by Brooke Smith | 7 February 2024 8:00 am
Since self-adhering roofing underlayments were introduced more than 30 years ago, there have been many changes to the composition of these waterproofing membranes. Today, there are any number of choices when considering a self-adhered product, including various mastic formulations, wet or dry edge, and extended UV ratings.
For metal roofing systems specifically, a high-temperature rated underlayment is preferrable due to the extreme heat generated beneath the panels. Most roofing contractors will opt for a product that is rated at a minimum of 107 C (225 F) for metal, steel, and aluminum systems. It is always best to contact the manufacturer for installations under copper or zinc roofing due to a higher heat level generated from
these materials.
What has not changed is how these underlayments are installed on the roof deck.
Once the roofing contractor has selected a self-adhered product based on all applicable building codes and meets the requirements of the final roofing system, they should follow these useful tips for a proper installation. It is important to read and fully understand the manufacturer’s application instructions before installing.
Temperature ranges
Most manufacturers recommend a minimum ambient surface and air temperature before installation. This is based on the mastic formulation, which differs from product to product. When installing below the minimum temperature range, some mastics will become brittle and crack, and may even cause the top surface to split, which will result in potential leak points. Most manufacturers will provide cold weather instructions for these unavoidable situations.
UV ratings
The underlayment manufacturer will provide a UV or exposure rating that needs to be followed very closely. Extended exposure to sunlight will eventually break down the chemical composition of the underlayment surface. The result may be a delamination of the film surface from the mastic. This results in the product losing its waterproofing properties, leaving the structure unprotected. Oftentimes, delays in receiving the final roofing system cause the membrane to be exposed longer than recommended. It is best to contact the manufacturer before the UV rating expiring to take corrective actions so as not to void the warranty.
Surface preparation
Ensuring the roofing substrate is clean, dry, and free of waxes, dirt, or debris is critical in proper adhesion. Dust and dirt will impair the adhesive properties of the self-adhered membrane. Weathered surfaces require the use of a primer before installation to provide a clean and smooth surface. Contact the underlayment manufacturer for their list of approved primers.
Tight overlaps
Self-adhered underlayments are designed to overlap onto themselves for a seamless barrier against the elements. Generally, side laps require a 70-mm (3-in.) or 100-mm (4-in.) overlap, whereas end laps require a minimum overlap of
150 mm (6 in.). These membranes will expand and contract with the building itself, so having enough overlap to accommodate this movement is important for the integrity of the roofing system.
Compatibility
For asphalt-based adhesive products, avoid installing the membrane over solvent-based or silicone caulking, polyvinyl chloride (PVC), or silicone roofing materials, as well as tongue-and-groove boards that contain tree resin. These products can react adversely to the asphalt adhesive, causing the adhesive to liquefy and bleed or run out.
Ventilation
Self-adhered underlayments are typically vapor barriers requiring proper ventilation of the roof system. Without proper ventilation, condensation can form and cause damage to the interior structure. Items such as roof or soffit vents will need to be installed, ensuring the flow of air to properly vent the roof.
Safety first
All roof work is dangerous. Most self-adhered underlayments are treated or embossed with a traction surface to avoid slips or falls. However, when underlayments are wet, chances of accidents increase. Always follow all safety precautions such as safety harnesses, roof jacks, and other safety-related equipment.
Reach out to the experts
When in doubt, always contact the underlayment manufacturer should there be any questions concerning the proper use and installation of their products. Years of research and development go into these products before being introduced to the market; therefore, when seeking an expert opinion, consult the manufacturer directly.
With proper information before the job, contractors stand a much greater chance of eliminating problems down the road for themselves and, more importantly, for their customers.
David Delcoma is the operations manager at MFM Building Products Corp. Delcoma joined MFM in 2011 as the product marketing manager after serving more than 20 years with an industrial advertising agency. He can be reached at ddelcoma@mfmbp.com.
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