Best Underlayment Options for Roofs

When people, homeowners specifically, think about roofing, the spotlight usually falls on the grand finale.

The stylish asphalt shingles, rustic clay tiles, or sleek metal panels that gleam in the sun are often the highlights. However, underlayment is one of the overlooked parts of a roof that helps keep your home dry and protected.

It is the hidden layer installed between your roof deck, the flat base typically made from plywood or OSB, and the outer roofing material.

Roofing underlayment provides an additional layer of protection against water intrusion and other environmental exposure. Depending on the roofing system, it can also contribute to the overall durability and performance of the roof assembly.

But with so many products available, choosing the right option can get complicated, especially for new homeowners working within a budget.

So, let's dig into the builder's world and explore some of the most common roof underlayment options.

What is Roof Underlayment and Why is it Important?

Just a quick reminder… roof underlayment is the layer of material installed directly onto your roof deck. It lies beneath the final roofing material, which could be metal, tile, or shingles.

The major function is that it stands guard against water, ice, wind, and even UV damage—a hand-in-hand co-operation with the outer roofing. 

The one you need for your roofing project can depend on a few factors, including cost, the slope of your roof, and even the climate. For instance, the roof underlayment options in Florida will vary from those in arid and windy climates. 

Depending on the roofing system and product, underlayment can:

• Provide an additional water-shedding layer
• Help protect the roof deck from moisture
• Provide additional protection during roof installation
• Help meet specific roofing-system and building-code requirements

Note:

Suppose you are going for metal roofing, questions such as, “Can you put metal roofing directly on rafters” are going to be common. 

Different Roof Underlayment Options

The best underlayment options for roofs vary. But for metal roofs, bare rafters are not one of them.

So before you start laying panels or driving in screws, let’s take a closer look at the different roof underlayment options available. 

They include: 

1. Asphalt-Saturated Felt

Asphalt-saturated felt, or more commonly, felt paper or tar paper is a classic choice.

Made from organic or fiberglass substrate saturated with asphalt, it has been used for decades with reliable performance under many roofing systems. 

For those wondering what goes under a metal roof, asphalt-saturated felt is one traditional option, but the appropriate underlayment depends on the specific metal roofing system and the manufacturer's installation requirements.

Asphalt-saturated felt is still used with some metal roofing systems, although the appropriate type and weight depend on the roofing manufacturer's installation requirements.

2. Synthetic Underlayment 

Made from polypropylene or polyester, synthetic underlayments are a modern upgrade from felt.

Its design is engineered with polymers to deliver better strength, water and tear resistance. Even though they have a slip-resistant surface, they can still pose a challenge for installers to walk on while working. 

Synthetic underlayment can be a practical option for metal roofing because it is lightweight, durable, and resistant to tearing. However, products vary considerably, so make sure the specific synthetic underlayment is approved or certified by its manufacturer for use with metal roofing.

It resists the kind of wear and tear that can happen during installation. Plus, it won’t give up if exposed to the elements during delays.

3. Self-Adhering Membrane Underlayment

Self-adhering membranes, also known as peel-and-stick underlayments, are designed to adhere directly to the roof deck without relying solely on mechanical fasteners.

They are commonly used in areas more vulnerable to water intrusion, such as valleys, eaves, roof penetrations, and other critical transitions.

These membranes are often made with rubberized asphalt or other polymer-based materials and can provide strong water resistance.

Some products are also designed to withstand the higher temperatures associated with metal roofing.

However, not every self-adhering membrane is suitable for every roof.

If you're installing a standing-seam metal roof, check the roofing manufacturer's specifications to make sure the membrane is compatible with the roofing system and approved for the intended application.

For metal roofing, pay particular attention to temperature ratings. Some self-adhering membranes are specifically formulated for high-temperature applications, while others may not be suitable for direct contact with metal panels.

Always check the manufacturer's temperature rating and installation requirements before choosing a product.

Common formulations and performance characteristics include:

A. Rubberized Asphalt / Modified Bitumen Underlayment

Rubberized asphalt and modified bitumen underlayments are heavy-duty membranes designed to provide additional protection against water intrusion.

They combine asphalt with polymers or rubber modifiers to improve flexibility, durability, and resistance to changing temperatures.

These underlayments are often used in areas that are particularly vulnerable to leaks, including valleys, eaves, roof edges, and around penetrations.

Some products are self-adhering, while others are mechanically fastened, so the installation method can vary by product.

For metal roofing, make sure the specific membrane is compatible with the roofing material and temperature conditions.

Some asphalt-based products may not be appropriate for direct contact with certain metal roofing systems, while high-temperature-rated products are specifically designed for applications where greater heat resistance is required.

B. High-Temperature Underlayment

Metal roofing can reach high temperatures when exposed to direct sunlight, so the underlayment needs to be able to handle the temperatures produced by the roof system. 

High-temperature underlayments are specially formulated to resist heat without softening, degrading, or losing their protective properties.

These products are available in different forms, including synthetic and self-adhering membranes.

They can be particularly useful beneath metal roofing, especially in areas with high sun exposure or where the roofing manufacturer's specifications call for a high-temperature-rated underlayment.

However, not every high-temperature product is suitable for every metal roof.

Always check the roofing manufacturer's installation requirements to confirm the approved underlayment and whether it can be used in direct contact with the specific metal panel.

4. Breathable Synthetic Underlayment

For homeowners looking for a roofing underlayment that can help manage moisture while offering modern performance, breathable synthetic membranes are another option to consider.

These products are engineered to allow water vapor to escape while helping prevent wind-driven rain and other moisture from reaching the roof deck. However, their suitability depends on the specific roofing assembly, climate, ventilation design, and the manufacturer's installation requirements.

For standing-seam metal roofs in particular, don't assume that any breathable membrane will work. Check the metal roofing manufacturer's installation instructions to confirm which underlayments are approved for the specific panel system.

5. Fire-Rated Underlayment

Fire-rated underlayments are designed to provide an additional layer of protection against flame spread and fire penetration through the roof assembly.

They can be particularly important in areas where building codes require a higher fire classification, such as certain wildfire-prone regions.

Some products are specifically tested and listed for use in fire-rated roof assemblies. However, the fire rating applies to a specific roof assembly rather than the underlayment alone.

The deck, underlayment, roof covering, and installation method may all affect the final classification.

For metal roofing, check the manufacturer's documentation to determine which underlayment and roof assembly are approved for the required fire rating.

For example, ASC Building Products specifies particular fire-resistant underlayments or non-combustible substrates for achieving a Class A assembly over wood decking with some of its metal roofing systems.

Choosing Underlayment for Metal Roofing

Choosing an underlayment for a metal roof isn't simply a matter of picking the thickest or most expensive option. The right product depends on the type of metal roofing, roof slope, climate, temperature exposure, and the design of the entire roof assembly.

Before installation, consider these factors:

• Check the manufacturer's requirements. Start with the installation instructions for your specific metal roofing system. The manufacturer may specify approved underlayments or particular performance requirements, and following those instructions can also help protect the roof's warranty.

• Consider temperature exposure. Metal panels can reach high temperatures under direct sunlight. If the manufacturer requires or recommends a high-temperature underlayment, choose a product specifically rated for the expected conditions.

• Consider the roof's slope and design. Low-slope areas, valleys, eaves, penetrations, and other vulnerable locations may require additional waterproofing or a specialized self-adhering membrane.

• Account for your climate. Heavy rain, snow, wind, humidity, and prolonged sun exposure can all affect underlayment selection. A product that works well in one climate may not be appropriate for another.

• Check compatibility. Some underlayments may not be suitable for direct contact with certain metal roofing materials. Verify compatibility with the panel manufacturer before installation.

• Follow local building requirements. Local codes may specify requirements for water resistance, fire classification, or the type of roof assembly permitted in your area.

Ultimately, there is no single underlayment that is right for every metal roof. The best approach is to choose a product that meets your local requirements and is specifically approved or recommended for the metal roofing system you plan to install.

General Tips for Installing Underlayment with Metal Roofs

If you want to get the most out of your metal roof, you should keep these expert-level pointers in your back pocket:

• Use the right size furring strips based on your roof pitch and panel style. They create ventilation channels, which reduce condensation and boost energy efficiency. 
  

• Always install a proper deck and an underlayment layer before laying down metal panels. Traditional asphalt-saturated felt can be suitable for some metal roofing systems, but always check the panel manufacturer's requirements before installation.

• Follow the roofing manufacturer's requirements. Metal roofing systems can differ significantly in how they are supported and what underlayment they require.
• Don't assume a metal panel can simply be installed over bare rafters. Many residential metal roofing systems require a suitable roof deck and underlayment, while some structural systems are designed differently. Always follow the specific panel manufacturer's installation instructions.
• Use the correct underlayment for the roof assembly. Traditional asphalt-saturated felt, synthetic underlayment, self-adhering membranes, and specialized products can all have different applications and limitations.
• Pay attention to vulnerable areas. Valleys, eaves, penetrations, and other transitions may require additional waterproofing or a self-adhering membrane, depending on the roofing system.
  

Endnote: The Unsung Layer of Lasting Protection

Choosing the right roof underlayment isn't just another item on a contractor's checklist. It is an important part of the overall roof assembly that can help protect the roof deck from moisture and weather exposure.

Depending on the roofing system, you may use traditional felt, a synthetic membrane, a self-adhering product, or a specialized underlayment designed for particular performance requirements.

The most important step is to check the roofing manufacturer's installation instructions and your local building requirements before choosing a product. The right underlayment isn't necessarily the most expensive option. It's the one that is compatible with your roofing system and appropriate for your climate, roof design, and required performance.