Roofing Underlayment Guide – Felt, Synthetic and Ice / Water Shield

In most regions, a roof will not be able to adequately protect the house from exposure to the elements without a layer of underlayment installed right under the external roofing material. As a homeowner, you need to make sure that your roof underlayment is made from high quality durable material and is installed properly, so that it will be able to deter water from getting inside the house. If you are planning to do a re-roof or install a brand new roof on your home, you need to make sure you understand the different underlayment options available on the market. It is important for you to choose the best quality underlayment that will provide long lasting protection for your home, suitable for the specific type of climate you live in.

How do I choose the best underlayment for my home?

In choosing underlayment, you goal is to get one that will provide you with the most protection against leaks at a price that you can afford. For example, a high quality synthetic underlayment (such as GAF Deck Armor) can extend the life of your roof by an average of 5 to 10 years, at an additional expense of only a few hundred dollars over felt underlayment. If you live in any of the states that get a lot of snow, ice and rain, it is wise to invest more money and install an ice shield in combination with synthetic underlayment. Synthetic underlayment or an ice shield or a combination of both should always be used for metal roofing, clay tile, slate, and other premium (expensive) roofing materials to provide a long lasting secondary layer of roof protection.

Initially spending more money on premium underlayment material will actually help you save a lot of money down the road. You will not need to spend extra to fix your leaking roof, or buy a whole new roof if yours will end up being in a condition beyond repair. If you live in a mild climate with minimal rainfall, your roof should be adequately protected with felt underlayment.

Roofing Underlayment video – overview of underlayment types and installation guidelines:

What exactly is Roofing Underlayment?

Roofing underlayment is a special barrier that is installed between the roof substrate and the surface roofing material (shingles, tile, membrane, etc). It provides added protection to the roof structure and the interior of the home against water leaks caused by rain and ice dams.

What is the proper way to install roofing underlayment?

Underlayment is installed in an overlapping sequence from the bottom of the roof going up. Typically, it comes in rolls and can be self adhered or nailed down to the substrate. If you are installing ice and water underlayment, it needs to be installed first. Second, you install the first course of either synthetic or felt underlayment, overlapping the ice and water underlayment by at least 6 inches. If you are installing felt, consecutive courses should overlap by 50%. If you are installing synthetic underlayment, 6 inch overlap is sufficient. It is advisable to use plastic cap nails to fasten down the underlayment with 12 inches on center spacing at the top and bottom of each course. The vertical overlap should be at least 2 feet or 24 inches. Underlayment should cover the entire roof deck. When transitioning into the valley, underlayment should overlap the ice and water shield(previously installed along the valley) by 6-8 inches. All nails should be placed at least 12 inches away from the center of the valley.

What are the different types of roofing underlayment?

There are three types of underlayment that can be installed on a roof: felt, ice shield, and synthetic.

1. Felt is tar or asphalt saturated paper made with some fiberglass. It is the most widely used and the cheapest roofing underlayment. While felt offers a general level of protection against leaks, it may not be enough if you live in an area that gets heavy snow and/or rain fall. Felt underlayment is not as high quality material as synthetic underlayment, does not last as long, does not offer the same level of protection and is less tear proof. Felt or tar paper underlayment is typically installed with asphalt roof shingles. For premium roofing products, contractors and manufacturers usually recommend using synthetic underlayment (read below for more details).

2. Ice Shield (also known as ice and water shield, or I&W) is a thick asphalt or bitumen based reinforced membrane. This membrane has a sticky surface, that glues to the roof deck, and does not tear after installation. In Northern states and Canada, building codes require the usage of ice shields, because it protects the roofing structure from water penetration caused by wind-driven rain and ice dams. It is essential to use an ice shield in critical areas such as eaves, rakes, ridges, valleys, dormers and skylights. It is best to install the ice shield when air, roof deck and membrane temperatures are at or above 40°F (4.4°C). For best protection, an ice shield should be installed at the bottom 3 ft of the roof. The rest of the roof can be covered by regular underlayment (felt or synthetic)

The down side of an ice shield is that when it is installed over the entire roof deck, it also acts as a vapor barrier, which means that it traps water under the roof. Trapped water can cause the substrate to rot and will also lead to the formation of allergenic mold and mildew. As a result, it is generally recommended to avoid covering the entire roof with an ice shield.

3. Synthetic is an expensive substitute for felt underlayment and has a number of advantages. It lasts longer, is more tear proof, and usually does not deteriorate. This underlayment can be breathable or non-breathable. Non-breathable underlayment acts as a vapor barrier, trapping moisture under the roof. Consequently, it is best to avoid it and not have to deal with the problems of rotting roof substrate and mold that invariably occur when there is too much water trapped under the roof.