Pro-Grade 988 Silicone Roof Coating

Pro-Grade 988 Silicone Roof Coating is a Henry product in the roof coatings and sealants range. It is a 100% silicone, high-solids, solvent-free, one-component, moisture-curing silicone rubber roof coating system. The technical data sheet carries a revision date of 09/27/2023. The coating is applied over existing roofs. Listed substrates include smooth asphaltic built-up roofing (BUR), smooth or granulated cap sheet, single-ply membrane, well-adhered acrylic coating, metal, concrete, sprayed-in-place polyurethane foam and various aged membrane roofing. As a silicone roof coating, it is formulated for use on both low-slope and pitched roofs, on commercial and residential buildings.

Because of its high solids content and the absence of hydrocarbon solvents, the coating can be applied in excess of 50 mils in a single coat without blistering while keeping maximum adhesion. Typical values and their test methods follow.

• Solids content by volume: 92% +/-3 (ASTM D2369).
• Durometer hardness: 42 Shore A (ASTM D2240).
• Tensile strength: 320 psi (ASTM D412).
• Elongation at break: 280% (ASTM D412).
• Low temperature flexibility: -15 F (-26.1 C) (ASTM D522).
• Permeability: 4.6 perms (ASTM E96).
• Water absorption: 0.0005% (ASTM D471).
• Flash point: 140.9 F (60.5 C) (ASTM D93).
• VOC content: 10 g/l maximum.
• Flame spread: Class A (ASTM E108/UL790).

Water leakage resistance is a pass at 22 dry mils or more (ASTM D7281). Water resistance under hydrostatic pressure is 100 psi at a minimum of 24 wet mils (AATCC 127, option 1). Accelerated QUV weathering for 5,000 hours (ASTM G154) shows no degradation. The coating is listed as permanent ponding water resistant, mold and mildew resistant, and resistant to ozone, ultraviolet radiation and temperature extremes. It chemically bonds with roof substrates as it cures. Fungi resistance is rated excellent under ASTM G21, and the coating meets or exceeds Henry Dirt Pick-Up Resistance Test Method MOT P-14. The formula uses Dirt Pick-Up Resistance (DPUR) technology.

The reflectance values apply to the white coating color only.

• Solar reflectance: 87 initial and 84 after 3 years aged (ASTM C1549; ASTM D7897 with C1549).
• Solar Reflectance Index (SRI): 110 initial and 106 after 3 years aged (ASTM E1980; ASTM D7897 with E1980).
• Thermal emittance: 0.90 initial and 0.90 after 3 years aged (ASTM C1371; ASTM D7897 with C1371).

The packaging offers three colors: white, gray and tan.

• CRRC listing: Product ID# 0620-0053 (Cool Roof Rating Council rated).
• Meets the requirements of California Energy Commission (CEC) Title 24 Section 118(i)3.
• Florida Product Approval FL10889-R3.
• Miami-Dade County Product Control Approved, NOA# 23-0316.02.
• UL Classified: cements and coatings for roofing systems as to external fire exposure <74LM>.
• FM Approved as a roof coating under its conditions of approval, when installed as described in the current edition of the FMRC approval guide.
• Tested and certified by NSF International in accordance with NSF Protocol P151, Health Effects from Rainwater Catchment System Components.
• Meets or exceeds ASTM D7281, the test method for water migration resistance through roof membranes.
• Meets or exceeds ASTM D6694, the specification for liquid-applied silicone coating used in spray polyurethane foam roofing systems.

Under NSF Protocol P151 the coating is applied in 1 to 3 coats to a maximum field-use dry film thickness of 30 mils, with a maximum of 15 mils per coat. Recoat cure is 24 hours at 100 F (38 C) and final cure is 48 hours at 100 F (38 C).

The roof is washed with a non-filming detergent such as TSP or a TSP substitute, using a pressure washer of at least 2,000 psi. Water is not to be injected into the roof substrate. Stubborn dirt or grease is scrubbed with a stiff bristle brush or broom, mildew or mold is treated, and the roof is rinsed and allowed to dry completely. A test area of coating is applied over the existing membrane to check adhesion before work starts.

• Over new or aged asphalt materials, BUR and modified bitumen membrane, Pro-Grade 294 Base Coat and Sealer is used before silicone repairs to prevent bleed-through, discoloring and staining.
• On metal roofs, all rust is removed and a rust-inhibiting spot primer is applied.
• An adhesion test patch is made over smooth modified bitumen or BUR, EPDM, TPO, PVC, Hypalon, existing coated roofs and metal roofs. If the result is not at least two pounds, Pro-Grade 941 Primer is applied and the test is repeated.
• Splits, cracks, blisters, deteriorated flashing and cracked metal edging are repaired. Penetrations, curbs, flashings and transitions are sealed with Pro-Grade 920 Silicone Roof Sealant, Pro-Grade 923 Butter Grade Silicone Roof Sealer, Pro-Grade 957 Silicone Fibered Roof Sealer, or a three-course patch with Pro-Grade 988.

The coating can settle in storage and is mixed during use with a drill and paddle. A paddle of at least 3 inches in diameter is used for 5 gallon pails and at least 6 inches for drums. The surface must be clean, dry, frost-free and prepared. Application is by 1/2 inch to 1 inch nap lint-free roller, brush or commercial airless spray rig.

• Application temperature: 35 F to 120 F (2 C to 49 C). The surface temperature must be at least six Fahrenheit degrees (three Celsius degrees) above the dew point and rising.
• Rain-safe time: 15 minutes. Tack-free time: 1 to 2 hours. A full cure may take 2 to 6 hours depending on weather.
• Spray rig: at least 3500 psi at the gun tip, at least 3 gallons per minute pump output fed by a 5:1 transfer pump, hoses with a minimum inside diameter of 3/4 inch, and a 5000 psi gun with a reverse-a-clean tip of 0.030 minimum orifice and 50 degree fan.
• Cooler weather below 50 F (10 C) with relative humidity under 35% typically slows cure.
• The product is not thinned. If a skin forms in the container it is removed and the rest is used.

Minimum coverage is 1.5 gallons per square, with a dry film thickness of at least 22 mils. Each additional coat is applied perpendicular to the previous coat. Packaging is 5 gallon and 55 gallon. Unopened shelf life is 24 months from the date of manufacture in a cool, dry, shaded location. The coating is not for use over plywood, walking decks, gravel, degraded polyurethane foam, Kynar or Hylar coated metal roofs, shingles of any kind, or old roofs too dry and brittle to withstand shrinkage stress. It is not applied to wet or saturated roofs and is not for interior use. Silicone coated surfaces are extremely slippery. Clean-up of spray equipment that holds uncured material uses VM&P naphtha or mineral spirits. Because the product cures with moisture, it is not left in guns, pumps and hoses for long periods unless the equipment has moisture lock hoses, fittings and seals.