The Role of Roof Deck Tape

One of the most overlooked items in terms of waterproofing the roof deck is sealing the seams on the substrate boards of steep-slope roofing projects. This has caught the eye of several building code agencies who have already implemented codes to seal these seams with a roof deck flashing tape.

Added protection for roofing applications

By David Delcoma

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Some general contractors, both residential and commercial, have included sealing the roof deck seams as a general practice on their projects, regardless of whether it is code or not in their locale. Insurance companies are also taking notice of this practice as a good building strategy when it comes to protecting the building structure.

Changes in state and local building codes have already been implemented and more are sure to follow. Here is an overview of how to properly seal these seams for additional waterproofing protection.

The Basics

The roof deck is the structural substrate to which the final roofing system will be applied. Typically, these are wood-oriented materials such as plywood or oriented strand board (OSB). When installed, the seams between all these boards are potential leak points for water, moisture and air to infiltrate the building structure.

Typically, if water enters these seams during construction, it does not pose any real problems. However, when the roof is completed and interior work begins, this is when a roof deck tape provides the highest value to the contractor and customer.

Membrane Types

For the locales that have adopted this code, the general rule is the use of a 4-inchwide, 40-mil, self-adhering waterproofing membrane that meets ASTM D 1970. In some cases, membranes that meet AAMA 711 are also accepted. You will need to investigate your building code specifics or if not code in your locale, seek a suitable flashing membrane that meets ASTM D 1970 at a minimum.

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In essence, the code wants the membrane to be self-adhering and self-sealing around common roofing fasteners when the final roof system is installed. The self-sealing aspect is critical in terms of maintaining complete waterproofing protection before, during and after the roofing system has been installed.

What is typically not specified, but is a good idea, is to find a self-adhering flashing membrane that offers good foot traction. Make sure the membrane you choose offers this feature, whether it is an embossed traction surface or is chemically enhanced to promote better traction. Roof work is dangerous and the potential to slip on a smooth polymer facer membrane should be taken into consideration when selecting a product.

Most codes will also address what types of underlayments can be used after sealing the deck seams. These include saturated felt, synthetic or self-adhering products. Although more economical, felts and synthetic underlayments offer limited waterproofing protection as compared to self-adhering underlayments. What is interesting to note is that even when using a self-adhering underlayment, the codes still require the use of a flashing tape on the deck panel seams.

Best Practices for Sealing Deck Seams

As with most roofing projects, start at the eave and work towards the ridge. The same is true when sealing the seams on roof deck panels. Tools required for installation include a sharp utility knife, measuring tape, hand roller and any necessary safety equipment when working on a pitched roof.

Before using any self-adhering product, be sure to read and understand the manufacturer’s complete installation instructions. Most brands indicate a minimum installation temperature and maximum UV-exposure time. Depending on the manufacturer, some surfaces may require priming or there may be warnings about the types of caulks or sealants used in conjunction with the self-adhered membrane.

In general, when working with self-adhering products, roll the entire membrane thoroughly with a hand roller. This ensures maximum contact of the adhesive system to the substrate. Always pay special attention to any overlaps when rolling the membrane. On long runs, you may need to overlap the membrane to finish a section. On these end laps, the material should overlap by at least 4 to 6 inches and rolled firmly.

To most, it may seem like an unnecessary step to seal the roof deck seams. However, if it is a required code, you will want to follow the steps above to ensure the job is done right. If it is not required, it may be soon, or other builders may be adding this step as a selling feature to customers. All in all, it is just another step in the never-ending battle against Mother Nature.

David Delcoma is product marketing manager at MFM Building Products, Coshocton, Ohio. To learn more, visit, call (800) 882-7663 or email

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