have a minimum tear strength of 20 lb per ASTM D1970 or ASTM D4533.

Option 6: Closed Cell Polyurethane Spray Foam (Existing Roof)

Install a closed cell polyurethane spray foam applied to the underside of the roof sheathing at the joints between the sheathing panels and along all intersections between roof sheathing and all roof framing members.

The minimum requirements for the closed cell spray polyurethane foam spray are:

  • Two-component spray polyurethane foam system with a minimum core density of 1.5-3.0 pcf in accordance with ASTM D1622, Standard Test Method for Apparent Density of Rigid Cellular Plastics.
  • Spray polyurethane foam adhesive system should be installed by a properly trained and qualified applicator in accordance with the manufacturer’s maintenance and installation guidelines.