Technical Letter 5
GCP’s self-adhered underlayments (GRACE ICE & WATER SHIELD® , GRACE ICE & WATER SHIELD® HT, GRACE ULTRA™, GRACE SELECT™, GCP granular underlayments and ROOF DETAIL MEMBRANE™) are chemically compatible with most materials that are normally present in roof-top environments. GCP underlayments are compatible with acids, bases, and salts. They are also compatible with asphalts and dried, fully cured coal tar. Generally, there is no chemical compatibility issue if a material contacts the high-density polyethylene film of the GCP smooth surface self-adhered underlayment. If the contact area is the adhesive component of the underlayment or the granular surface of GCP granular underlayment, there is need to further investigate. As a general rule, the material must be sound, functional and firmly bonded to the substrate.
Roof Deck Materials
GCP’s self-adhered underlayments are applied directly to a clean, dry, continuous roof deck. Some suitable deck materials include plywood (including fire treated plywood), oriented strand board (OSB), wood plank, metal, concrete, exterior gypsum board, glass mat surfaced gypsum board, and rigid roof insulation board. Refer to Technical Letters 10, 12, and 13 for additional surface preparation requirements for some of these deck materials. GCP’s underlayments are compatible with chromated copper arsenate (CCA), alkaline copper quaternary (ACQ), ammoniacal copper zinc arsenate (ACZA), pressure-treated woods and pressure-treated plywood. GCP’s underlayments are not compatible with high concentrations of resin or “pitch” that may bleed out of resin canals in softwoods such as pine, spruce, cedar, larch, or Douglas fir. A ¼” or thicker plywood overlay should be installed over the existing deck when roofing over wood plank decks which are suspected to be of one of the above species, or are of an unknown origin. Tongue and groove roof deck material (regardless of species) that is directly exposed to the interior living space of the building should be overlaid with minimum 1/4” plywood prior to applying the GCP self-adhered underlayment. GCP underlayments should not be installed over unsupported areas of roof deck. Gaps between wood planks are frequently encountered on wood plank decks. These gaps must be repaired prior to installing the GCP underlayment. Alternatively, a plywood overlay may be installed over the wood plank decking to cover the unsupported areas.
Most roofing cements/sealants are chemically compatible with GCP’s underlayments provided that they are applied to the high-density polyethylene film component of the underlayment. If the roofing cement/sealant is to contact the adhesive side of the underlayment, ensure it is fully cured prior to applying the underlayment, and avoid cements/sealants high in solvent.
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GCP Canada, Inc., 294 Clements Road, West, Ajax, Ontario, Canada L1S 3C6.
This document is only current as of the last updated date stated below and is valid only for use in the Canada. It is important that you always refer to the currently available information at the URL below to provide the most current product information at the time of use. Additional literature such as Contractor Manuals, Technical Bulletins, Detail Drawings and detailing recommendations and other relevant documents are also available on www.gcpat.com. Information found on other websites must not be relied upon, as they may not be up-to-date or applicable to the conditions in your location and we do not accept any responsibility for their content. If there are any conflicts or if you need more information, please contact GCP Customer Service.
Last Updated: 2023-06-29