Underlayment is an essential roofing component that must be included in every roof replacement.

Available in felt and synthetic, roofers can use either depending on a homeowner’s budget and local weather conditions.

To make a highly informed decision on which underlayment is the better choice, roofing companies recommend that homeowners understand its purpose and the difference between the two types.

What Is Roofing Underlayment and What Does It Do?

Roofing underlayment is a critical layer of waterproofing that is installed under the exterior roofing materials but on top of the roof decking.

A high-quality underlayment prevents moisture from soaking through to the decking in areas that may experience minor leaking from the outside.

It provides insulative qualities, acts as a noise barrier, and protects the underside of the roofing materials.

Underlayment can be made from felt or synthetic fabric and is an essential, protective component that roofers must install based on roofing materials manufacturer requirements and local building codes.

It also serves as an extra layer of protection if the roof is damaged until professional repairs can be made.

Felt Underlayment - Advantages and Disadvantages

Felt underlayment has been in use by roofing companies since the 1930s, although its quality has improved greatly over the decades.

Today, it is made from asphalt-saturated cellulose and paper fibers that are made into a water-resistant fabric in two thicknesses, 15# and 30#, with the latter being the most resilient.

Advantages - Felt underlayment is easy to lay down and cut to shape around roofing protrusions, breathable to prevent moisture buildup under the roof, biodegradable, and less expensive than synthetic underlayment, and ideal for use with most pitched roofs.
Disadvantages - The main disadvantages are that felt underlayment can degrade when exposed and allowed it to dry out, crack, as well as lose its water-resistant properties plus it is heavier and generally less durable than synthetic products. It can also wrinkle when affected by wind during installation.

Synthetic Underlayment - Advantages and Disadvantages

Synthetic underlayment has been in use by roofers since around the year 2000.

Made from a variety of materials including polypropylene plastic, polyester, and fiberglass, it provides several advantages over standard felt underlayment in certain situations.

Advantages - Synthetic underlayment is very lightweight in comparison to felt, less prone to wrinkling during installation, more UV-resistant and less likely to crack, and has a much higher tear-resistance than felt. It is also totally impermeable, making it more suitable for roof architecture with lower slopes.
Disadvantages - Synthetic underlayment costs considerably more than felt, which is the main reason why roofing companies see their customers opting against it. In addition, it is made from petroleum-based products and is not biodegradable. Because it is impermeable, synthetic underlayment is also non-breathable, which can create moisture issues for roofs that are not properly ventilated.

Roofers Can Advise Which Underlayment Is Best For A Roof

When doing a new roofing installation, contractors must install a high-quality underlayment between the decking and top materials to ensure a complete and serviceable installation.

Both felt and synthetic underlayment offers homeowners various benefits at different price points.

Roofers find that the best option will depend on multiple factors including the weight, slope, and shape of the roof, local weather conditions, and the homeowner’s roofing budget.

An experienced local roofing company can help customers understand the differences between the two types of underlayment and decide on the product that will provide better protection for their specific roof!