Underlayment: The Most Important Roofing Layer You’ll Never See

At its core, a roof is a system of layers and materials that works together to do what a roof is supposed to do – that is, keep water out, keep the weather outside where it belongs, keep the temperature comfortable, and make sure that animals and plants don’t make their way into your home uninvited. When we think of a roof, we often think of the outside layer – in most modern roofs, that’s the shingles or tiles that you see when you look at a roof. But underneath that, there is actually an equally important layer called the underlayment.

You’ve probably seen underlayment before if you’ve ever passed by a roof that’s under construction. It is a sheet of material that goes between the wooden structure of a roof and the shingles that greatly assists in making sure that the roof remains waterproof for its full lifespan. Your underlayment is the last defense against water before it gets into your home. There are a few different types of underlayment that are common, and it’s important to realize that they are not created equal.

Types of Underlayment

In general, there are two kinds of underlayment that we see in common use: felt and synthetic.

Felt underlayment is something that we commonly see on our competitors’ proposals, and the most common one is 15-pound felt. This is a felt paper infused with tar, and it is usually the cheapest way to meet building code as well as the manufacturer minimum requirements for an underlayment. The tar allows it to grip a nail, making it a bit harder to remove than if the nail were going through regular paper, however there are much better options available. 15-pound felt is prone to ripping and can be torn easily by hand. Additionally, it is not as resistant to weather, and can be damaged easily by UV light from the sun. 30-pound felt is another option that can be thought of as a heavy-duty version of its lighter cousin. While it does offer advantages in terms of its strength, 30-pound felt can still be ripped by hand and is weak against UV exposure.

For reference, the ‘pound’ rating of the roofing felt paper is measured by how much 100 square feet of the material weighs on a scale, therefore showing you the density or thickness of the material. The felt paper that is used behind stucco or siding on a house is 5-pound material, while construction paper, or red rosin paper typically comes in at about 2-pounds.

The second type is a synthetic underlayment, and it is made of either a woven or spun polymer composite. These have several advantages over traditional felt underlayments, but the most important is that it is many times stronger. Synthetic underlayments cannot be ripped by hand – and won’t be damaged by things like boots as workers walk over your roof during the installation process. Additionally, while felt underlayments are water absorbent, synthetics actually repel water. Additionally, because of its polymer construction, it is resistant to mold, whereas felt is paper based, which mold can actually use as food.

In addition to standard synthetic underlayment, we also use a special type of synthetic underlayment in the valleys of a roof, where two sections come together, called an ice and water shield. This sticks to the wooden structure of the roof and is specially designed to handle the flow of water that comes through the valleys during rain. It helps make sure that there are no weak points in your underlayment and will make sure that you won’t have to deal with leaks that can occur when water builds up in the valleys of your roof where rain likes to flow during storms.

There are only a few situations in which we will recommend a felt underlayment, and in these situations, we still never use 15- or 30-pound felt. We will always double-layer 40-pound felt to make sure that you are getting the maximum level of protection that felt underlayment can offer. We recommend an ice and water type of underlayment, or a double 40-pound underlayment for tile roofs, because they tend to be more resistant to wear caused by the movement of tiles as they shift around from wind, and small earthquakes.

Outside of this specific case, we always recommend synthetic underlayment. It has massive advantages in terms of longevity and strength compared to felt underlayment and is an integral part of our leak-free guaranteed roofing system. Synthetics only cost slightly more than felt underlayment, but the benefits that they bring can mean thousands of dollars in saving later from repairs that you will never need to do, leaks you will never need to deal with, and years added onto your roof’s lifespan.