Critical Mass: Site mixing of catalyzed resin waterproofing

Keith B. Nelson, Joseph D. Rogers

The recent rise in popularity of catalyzed resin roofing and waterproofing products has moved many specifiers and installers onto the steep part of the learning curve, collecting new experiences and learning new lessons about their installation. It is not unusual to see deficiencies in mixing the components for this technology, some of which become apparent almost immediately upon installation.

Catalyzed resins, including materials such as poly methyl-methacrylate (PMMA) and polyurethane-methacrylate (PUMA), require site mixing of two or more

components to initiate a chemical reaction that cures the product. The ratio of these components is dictated by the manufacturer and will commonly vary based on the air, material, and substrate temperatures at application. It is critical the correct ratio of resin and catalyst be selected for the conditions and that both the base resin and the catalyzed resin be thoroughly mixed so there are no pockets of varying concentration in the resulting mixture. Selection of improper component ratios or inadequate mixing produces either a resin that remains tacky or liquid and fails to cure or a resin that cures too quickly. These products are not always shipped with the tools necessary to properly measure and mix the components, especially in cases of detail or repair work where contractors may prefer to purchase in bulk but only mix partial batches or pails of material.

With one-component products or products with a 50/50 ratio by volume, preparation for installation is relatively straightforward. Catalyzed resins are typically specified by ‘mass’ and are mixed at ratios far less than 1:1. For some resins, addition of catalyst at a rate of 0.5 to 1 percent by mass may be required. It is virtually impossible to ‘eyeball’ this amount correctly, and it is therefore necessary to establish a set of tools and a procedure for mixing prior to application. It is critical the installer review installation instructions and, if necessary, bring graduated containers and/or scales to determine the quantity or mass of each component to facilitate the proper mixing ratio.


Manufacturers take several approaches to providing instructions for mixing their product. Some manufacturers specify narrow temperature ranges and provide product only in premeasured ‘workpacks’ with strict instructions to always mix full packs (i.e. no field measuring of partial batches). Other manufacturers may provide directions for quantities of catalyst to mix with a full pail of resin, and still others provide volumetric or weight ratios to be applied as necessary.

To sanity-check a field installation, monitor the measurement and mixing technique and check them against manufacturer literature. Ensure mixing times are sufficient and performed with appropriate agitators. Look for material to ‘kick’ (get hot and thicken) in the pot at or near the end of the specified pot life. If a manufacturer indicates a product should have a 20 minute working time but 90 minutes later the batch remains a liquid, the resin will likely not cure correctly. Check the skin-over, watertight, and traffic-ready times to gain an understanding of the timeline for membrane cure. If the material remains soft and tacky well beyond this timeline, consult manufacturer literature or a technical representative to determine how to remedy this situation. Typically, any uncured or improperly cured material must be completely removed from the substrates and new membrane applied.

To avoid this failure, start the process early, review mixing ratios, and bring scales or graduated containers if necessary to ensure all product is portioned and mixed in accordance with manufacturer instructions.

Keith B. Nelson is a consulting engineer at Simpson Gumpertz & Heger in Washington, DC. Nelson has expertise in the investigation, design, and rehabilitation of modern and historic building enclosures. He specializes in plaza waterproofing and roofing repairs and historic masonry restoration. He can be reached via email at

Joseph D. Rogers is an associate principal at Simpson Gumpertz & Heger in Washington, DC. His work includes a general focus in investigation and repair of building performance issues and rehabilitation of monumental and historic structures. Rogers is also a lecturer in the Department of Civil and Systems Engineering at the Johns Hopkins University. He can be reached via email at

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier or CSI.

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