Why Metal Roofs Need a Vapor Permeable Underlayment

Protecting Metal Roofs and Cladding with Vapor Permeable Underlayments


Building or installing a metal roof or metal cladding? Certified Passive House Consultant and experienced technical services manager Krzysztof Apriasz shows how a vapor permeable underlayment like DELTA®-TRELA is essential for effective protection against moisture and corrosion, ensuring any metal roof or cladding lasts longer.

He also talks about some handy DELTA® components like DELTA®-MULTI-BAND tape, and how they can be used to ensure a tight seal and proper installation of the underlayment.


Why does a metal roof need an underlayment?


For commercial builders and owners, there are a variety of benefits to metal roofs in building design. A properly installed metal roof is extremely durable and can last as long as 40-70 years (exceptionally longer than traditional shingles). They’re also known to withstand strong winds and, thanks to their reflectiveness, can be very energy efficient and reduce heating and cooling costs.

However, if the metal roof isn’t fastened properly to the deck or sheathing, moisture can enter, which can lead to rust and damage. Metal roofs are prone to having condensation form on the underside.

Having a metal roof without an underlayment can cause rust and damage, reducing its performance. Adding an underlayment to a metal roof provides an air gap that allows any condensation and moisture to dry out, eliminating the risk of corrosion or damage.


Does it have to be a ‘breathable’ metal roof underlayment?


Metal roofs aren’t completely waterproof, so there needs to be an underlayment that allows for adequate airflow to effectively dry out any moisture. 

A system like DELTA®-TRELA is a breathable, or permeable, underlayment that’s an all-in-one solution thanks to its built-in air gap. 


About DELTA®-TRELA vapor permeable underlayment


DELTA®-TRELA roof underlayment


To combat this, DELTA®-TRELA is a vapor permeable underlayment that features two layers of polypropylene and is specifically designed for metal roofs and facades. This waterproof, breathable membrane protects against moisture and corrosion, provides effective ventilation and drainage, and reduces rainfall noise by up to 15 dB.


If you’d like to learn more about underlayments for metal roofs, check out The Build Show’s Matt Risinger’s review of DELTA®-TRELA and how he installs the system.


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