October 4th, 2004

Article

This article, written by Andy Lodge, Chair of CASMA's technical committee, was originally published in the premiere issue of Homebuilders, the official magazine of the Greater Windsor Homebuilders' Association.

Shingle Underlayment Use in Canada

By Andy Lodge, Chair - CASMA Technical Committee

Asphalt shingles have been the most popular steep roof covering material in Canada for decades. Often they are used along with an asphalt-saturated roofing felt, which is installed beneath the shingles, hence the term “underlayment”. This article will explain some of the benefits of underlayment felts, their composition, and current requirements according to major shingle manufacturers and the National Building Code.

Historically, organic felt-reinforced shingles have been the most popular shingles used in Canada. They are routinely installed without an underlayment felt and perform very well, due in part to their added asphalt mass and thicker, stronger reinforcement. In some instances, particularly in lower slopes or in severe climates, underlayment is recommended for organic shingles. Glass fibre shingles are becoming more common in Canada, especially in the more temperate regions although they do not have any significant history in Canada as it pertains to use without underlayment felt.

Underlayment felts are used for a variety of reasons. The most common reason is that they provide the roof system with a secondary layer of protection, should the primary layer (typically asphalt shingles) become compromised due to wind blow off, excessive wind-driven rain, and other roof problems. Other benefits are outlined in the Canadian Asphalt Shingle Manufacturer’s Association’s (CASMA’s) Technical Bulletin #3 “THE USE OF UNDERLAYMENT WITH ASPHALT SHINGLES”, and are expanded upon below:

Underlayment protects the deck from moisture penetration until the shingles are applied, reducing problems that can arise if shingles are installed on wet lumber. Such problems may include buckling and ridging of the felt, which can show through the finished shingle layer.

Many asphalt roofing manufacturers specify the use of asphalt-saturated felt to validate their warranties. Consumers should read the manufacturer’s limited warranty very carefully because it usually states all the application requirements that must be followed – if the use of underlayment is one of those application requirements, it should be used. One imported glass fibre shingle manufacturer’s instructions state - “Your failure to follow these instructions may void the product warranty” and the underlayment portion of their instructions begin with the statement “apply underlayment”. On the wrapper from another imported shingle: “Failure to follow these instructions may result in serious damage to the application and life of this roofing product, resulting in the termination of any warranty, express or implied”. This is followed by their underlayment instructions, which begin: “Underlayment must be applied flat and unwrinkled.” Use of underlayment is a requirement for these glass shingles. It appears foolish to ignore such a clear warning from the manufacturer when making your purchase.

Asphalt-saturated felt should be used beneath asphalt shingles reinforced with glass fibre felt, since this is a requirement for Class A fire resistance. Fire resistance ratings and testing is performed on a roof system, which includes the deck, the underlayment, and the shingles. Traditional organic felt shingles have been successfully tested for their fire resistance without felt, but the Class A tests for glass fibre shingles includes felt. Since glass shingles are becoming more common in some parts of Canada, it is increasingly important for roofing contractors and homeowners to be aware of this.

Asphalt-saturated felt helps to minimize "picture framing", i.e. the visible outline of deck panels caused by irregularities in roof decking thicknesses. “Picture framing” is also minimized through the use of thicker, better-quality shingles.

Asphalt-saturated felt also offers protection to the shingles from resins that can be released by wood board decking materials. These resins can soften the asphalt in asphalt shingles. Since CASMA’s Bulletin #3 was originally issued in 1992, it has been learned that there have been some instances where glass fibre shingles have formed shallow horizontal “troughs” on lower sloped roofs (approximately 4:12 and 5:12 in slope), potentially channeling water to shingle joints. Underlayment felt is therefore recommended beneath glass shingles more than ever, to provide secondary leak protection.

The most common underlayment used on Canadian roofs is typically referred to as No. 15 Felt. These products generally weigh approximately 40 – 50 lbs. for a 40 square metre roll, and are supplied with laying lines on their top surface to assist in proper alignment on the roof. They consist of organic fibre felt mats, which have been saturated to an adequate level with asphalt, and are generally overlapped 2” during installation. Note that in the eave area of the roof and in roof valley flashings, other products are used, and are sometimes also referred to as “underlayments”. These membranes are typically self-adhesive modified bitumen membranes and although they can be used over the entire roof surface, they are usually used in the eave area only, with asphalt felt more commonly used in the field of the roof.

It is important for consumers to be aware that the current Canadian National Building Code does not require that underlayment be used beneath shingles but it does prescribe that when it is used, it should satisfy certain material weight requirements. And although the code allows “perforated” asphalt felt, non-perforated felt is normally specified by shingle manufacturers and should therefore be the material used. These requirements are contained in the National Code’s section 9.26.6. CASMA’s Technical Committee is currently working with Canadian building code officials to clarify and improve the underlayment requirements as they apply to asphalt shingle roof systems. CASMA member companies promote the use of asphalt saturated felt underlayment, and homeowners should confirm specific requirements, which may vary depending on shingle type, brand and the detailed roof design.

For further information on the use of asphalt shingle underlayments, or any other asphalt shingle roofing technical issue, please contact CASMA through their website at www.casma.ca.

PULLQUOTES

Underlayment felts are used for a variety of reasons. The most common reason is that they provide the roof system with a secondary layer of protection, should the primary layer (typically asphalt shingles) become compromised due to wind blow off, excessive wind-driven rain, and other roof problems.

The most common underlayment used on Canadian roofs is typically referred to as No. 15 Felt. These products generally weigh approximately 40 – 50 lbs. for a 40 square metre roll, and are supplied with laying lines on their top surface to assist in proper alignment on the roof.

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