Modified-Bitumen Cool Roof Cap Sheets – March 2020

April 1, 2020

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TITLE: Modified-Bitumen Cool Roof Cap Sheets


OBJECTIVE: To provide an informative advisory regarding maintaining the solar reflectivity of bright-white-colored, granule-surfaced modified-bitumen cap sheets.


Modified-bitumen material manufacturers have developed bright- or true-white-colored, granulesurfaced modified-bitumen cap sheets to provide higher solar reflectivity and thermal emissivity values. The granules used on these cap sheets are a “bright” white color compared to the traditional “white”-colored, granule-surfaced cap sheets, which are actually light gray in color. Both APP and SBS modified-bitumen sheets are currently available with these white granules that can be applied in hot asphalt or cold adhesive or applied with heat-fusing techniques or self-adhering technology.


An important feature of white-colored, granule-surfaced modified-bitumen cap sheets is that they meet the specified solar reflectivity upon initial installation. This requires that the applicator implement certain precautions during the handling, storage, and installation process to prevent damage or soiling of the cap sheet, which can diminish its reflectivity.


Installation Concerns and Precautions Use the following methods to protect the cap sheet.

  • Use sacrificial sheets of the top ply (placed surfacing side down) under mop carts, material pails, and other equipment within the subject area.
  • Avoid placing a sheet with a bituminous surface in contact with the top surface of the installed finish ply.
  • Place protective material under pails, fire extinguishers, and similar items.
  • Use disposable “booties” on worker’s shoes.
  • Keep metallic items, such as carbon-steel fasteners, shavings from cutting or drilling sheet metal, from the surface to minimize corrosion stains. Delay the installation of the cap sheet until other ancillary work, such as installation of saw-cut reglets, masonry work, sheet metal flashing installation, MEP-related rooftop work, can be completed.
  • Embed white-colored granules into bituminous bleed-out at seams to prevent tracking the bituminous material across the roof.
  • Utilize proper torching techniques and application procedures during the installation process to maintain a uniform color of the finish ply.
    • Avoid “scorching” of the adjacent sheets while fusing side laps.
    • Direct the flame toward the subject sheet by extending the arm/torch over the adjacent sheet and pointing back at the subject sheet to melt the back coating, thus achieving a properly fused sheet without scorching in-place sheets.
  • Ensure properly sequence work to avoid water runoff from previously installed smoothsurfaced modified bitumen base plies from adjacent or higher roofs onto an installed cap sheet.

Material-Related Issues

“Tobacco-juicing,” a visible, brown-colored staining, has been observed on these bright-whitecolored, granule-surfaced cap sheets after removal from manufacturers’ wrapping upon delivery to a jobsite in preparation for application. When it occurs, the staining is prevalent and widespread throughout the sheet. Manufacturers have indicated that the staining is due to light oils from the bitumen migrating from within the sheet and depositing on the granules during storage. This condition most commonly occurs in unconditioned warehouses. Manufacturers have stated that the staining will “bleach-out” after installation and exposure to sun and rain over some period of time (three to six months), depending on geographic location, time of year, and the weather. While this event may occur, the color of the cap sheet upon completion of application and acceptance by the building owner would not meet the published solar reflectivity. Even after the sheet experiences the bleaching-out, the resulting solar reflectivity may not meet the published initial solar reflectivity. Manufacturers have reported that actions to avoid this condition may include storing this product in conditioned warehouse space or manufacturing the product upon demand, avoiding storage in an unconditioned warehouse, and shipping the product direct to the project. It is unclear at this time if storage on site can cause this phenomenon to occur or how long it takes it to occur. One practice that appears to be successful in maintaining the bright white color of the granules is the placement of a thin plastic film over the granule surfacing while the material is rolled, following completion of the manufacturing process, thus preventing direct contact of the bright white granules with the smooth-surfaced bituminous underside of the sheet.

In-Service Related Issues

• The in-place reflectivity of these types of sheets may vary. Maintaining a uniform granule embedment is critical for these sheets to provide the appropriate solar reflectivity. Loss of granules and exposure of the underlying black-colored bitumen greatly diminish the solar reflectivity characteristics and reduce the service life of the material.

• Dirt or other atmospheric deposits on the cap sheet can also diminish its solar reflectivity, but this condition can readily be rectified by appropriate cleaning methods.

RECOMMENDATION Designers should be cognizant of and specify the required protocols and measures needed to be implemented during the design, storage, handling, installation, and maintenance of these types of cap sheets in order to optimize the published reflectivity for the finished product. The photos that follow are examples of conditions of concern.

Figure 1 – Discolored granule surfacing on white-colored, cool roof cap sheet. 2

Figure 2 – Discolored granule surfacing on white-colored, Cool roof cap sheet and top-ply base flashing.

Figure 3 – Asphaltic bleed-out along side and end laps of white-colored, cool roof cap sheet.

Figure 4 – Asphaltic bleed-out along side lap of white-colored, cool roof cap sheet.

Figure 5 – Corrosion stains on white-colored, cool roof cap sheet.

Figure 6 – Corrosion stains on white-colored, cool roof cap sheet.

Figure 7 – Discoloration of white-colored, cool roof cap sheet from “rust-stained” drainage from rooftop piping.

Figure 8 – Discoloration of white-colored, cool roof cap sheet from emissions from rooftop unit. Figure 9 – Discoloration of white-colored, cool roof cap sheet from ponding water in valleys.

Figure 10 – Discoloration of white-colored, cool roof cap sheet from ponding water.

Figure 11 – Plastic film applied on roll between granules and back of roll.

Figure 12 – Granules embedded in bleed-out along side and end laps of white-colored, cool roof cap sheet.