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Do Knoxville Metal Roofs Require Underlayment?

Sep 04, 2020

When you decide to install a durable, long-lasting metal roof to add superior waterproofing, fire resistance, and value to your Knoxville home, you’ll need to make a variety of choices regarding your roof’s appearance, including color, panel type, gauge, and more. However, one aspect of your metal roofing system you may have overlooked is the roof’s underlayment – the water-resistant material installed between your metal roof and the roof deck below. Underlayment can take many forms but is always a relatively thin, flexible layer designed to enhance your roof’s protective capabilities.

This additional layer is required in most building codes, and some even specify which type of underlayment must be used. In addition, some roofing types require the use of underlayment to ensure warranty protection – if you choose not to use underlayment in these situations, your roof’s warranty may be void. Even if your municipality or warranty provisions do not specifically require underlayment, we recommend its use for the multiple benefits it can bring to your roof.

Advantages of Underlayment

Installing an underlayment layer on your Knoxville roof deck holds multiple advantages, even though it cannot be seen once your roof’s installation is complete. Advantages include:

  • Uniform surface for roofing materials. Your roof’s decking is not always completely level, which may pose a problem when it comes to installing your metal panels. A layer of underlayment material helps to provide a more even surface, so your roof installation appears uniform.
  • Secondary moisture protection. Your roof’s underlayment helps to prevent moisture and condensation from making contact with the roof decking, protecting your interior for moisture infiltration. In addition, if you experience catastrophic damage to your roof’s surface – such as a fallen tree – underlayment can provide an additional protective layer.
  • Secondary insulation. Although your metal roof already provides excellent temperature control, a proper underlayment helps to further reduce heat gain and loss during the warmest and coldest months. As a result, your heating and cooling bills will remain more consistent.
  • Noise control. Providing an additional barrier between your roof decking and a metal roof can act as a sound blocker. This extra layer serves to help muffle exterior sounds and make them less intrusive to your home’s occupants.

Deciding When to Use Underlayment

While you’ll always need to use underlayment when specified by the Knoxville building codes or warranty requirements, there are a few instances when underlayment may not be required (though it is always recommended). For instance, if you’re installing a metal roof on a porch or covered patio without sheathing, you may not need an underlayment. Similarly, if you’re installing your metal roof over an existing shingled roof, an additional underlayment may not be necessary.

However, if you are building a new home and won’t be installing the finishing components of your metal roof right away, underlayment is necessary to waterproof the roof during the building process. In addition, if your roof has a low slope and may gather moisture after a heavy rain, you’ll need underlayment to provide a crucial waterproof layer to protect against water damage.

Types of Underlayment

When you’re choosing the ideal metal roofing components for your home’s roof, it’s important to consider the multiple types of underlayment available. In the past, metal roofing companies in Tennessee utilized the same type of felt paper underlayment as was recommended for most asphalt shingled roofs. However, other, newer underlayment options may provide heat and UV resistance that is more ideal for your area.

The most common underlayment options offered for metal roofing systems include:

    • Asphalt-saturated felt paper. More commonly known as tar paper, this is the material traditionally used under roofing systems of all types, including asphalt shingled roofs. Felt paper consists of either an organic cellulose substrate or a synthetic fiberglass substrate, soaked with asphalt material to enhance waterproofing. You can find felt paper in either 15- or 30-pound weights, though it often needs to be doubled up to provide adequate water resistance.Felt paper is the most inexpensive option and is generally simple to install but comes with a few disadvantages as well. In particular, it does not hold up to UV exposure as well as other options and tends to transfer heat at a higher rate than other materials. Additionally, felt paper is susceptible to tearing around nail intrusions and is only water-resistant, not waterproof.
    • Rubberized asphalt underlayment. When a manufacturing company modifies asphalt with bitumen, it creates a rubberized product with plastic properties. This substance can be applied to a roof in panels and then heat sealed or fused together, creating a rubberized membrane with additional insulative and waterproof qualities. In addition, the underside of the underlayment is self-adhesive, meaning it clings to the roof decking and seals completely around nails and other roof fasteners.Rubberized asphalt provides better heat resistance, superior waterproofing, and is highly rated for UV resistance. It is also extremely flexible and ideal for climates that experience weather extremes. While this underlayment is thicker to provide these benefits, it is also heavier and, therefore, more expensive and labor-intensive to install.
    • Synthetic underlayment. There are a few different types of synthetic underlayments available on the market. Synthetic underlayments usually consist of polyethylene or polypropylene, woven together to create a substrate for installation on your roof. These materials make synthetic underlayments highly UV resistant, fungus and mold resistant, and water-resistant.Synthetic underlayment’s composition makes it lighter and non-slip – meaning it is far easier to install than other types of underlayment. However, it is also the most expensive type of underlayment. In addition, it does not seal around nails and other fasteners as well as rubberized asphalt underlayment.

What Underlayment Is Best For My Metal Roof?

The type of underlayment that is right for your roof will depend on a variety of factors, including your roof decking, local climate, metal roofing materials, budget, and more. In addition, be sure to inquire with your Knoxville metal roofing contractor to learn about building code requirements, warranty information, and the types of underlayment recommended for your area. In the end, choosing a quality underlayment product can help your metal roof protect you and your home for years to come.

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