Roof underlayment fits between the roof deck and the shingles to provide additional insulation to the top of the home while also acting as a waterproof barrier to prevent moisture from entering your attic. It is one of the most overlooked parts of your roof. In fact, most people would not be able to tell you what roofing underlayment is, despite the majority of residential roofs consisting of a roof deck, roofing underlayment, and shingles.
Similar to your shingles, the roof underlayment needs to be replaced at semi-regular intervals based on the condition of the roof. Underlayment was traditionally only available in one type known as asphalt-saturated felt, but over the years additional types have been developed. This guide will help you discover the differences between the various types of roof underlayment, so you can select the right type for your home.
What Is Roof Underlayment?
Roof underlayment is a layer of synthetic fiber or felt that is installed over the roof deck and under the shingles. This layer acts as a waterproof barrier to prevent moisture from seeping into the home and also helps to insulate the roof.
- Best for: Inexpensive roof repairs, slate shingles, and tile shingles
The classic type of roof underlayment is known as asphalt-saturated felt. It's similar to tar paper, except that instead of tar, the waterproofing ingredient is asphalt. This style of roof underlayment has been used for decades to provide adequate moisture protection and insulation, but it just doesn't measure up to the flexibility and durability offered by rubberized asphalt or synthetic roof underlayment.
Asphalt-saturated felt is also known as roofing felt or felt paper. As the most affordable option for roof underlayment, roofing felt remains a popular option that can improve the durability of the roof deck in order to protect against impact damage from hailstones or airborne debris, like tree branches. It comes in two standard weights, including 15 pounds per hundred square feet (No. 15 felt) and 30 pounds per hundred square feet (No. 30 felt).
It's recommended to use No. 15 felt for light-duty projects, like finishing the roof of a shed or detached garage, while No. 30 is the best option for protecting your home from moisture. If you are concerned about your choice, just keep in mind that roof underlayment can be applied in multiple layers to provide additional protection to the home.
- Best for: Premium roofing protection, leak prevention, and extreme temperatures
Rubberized asphalt is a top-quality option that provides the highest level of waterproofing. Install this underlayment material around leak-prone areas, like eaves, valleys, vents, chimneys, and skylights, to help prevent water damage before it can become a problem. The underlayment is made primarily of rubber polymers and asphalt, but the exact composition varies from product to product.
This variability allows manufacturers to slightly alter the design to provide improved waterproofing, fiberglass reinforcement, or even sound absorption. Additionally, if rubberized asphalt is damaged or there is a leak, it can be patched, though the outer roofing layer will need to be partially removed for access. However, the drawback to installing this type of underlayment is that it is the most expensive type, so if you are looking for a way to cut costs, rubberized asphalt underlayment isn't the right choice.
Rubberized asphalt roof underlayment is also known as self-adhering underlayment because it comes with an adhesive backing that sticks directly to the roof deck. The adhesive helps to form a waterproof seal between the deck and the underlayment, preventing moisture from penetrating through the roof. The composition of rubberized asphalt underlayment allows it to expand and contract without cracking or breaking. This makes it an excellent choice for regions that experience extreme temperature changes throughout the year.
- Best for: Durability, tear-resistance, and mold-resistance
The most recently developed type of roof underlayment, synthetics are made commonly made from woven or spun polyethylene or polypropylene in order to significantly increase the durability and tear-resistance when compared to saturated-asphalt underlayment. This type of roof underlayment costs more than asphalt-saturated felt, but is still more affordable than rubberized asphalt underlayment, making it a good choice for individuals that are looking for a less expensive option without sacrificing durability.
Synthetics aren't just designed for increased durability and tear-resistance, this type of underlayment is also very effective for waterproofing, though it doesn't quite reach the level of rubberized asphalt. If you have a home that is prone to mold, synthetic roof underlayment is the recommended option because this material repels water and it's inert to mold.
Use of synthetic roof underlayment has gradually increased since it was introduced to the market. It has anti-slip properties that make the underlayment easier to walk on during roofing projects, and an elasticity that helps the underlayment expand and contract with temperature changes to prevent tearing. The rolls are typically lighter than other underlayment options and will often come with clear lines and overlap guides for quick, efficient installation.
Choosing a Roof Underlayment
Even after learning more about the three main types of roof underlayment, you may be unsure about which type is the right option for your home. In order to determine the best roof underlayment, it's necessary to consider a few important factors, including the climate, top roof layer material, durability, water-resistance, and cost.
The climate impacts any materials that are installed outdoors because they are exposed to the seasonal temperature changes. For homes located in areas with relatively constant temperatures, asphalt-saturated roof underlayment is a good option, but if you live in an area that experiences extreme temperature changes throughout the year, then rubberized asphalt is the best choice. Synthetics are also a suitable option for areas with broad temperature changes.
Roofs built with traditional asphalt shingles tend to pair well with rubberized asphalt and asphalt-saturated roof underlayment. Similarly, rubberized asphalt and asphalt-saturated underlayment is also suitable for slate shingles. However, if you have a metal roof, then it's recommended to invest in synthetic roof underlayment because this material is more heat-resistant than either rubberized asphalt or asphalt-saturated underlayment.
Opt for synthetic roof underlayment if durability and tear-resistance is necessary for your roof, but if you want to prioritize waterproofing, then spend a little more to install rubberized asphalt. Just keep in mind that if cost is a factor, then it's important to know that asphalt-saturated underlayment is the most affordable choice. Synthetic roof underlayment is the next step up in price, while rubberized asphalt is the most expensive option.
However, before installing any roofing materials, make sure you have obtained the proper building permit(s) if required in your locale, and refer to the building code in your area to ensure you're using acceptable material.