What Is The Best Underlayment For A Metal Roof

The underlayment product provides a weather barrier for the interior of a building both during construction and after.
A waterproof underlayment is used to protect the structure during construction from infiltration of water before installing the final roof covering.

The underlayment is used to shed any water that might find its way through the roof system.

Underlayment Contractor

Let’s begin by explaining the three main types of underlayment.

1. The felt paper is mechanically attached.
2. Mechanically attached synthetic underlays.
3. Peel and stick adhesives are used to attach synthetic underlayments.
There are pros and cons to each underlayment, particularly when dealing with metal roofs.

Felt Paper Underlayments

The felt paper is usually made of wood or fiberglass, and then coated with an asphalt-like product to repel water. This product family can also be referred to by the names roofing felt and tar paper.

In the past, felt underlayments saturated with asphalt were the most popular choice for roof underlayments. Over the past decade, however, the market share of these products has dramatically decreased.

Benefits

Underlayment that is most economical

Disadvantages

• Small Roll Size: The most common size of felt paper is 2 square rolls, or enough to cover an area approximately 10′ x20′. Synthetic underlayment, for instance, can cover 10 squares or five times as much in a single sheet.
• Weight Increase: This type of underlayment is heavier than any other option. A 2 square roll #30 felt will weigh around 43 pounds and a synthetic roll 10 square feet in size should weigh about the same.
• Increased Labor:  Small roll sizes and weights increase labor costs.
  Installing asphalt-based products can be difficult as they tend to buckle or wrinkle.
• Recyclability: These products are made from asphalt, as their name suggests. Their use is therefore environmentally harmful. Recycling is also not common.
• Permeability: Asphalt-based underlayment products have a greater permeability than other materials. When a product has a higher permeability, it can absorb more water and cause roof deck rot if not dried completely before installing the final roofing material.
• Color: Asphalt-based products are black in color, meaning they absorb more sunlight and therefore introduce more heat than synthetic underlayments which tend to have a grayish hue.
• Durability – UV Resistance: Ultraviolet radiations can cause felt paper to degrade very quickly. Install the final roofing product as soon as you can after installing underlayment. Although felt manufacturers suggest a 30- to 60-day exposure period, the real world is usually one to two weeks.
• Long-Term Durability: Metal roofing is more durable over the long term than felt paper products. Metal roofing has a life expectancy of 60 years, while asphalt underlayments have a life expectancy of 15 to 20 years. It doesn’t make any sense to add a more durable product over a less durable one.
• Availability: Asphalt-based products are not very flexible when it comes to schedule changes, manufacturing problems, or delays. This is especially true in light of current labor and supply chain issues.

Recommendation General

When product performance is considered in the purchasing decision, the attraction of low prices quickly diminishes.

We do not recommend the use of asphalt-based roof products underneath metal roof panels.

Synthetic Underlayments-Mechanically Attached

Synthetic underlayments are made from polypropylene, polyethylene or laminated plastics. The underlayment will then be mechanically attached to your roof.

Synthetic underlayments have been around for 20 years and offer many advantages over the traditional felt paper made of tar or asphalt.

Advantages

Strength: Synthetic underlayments have a 10-15% higher strength than felt underlayments. This product strength reduces blow-off and tears.
Installing: 10 square rolls (compared to two square rolls felt paper). A single roll can cover 5x as much asphalt and is lighter. The larger rolls allow for faster installation, and less trips to obtain more product. The installed cost is positively impacted by both.
Installing the product: This product is easier to install than asphalt-based products.
Cold Weather Installation: The system remains flexible even in cold temperatures, making it more contractor-friendly.
Longevity: Underlayment made of synthetic materials can be exposed for a much longer period to UV rays without affecting its performance. Synthetic products can be exposed to ultraviolet rays for up to nine months without affecting their performance.
Better Walkability: Synthetic materials offer improved walkability, and installer safety.
Color: The majority of synthetic underlayments come in gray. They absorb less heat as a result than black asphalt-based products.

Disadvantages

• Some product installations use capped nails. When using flat panels (without striations), these caps may occasionally transmit through the roof. It’s important to test a small area first. Check with the manufacturer if cap telegraphing poses a problem.
• The holes created during the installation of the final roofing material, in this case metal, remain unsealed.
  General Recommendation

This category of underlayment provides cost-effective protection in the roof’s field, but it may fall short in areas such as the eaves and valleys.

It is therefore combined with peel-and-stick synthetic products.

Peel & Stick Technology for Synthetic Underlayments

The three-layer method of construction is typical for synthetic peel and stick underlayment.

• Top Layer – Synthetic Underlayment
• Middle Layer – Modified Bitumen
• Bottom Layer – Adhesive

This family of products can self-seal small penetrations, such as those caused by fasteners during the installation of primary roofing materials. GCP Applied Technologies, which markets these products, is often referred to as Ice and Water ShieldR.

Benefits

Synthetic peel-and-stick products also offer the same advantages as mechanically adhered synthetic underlayment.

• Product Strength
• Installation Ease/Lightweight
• Longevity
• Improved Walkability
• Color

Peel and Stick products also provide additional benefits, as highlighted below.

• Sealing around the fasteners that are used to attach primary roofing materials.
• Rarely experience blow-off failures.
• A thicker product (normally between 40 and 48 mils) offers greater protection. Comparatively, the thickness of mechanically attached synthetic underlayments is usually 6 mil.

Disadvantages

Peel and Stick products are the most expensive option for underlayment.
It is important to use hi-temp when paired with metal roofing. Low-temperature products are less expensive but they tend to melt or fail when used with metal roofs.

General Recommendation

Peel and stick underlayments offer the best performance but are often used in only critical areas, such as valleys and ridges. They can also be used on eaves and eaves.

They can also be used to cover the entire roof surface if desired.

To maximize performance and minimize costs, it is common to combine mechanical and peel-and-stick attachment methods. Read more info about – Metal Roofing in Bow.