Roof Underlayment – Everything You Need to Know About

When clients are interested in having their roofs replaced; the subject of roof underlayment typically comes into the conversation.

Roof underlayment; is an important and integral part of a complete sloped roofing system.

While some shingle manufacturers do not insist on the use of roof underlayment on steeper pitches, and while most subdivision homes do not have underlayment used at the time of new construction, in favour of keeping costs down; the investment is highly worthwhile.

It is important to note that all shingle manufacturers do recommend the use of underlayment beneath their respective shingles. Further to note is that modern building codes are now specifying that such products should be used as the new standard.

The next thing to consider is what types of roof underlayment are available, and where they are supposed to be used.

The term “ice and water membrane”, refers to a highly adhesive (peel and stick),  sheeting which comes in a roll approximately three feet wide. Such material comes in either a form which has a sanded or granulated exposed, exterior surface; or a smooth surfaced finish. Traditionally, the smooth surfaced material tends to be more adhesive (much like handling fly paper); whereas the surfaced materials tend to be less tacky; and can be somewhat easier to work with in hot temperatures, as it is more forgiving to becoming stuck to itself during handling.

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Ice and water membrane is intended for use at the bottom roof eaves areas of a sloping roof. Its intended purpose is to help avoid water ingress during winter, as a result of snow and ice damming which often forms at the gutter, thus accumulating snow and or ice at the bottom edge of the roof. As shingles are deemed water-shedding products (not water-proofing); the membrane acts as a buffer between the bare roof deck beneath and the shingles above.

One of the characteristics of better quality ice and water membrane products is that they will help seal where the nails have been driven through the shingles. Proper installation of such products at the roof eaves areas involves the installation of as many courses as necessary to cover a minimum of 24” past the heated interior wall. This can result in a minimum of three feet at the bottom roof eaves areas, and in some instances can involve installation covering up to nine feet from the roof eaves. It is dependent on the pitch (steepness) of the roof and the length of the overhang on the building.

Ice and water membrane can be applied in valley areas of the roof, to surround skylight curbs, chimneys, and various protrusions through the roof such as pipes and vents; which represent common leak threat areas. Adding the material around such items can dramatically help resist water ingress on a roof, especially from wind-driven rain, severe storm events, and unusual winter circumstances.

To note is that good quality ice and water membrane is widely used on lower pitched roofs (between 2/12 to 3/12 pitches); as a complete underlayment on the wood roof deck. Pitches below 2/12 should be treated as a flat roof.

When using ice and water membrane on all key roof components and specified areas, such shingle manufacturers as Certainteed™; can provide enhanced warranties to the consumer as a result.

The next category or roof underlayment is designed to be used on the balance of the sloped roof deck, where ice and water membrane has not been placed. By having an underlayment over the remaining roof deck; it supplies added protection in the event shingles become dislodged from the roof (such as in a severe storm event).

During last year’s (2019) windstorm, many homes that lost shingles due to high winds, and that did not have underlayment on their roofs, were subject to direct water ingress, resulting in interior damages. Those property owners that had underlayment; while still requiring shingle repairs, reported no interior damage typically. Such property owners also had the luxury of waiting a little longer for repair work as the roofing industry was plagued with service calls very quickly.

Underlayment designed to cover the roof deck is typically specified as one of three things. Fiberglass non-perforated felt, which is designed to work in concert with fibreglass shingles, and helps to provide a class “a” fire rating for the roof assembly.

Organic felt (good old fashioned tar paper); which can be classified as number 15 lb. or 30 lb. This material has been a mainstay underlayment for decades, used both as a roof and wall underlayment.

Such felts contain asphalt and as such can supply some short term water-shedding characteristics if installed correctly. Using organic felts in a roof assembly provides a class “c” fire rating.

In more recent years; shingle manufacturers have introduced their own versions of a synthetic underlayment, designed to have more aggressive water resistance capabilities. Many manufacturers promote the fact that when such products are installed in a water-shedding method, and secured using disc fasteners at a prescribed rate; that a roof can remain essentially waterproof for a period of six months. This product makes it a popular choice among custom home builders that require quick water protection, as they allow for trades to move forward with their agendas on the inside of the build project.

To note is that such synthetic underlayment has not been in the market place for as long as felt materials, but manufacturers do stand behind such products when used in conjunction with their same branded roofing products.

Under some circumstances, choosing felts versus synthetic products may be preferred where for example, issues involving condensation have been identified in a particular building.

The market has majorly shifted to using synthetic underlayment these days as a result of the fact that it is manufactured in 1,000 sq. ft. coverage rolls, and provides ease of installation. The wider roll product allows roofing contractors to water-tight a roof more quickly.

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There are other types of roof-related underlayment which are designed for specific purposes.

As an example, a product called “cedar breather™”, is designed to provide ventilation above the traditional underlayment, and beneath cedar shingles to be installed on a sloping roof. The mesh-like rolls of material to allow the cedar products to breath by promoting air circulation beneath the shingles.

Another type of underlayment is rosin paper, often used beneath copper panel roofs, which is designed to mitigate the formation of condensation, where the metal product would otherwise touch the bare wood substrate.

Yet another type of underlayment, which is used to wrap bare wood walls is a form of “house wrap”.  Among many trade name versions, popularly include “Tyvek™”; which is designed to help prevent cold air from migrating into a wood structure building, yet allowing moisture to escape through the material.

Such material is taped at joints to help create a tight seal of the exterior building envelope. In terms of modern building methodology, it is the most popular choice to be installed before bricking or siding a structure.

Other types of exterior underlayment include waterproofing products such as “Blue Skin™”, which is a highly adhesive, water impervious, membrane; commonly used as part of waterproofing exterior concrete walls, foundations, sills, wood walls near the ground, etc. Some versions of the product require the use of an asphaltic primer beneath, to provide for proper adhesion.

Such products may also be used in relationship to roofing work and repairs.

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