DisasterSafety — Re-Roofing Sealing Roof Deck

DisasterSafety - Re-Roofing Sealing Roof Deck

Benefits of sealing the roof deck

Contents

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 rains, which will eventually seep through attic insulation and damage ceilings and walls. Watch what happens to a home when the roof covering is lost and the roof deck is not sealed in this video from the IBHS Research Center .

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 water tight enough to keep water out of the attic after the primary roof cover is lost.

Selecting the Right Sealed Roof Deck Option

Determine what type of roof cover you plan to use: shingle, tile, metal or another type. The high wind and water resistant barrier that you use to seal the roof deck will vary according to roof covering type.

Evaluate your options :

Options for shingle or metal roofs :

Install a “peel and stick” membrane over the entire roof deck:

Cover the entire roof deck with a full layer of self-adhering polymer modified bitumen membrane (“peel and stick”) meeting ASTM D1970 requirements. In some instances, the ability of the self-adhered membranes to adhere to Oriented Strand Board (OSB) sheathing may be compromised by the level of surface texture, the amount of wax added to the OSB panel, and the job site conditions.

In applications where membrane adhesion to OSB is marginal, apply a primer to the OSB panels to ensure the proper attachment of the self-adhering membrane to the sheathing. It should also be noted that most manufacturers warn that attic ventilation must be good if these products are used because moisture in the attic cannot evaporate through these types of membranes.

Roofers are finding that shingles are bonding to many of these membranes and this could lead to damage of the sheathing when it comes time to replace the shingles. Consequently, the membrane should be covered with a bond break such as a 15# ASTM D226, Type I underlayment. This underlayment for the shingles only needs to be fastened sufficiently to keep it on the roof surface and provide safety to the roofers until the shingles are applied.

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. 

Install 4-in. to 6-in. wide “peel and stick” tape installed over all the wood roof panel seams, covered by a 30# felt underlayment over the entire roof.

Apply a self adhering polymer modified bitumen flashing (“peel and stick”) tape 4-in. to 6-in. wide directly to the roof deck to seal the horizontal and vertical joints in the roof deck. In some instances, the ability of the self-adhered membranes to adhere to Oriented Strand Board (OSB) sheathing may be compromised by the level of surface texture, the amount of wax added to the OSB panel, and the job site conditions.

In applications where membrane adhesion to OSB is marginal, apply a primer to the OSB panels to ensure the proper attachment of the self-adhering tape to the sheathing. Do not nail or staple the tape to the roof sheathing. Refer to the manufacturer’s recommendations for installation.

Next apply a code compliant 30# ASTM D226, Type II underlayment over the self-adhering tape. This underlayment must be attached using annular ring or deformed shank roofing fasteners with minimum 1-in. diameter caps at 6 in. on center spacing along all laps and at 12 in. on center in the field or a more stringent fastener schedule if required by the manufacturer for high wind installations. The minimum horizontal overlap between rolls is 2 in. and the minimum overlap at the ends of rolls (vertical) is 6 in.

Nails with plastic or metal caps may be used in areas where the design wind speed is less than 140 mph. Metal caps should be used in areas where the design wind speed is greater than or equal to 140 mph. A self-adhering polymer modified bitumen membrane complying with ASTM D1970 may be installed over this underlayment in order to comply with FEMA technical fact sheet No. 19, Option 1 which is recommended for areas with design wind speeds greater than 120 mph.

Install a high tear strength synthetic underlayment with all vertical and horizontal seams taped.

Apply a reinforced synthetic roof underlayment which has an ICC approval as an alternate to ASTM D226 Type II felt paper. The synthetic underlayment must have minimum tear strength of 20 lbs. per ASTM D1970 or ASTM D4533.

This underlayment must be attached using annular ring or deformed shank roofing fasteners with minimum 1 in. diameter caps at 6 in. on center spacing along all laps and at 12 in. on center in the field or a more stringent fastener schedule if required by the manufacturer for high wind installations. Metal caps are required for areas where the design wind speed is greater than or equal to 140 mph.

The minimum horizontal overlap between rolls is 2 in. and the minimum overlap at the ends of rolls (vertical) is 6 in.

All seams should be sealed with a compatible adhesive or a compatible 4-in. wide tape except for steep slope roofs with a 12/12 roof pitch (45 degrees) or greater.

Horizontal seams on steep slope roofs with a 12/12 pitch (45 degrees) or greater do not have to be sealed with adhesive or tape provided the overlap for horizontal seams is at least 18 in.

A self-adhering polymer modified bitumen membrane complying with ASTM D1970 may be installed over this underlayment in order to comply with FEMA technical fact sheet No. 19, Option 1 which is recommended for areas with design wind speeds greater than 120 mph.

Options for concrete and clay tile roofs :

Install a “peel and stick” membrane over the entire roof deck. Cover the entire roof deck with a full layer of self-adhering polymer modified bitumen membrane cap sheet (“peel and stick”) meeting ASTM D1970 requirements.

In some instances, the ability of the self-adhered membranes to adhere to Oriented Strand Board (OSB) sheathing may be compromised by the level of surface texture, the amount of wax added to the OSB panel, and the job site conditions.

In applications where membrane adhesion to OSB is marginal, apply a primer to the OSB panels to ensure the proper attachment of the self-adhering membrane to the sheathing. It should also be noted that most manufacturers warn that attic ventilation must be good if these products are used because moisture in the attic cannot evaporate through these types of membranes.

In addition, 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.

Install 4-in. to 6-in. wide “peel and stick” tape installed over all the wood roof panel seams, covered by a 30# felt underlayment over the entire roof. Apply a self adhering polymer modified bitumen flashing (“peel and stick”) tape 4-in. to 6-in. wide directly to the roof deck to seal the horizontal and vertical joints in the roof deck.

In some instances, the ability of the self-adhered membranes to adhere to Oriented Strand Board (OSB) sheathing may be compromised by the level of surface texture, the amount of wax added to the OSB panel, and the job site conditions. In applications where membrane adhesion to OSB is marginal, apply a primer to the OSB panels to ensure the proper attachment of the self-adhering tape to the sheathing.

Do not nail or staple the tape to the roof sheathing. Refer to the manufacturer’s recommendations for installation.

Next, apply a code compliant 30# ASTM D226, Type II underlayment over the self-adhering tape. This underlayment must be attached using annular ring or deformed shank roofing fasteners with minimum 1-inch diameter caps at 6 in. on center spacing along all laps and at 12 in. on center in the field or a more stringent fastener schedule if required by the manufacturer for high wind installations.

The minimum horizontal overlap between rolls is 2 in. and the minimum overlap at the ends of rolls (vertical) is 6 in.

Nails with plastic or metal caps are allowed in areas where the design wind speed is less than 140 mph. Metal caps are required for areas where the design wind speed is greater than or equal to 140 mph.

Finally, apply a self adhering polymer modified bitumen cap sheet complying with ASTM D1970 over this underlayment.

Another option :

Seal the roof deck by applying a closed cell spray polyurethane foam to the underside of the decking.

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.

Ensuring proper installation

Provide these installation instructions for sealing the roof deck to your contractor to ensure proper product selection and installation.


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