No. 3
1992-09-01
Revised: 1997-04-01
2001-06-04
2006-09-12
Printable version

THE USE OF UNDERLAYMENT WITH ASPHALT SHINGLES

There are many reasons why the use of asphalt-saturated felt as an underlayment prior to applying asphalt shingles makes good roofing sense.

•  Underlayments protect the wooden deck from moisture penetration until the shingles are applied, thus greatly reducing problems that can arise when shingles are installed on a wet surface.

•  Many asphalt roofing manufacturers specify the use of an underlayment over a wood roof deck to validate their warranties.

•  An underlayment should be used beneath asphalt shingles reinforced with glass fibre felt, especially on roofs with a slope less than 8:12. The use of underlayment is a requirement for Class A fire resistance.

•  Installing an underlayment helps to minimize «picture framing», i.e. the visible outline of deck panels caused by irregularities in roof decking thickness.

•  The water resistance of an underlayment provides secondary protection by helping shield the deck from wind-driven rain.

•  Underlayments also offer protection to the shingles from the resins that can be released by the wood board decking.

•  Underlayment material should preferably be asphalt-saturated felt, and if so, they should meet at least one of the following industry standards:
a.  CSA 123.3-M - No. 15 / ASTM D226 Type 1 (No. 15 felt)
b.  CAN 2-51.32 - Breather Type Sheathing Paper.

There are no CSA standard for synthetic membranes, but they should be at least as strong as the asphalt saturated felt.

The proper application techniques recommended by the manufacturers should be followed to ensure optimum performance of the underlayment and overlying shingles.

With recent advancements in construction materials and manufacturing processes, certain manufacturers are producing synthetic underlayment products. Synthetic underlayments are generally a suitable alternative to the proven asphalt saturated felt.

* Synthetic underlayments are installed in a similar fashion as asphalt saturated felts; they are temporarily tacked into place with fasteners prior to subsequent shingle application. However, unlike felt, where nails/staples are used for the installation of the product, some synthetic membranes may need to be installed with 1 inch diameter metal/plastic caps. As with felt underlayment products, sufficient side and end laps must be provided. Although many synthetic membranes are "breathable", some products may have very low vapor permeability, much like ice and water membranes. Thus, if a synthetic underlayment is used, it is recommended to check with the manufacturer and provide proper ventilation (check local building codes for compliance).

* Check with shingle manufacturer regarding compatibility of a specific synthetic underlayment with their specific asphalt shingles.

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