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The 3 Best Metal Roof Underlayment Options

There is one underrated design consideration most people skip when planning their home construction or remodel. While most people obsess on the best type of roofing material for their home, there is one part that always gets overlooked: roofing underlayment. 

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Choosing the best roof underlayment for your home is as important as deciding on what roofing material to choose. It’s a critical decision that you need to make depending on a lot of factors like maintenance, longevity, durability, and cost. Just like deciding on what roof material you wish to install, choosing your roof underlay is critical because it is second in command in protecting your home from the elements.

What is Roof Underlayment?

Roof underlayment provides your home with a layer of protection against the elements. Roof underlayment should not be confused with roof sheathing. Roof sheathing, also known as roof decking, refers to the surface on which your roof is laid on. On the other hand, roof underlayment refers to the waterproof sheet laid on top of the roof deck, right before the roof is installed. It creates a barrier that prevents the roof deck from water damage if the roof fails.

Why is Roof Underlayment Important?

Roof underlayment is usually installed in compliance with local building codes and laws. Besides this, it is an essential facet of roof construction because it creates an additional layer of protection from the elements.

The absence of a roofing underlay puts your house at a disadvantage. Skipping the underlay would seem like a cheaper solution, but it poses many problems in the long haul. Without this extra protective layer, leaks from your roof may damage the roof decking, calling for more costly repairs in the future.

Roof underlayment complements the effectiveness of your metal roof.  It battles the condensation formed under the metal panel that could affect the integrity of your roof deck or sheathing. Since the sheathing is usually made out of plywood or other wooden boards, it is susceptible to mould and mildew when exposed to water. 

Metal roofs tend to expand when exposed to high temperatures and contract when exposed to lower temperatures. The constant expansion and contraction make the seams and connections vulnerable to water seepage. This expansion and contraction usually occur during the day and night, respectively. It happens in such a short period, causing droplets of water to form underneath the metal roof. Without the underlay, the roof sheathing or decking will be exposed to droplets of water, causing it to rot or disintegrate at a higher speed.

Strong winds allow water to seep into a building in different directions. Since metal roofs tend to expand and contract constantly, seams created by overlapping metal roofing sheets tend to crack open, allowing water to be drilled into the openings. The absence of an underlayment exposes the roof decking to water damage which eventually leads to mould build-up over time. 

Best Roofing Underlayment Options

Felt Underlayment/ Asphalt Saturated Felt/ Felt Paper

If you are looking for a more traditional and affordable roof underlayment, asphalt saturated felt could be the best solution for you. It goes by various names like felt paper, roofing tar paper, felt underlayment, asphalt saturated felt, and others. 

Felt underlayment is characterized by a blend of organic cellulose fibres forming a sheet similar to paper. It is then drenched in an asphalt coating that provides water resistance.  

Due to the asphalt coating, felt underlayment is inherently heavy. There are typically two options for this, a 15-pound sheet and a 30-pound sheet, with the latter being thicker and heavier. The physical properties of this material also make it prone to buckling and wrinkling, making it an unpopular choice compared to its modern counterparts. Felt underlayment is typically paper topped with an asphalt coating. It tears apart easily, requiring careful installation. This material does not stand well with ultraviolet rays. Prolonged exposure to the sun may cause the underlay to fail.

Asphalt felt underlayment does not offer moisture resistance, unlike its modern counterparts which is why it is best suited for dry climates where humidity levels are low. It mainly provides water resistance alone.

One of the disadvantages of Asphalt felt underlayment is its lifespan. Since it is considered a paper product, it may tend to deteriorate quickly once soaked in water for an extended period. 

Synthetic Underlayment

With durability in mind, there is no doubt that synthetic underlayment is your best bet. It is made from thin strands of polyethylene or polypropylene, woven together to form a thin sheet. The absence of organic materials make synthetic underlayment mould-resistant. Since it is created from synthetic fibers, this type of roof underlay has inherently stronger qualities compared to its organic counterpart. Comparing it to felt underlayment, synthetic underlayment does not tear easily. 

Synthetic underlayment is significantly more cost-efficient when it comes to labor, because it weighs a fourth of felt underlayment. The absence of the asphalt coating makes it a lot lighter and easier to install. A roll of synthetic underlayment would cover a larger surface area than asphalt felt underlayment, making mobilization a lot easier since you will need fewer trips up the roof to bring in your material.

Cost-wise, synthetic underlay cost double of felt underlay. The price may seem steep at first, but once you look closely and examine the differences between the two, you will see that synthetic underlay is significantly stronger than felt underlay. It is a completely different product when put side by side with felt underlay.

Self Adhering Underlayment/ Rubberized Asphalt

A step beyond synthetic underlayment, this type of roof underlay is the most expensive. It is characterized by a synthetic and water-resistant material backed with a layer of adhesive that sticks easily to the roof deck. The adhesion creates a watertight barrier in between the underlay and the roof deck, preventing water seepage. This type of underlay works even better with metal roofs because its waterproof qualities are intensified when exposed to extreme heat.  

Due to its cost, this type of underlay is usually reserved for areas in the roof that are highly prone to leaking and water seepages like valleys, ridges, and seams. 

Like synthetic underlayment, self-adhering underlayment is created from various plastics and polymers, creating a lightweight and waterproof sheet that protects your roof deck from water damage. Many variations for this type of underlayment are available in the market, with one common feature: its adhesive characteristic.

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