Metal Roofing – Choosing the Right Underlayment

There are three basic types of roofing underlayment:

  • Felt underlayment

  • Rubberized asphalt underlayment

  • Synthetic underlayment

Felt Underlayment (also called “tar paper”): This is the lowest cost underlayment – and for a good reason:

· Absorbs moisture and wrinkles when exposed to rain or snow;

· Tears when exposed to high winds;

· Prone to tearing around fasteners;

· Is much heavier than many of its synthetic competitors;

Rubberized asphalt: A self-adhesive and self-sealing membrane. This is an excellent choice for low slope and water infiltration prone areas;

· Self-adhesive backer which makes for a watertight membrane if installed correctly.

· Self-sealing around nail heads and fasteners which is especially important for low-slope roofs.

· Can be left exposed to rain or snow for a period of time without fear of leakage.

· Specific brands are high-temperature resistant and well-suited to metal roofing.

Warning: Do NOT use granular coated membranes! As the metal panel expands and contracts with changes in temperature, the granules wear away both the primer and zinc / Galvalume coatings, which can lead to premature corrosion and nullify the panel manufacturer’s warranty.

Recommended Products: “Ice & Water Shield” by Grace , “LeakBarrier PS200 Ice and Water Armor” by Tarco

Synthetic Underlayment:

· A light-weight, very high-strength alternative to felt underlayment which results in decreased worker fatigue and increased safety;

· Breathable varieties ensure that moisture will not be trapped against the roof deck;

· Can be left for several months exposed to hot, wet and / or cold weather conditions;

· Available with a non-skid surface which results in increased rooftop safety.

Recommended Products: Tri-Flex 30 by Grace, Titanium UDL-25 by Interwrap

Summary:

· The use of an underlayment between the roof deck and the metal panel is always recommended as a barrier against condensation. This protects both the wood of the roof deck from potential rot as well as the metal panel from being in constant contact with a moist / wet surface.

· Follow manufacturer guidelines when installing the selected membrane. Overlaps, fastening patterns, weather conditions and roof deck preparation are all important factors in achieving long lasting results.

· We do not recommend the use of staples when applying underlayment. Use either plastic capped nails or traditional roofing nails. Both must be coated / plated (zinc, stainless) for corrosion resistance.

· We do NOT recommend the use of “tar paper” for metal roofing projects.

· If you have a roof that is susceptible to ice damming or a low slope roof, a self-adhesive membrane (such as Ice & Water Shield) is recommended.

· For high-slope applications, think about combining products – consider putting a self-adhesive around the perimeter, in valleys and potential water infiltration areas, and covering the balance with a light-weight synthetic.

· Never use a granular coated membrane beneath a metal roof.

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