the basic homeowner's guide to roofing underlayment

The Basic Homeowner’s Guide to Roofing Underlayment


PART IWhat Is Roof Underlayment?When you look at residential roofs including your own, they mostly appear the same. Giving it more attention and thought may actually be the last thing to come to mind, but when you do, you will realize that you are taking a crucial step to taking better care not only of your roof, but your entire home as well.


What Is Roof Underlayment?

1The roof is a system and for the uninformed homeowner, it can seem like a very complex one. However, your roof is worth understanding and learning more about. Now, let us take a closer look into an aspect that is probably new to your vocabulary: roof underlayment.

Roof Underlayment Defined

From the term itself, you can assume that it is something you can find under the roof. This is somewhat true, but there are other things worth considering and looking into.

Underlayment is a sheet of material installed between the roof deck and roof covering or shingles. It is mainly used to separate the roof covering from the roof decking or frame, and is traditionally made of felt and saturated with asphalt, creating a bond with the shingles.



1

THE IMPORTANCE OF UNDERLAYMENT

Knowing what roof underlayment does can help you understand its importance, especially in certain applications. Some homeowners may think that roofing contractors simply add underlayment as part of the project, but your contractor may actually have specific reasons for selecting and recommending a particular product.

Below are the primary roles of roofing underlayment.

Provides immediate protection to the open roof during installation

The underlayment helps prevent water from getting through and moisture from resting on the roof deck while the shingles are not yet installed.

Serves as ice and water barrier

Synthetic underlayment can be used on areas where overhangs meet heated surfaces of the roof deck, preventing ice back up and water ponding as the result of melting snow. This helps avoid the formation of ice dams.

Some underlayment materials are considered as a vital part of complete roofing systems and are specifically required for installation by roofing shingle manufacturers. Such underlayment materials are necessitated for the validation and maintenance of warranty conditions for particular shingle types.



1Helps prevent leaks

The underlayment creates an additional seal around the area where roof nails penetrate. Even though the nail areas may be covered, ice, snow, and rainwater might still be able to find its way underneath the shingles, and the underlayment can provide another effective barrier.

Enhances roof’s appearance

This is a lesser-known advantage of roof underlayment. Some roof decks may have a wavy appearance and the extra roofing layer between the deck and the shingles can cover up the wavy areas of the deck. This is especially beneficial when using three-tab shingles, which are thinner than architectural or laminated shingles.


PART 2The Different Types of Roofing UnderlaymentFor a roof system to perform optimally and for its service life to be maximized, the underlayment product to be used must be carefully selected. Improper selection of the underlayment may allow water leakage and moisture entrapment which can both cause premature deterioration of the concealed components, affecting the performance of the entire roof.


The Different Types of Roofing Underlayment

2TYPES OF ROOF UNDERLAYMENT

Below are the three basic types of underlayment used in residential roofing.

Felt Underlayment (Asphalt-Saturated Felt)

This is the most commonly used type of underlayment in residential applications, especially for homes with steep-sloped roofs. It can be made from either organic substrate or inorganic (fiberglass) substrate. Although the organic substrate is more common, the inorganic variety usually lies flatter and stands up better than the former.

Here are some of the characteristics of felt underlayment:

■ It is water-resistant but not waterproof.

■ Generally available in two thicknesses: 15# (pound) and 30# (pound). The 15# has a perm rating of about 5 but can rise in high-humidity conditions. Meanwhile, the 30# felt is known to be more resistant to damage as it is thicker and stiffer.

■ Felt underlayment provides little resistance to air and moisture vapor migration, so it is not an ideal air barrier.

■ Best used in conjunction with ice dam protection.



2 Synthetic Sheet Underlayment(Non-Bitumen Synthetics)

Synthetic plastic sheet underlayment is typically made from polyethylene, polyolefin, or polypropylene, which are materials that are also used in the production of other products such as containers and ropes. Popular for their durability, synthetic sheet underlayment also has the following characteristics:

■ Comes in thicknesses of between 8 and 30 millimeters.

■ Lightweight yet possesses notable strength.

■ Less susceptible to wind damage and less sensitive to cold weather installation compared to felt.

■ Lies flatter after being subjected to wetting once installed.

■ Most synthetic underlayment on the market is vapor resistant, but does not qualify as an air barrier.

■ Resistant to moisture preventing fungal growth.

■ Resistant to UV damage.



2Self-Adhering Membrane Underlayment(Rubberized Asphalt)

This is a type of underlayment commonly consists of rubberized asphalt, but may also use butyl-based adhesive. It is a self-adhering peel-and-stick product designed to adhere securely to the roof deck, and seals around nails, staples and other fasteners. Among its other characteristics, it may have the following:

■ Polyester or polyethylene bonded to the upper surface, providing non-skid and weather-resistant properties.

■ Fiberglass reinforcement.

■ A polymer film bonded to the weather surface that helps improve its moisture resistance.

■ Mineral coating on the weather surface.


PART 3The Basics of Roof Underlayment InstallationRoof installation does not start and end in the nailing of shingles. It involves installing solid decking for adequate support and applying underlayment that will serve as the waterproofing membrane.


The Basics of Roof Underlayment Installation

3 BASICS OF ROOF UNDERLAYMENT INSTALLATION

Roof underlayment installation procedures and intricacies vary per type. While your contractor is probably already adept in installing the material properly, it is still better for you to have an idea about it so you will know what to check for.

Felt Underlayment

■ In low-slope roofs, which include 2:12 up to 4:12, courses of felt underlayment should overlap at least 19 inches. This will make for a double layer of felt across the entire roof.

■ In steep-slope roofs, which include 4:12 and up, the upper courses should overlap the lower ones by at least 2 inches.

■ Felt is usually fastened using staples, but if you are in a high-wind area, plastic windstrips are recommended to be used along the edges to prevent tearing.

■ Plastic caps may also be used as fasteners for high-wind areas.

■ Felt underlayment should also overlap the edge metal at the eaves while being overlapped by edge metal on the rakes.


The Basics of Roof Underlayment Installation

3 Synthetic Sheet Underlayment

■ Slope limitations in the installation of synthetic underlayment vary by manufacturer. Some require overlap for low-slope roofs while others don’t.

■ Many synthetic underlayment products are meant to be wrapped around the roof edge, protecting the edges of the roof sheathing. This is also done to avoid problems caused by moisture.

■ Plastic caps or roofing nails are generally used for fastening synthetic underlayment. The use of staples is not recommended, as this type of underlayment is not self-sealing.

Self-Adhering Membrane Underlayment

■ This is practically a peel-and-stick product so installation is generally easier.

■ Once the adhesive is set, self-adhering underlayment is very difficult to remove – so laying it properly on the roof deck in the beginning is of utmost importance.


KRECH EXTERIORS5866 Blackshire Path, Inver Grover Heights, MN 55076

651-968-8888www.KrechExteriors.com

http://www.KrechExteriors.com

the basic homeowner's guide to roofing underlayment

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