Asphalt and Wood Shingle Roof Coverings

Many years ago, large uninterrupted sheets of roofing material (such as thin metal sheets or rubberized/vinyl membranes) had not been invented, were cost prohibitive, or difficult to install. As a result, many early roof coverings were made by building up a roof from smaller, overlapping elements. Roof shingles (sometimes also called “shakes”) are in essence small individual rectangular roofing components installed on pitched (sloping) roofs. To prevent water ingress through the joints where shingles meet, the top and side edges of the shingles overlap. This allows water to flow over the tops of the shingles and off the bottom of the roof, to ground level or into gutters and downspouts. In the U.S., shingles are extremely common as they are easy to install, are cheap, and provide a traditional high-quality finish. Shingles can be manufactured from different materials, but in this article, we will focus on the two most common types – wood and asphalt shingles.

Common Types of Shingle – Wood v Asphalt

hingles can be manufactured from both natural and man-made resources. Historically in the U.S., traditional wood shingles were once common, but these how now been largely surpassed by man-made asphalt alternatives (except where called for on a particular type or style of building). The following paragraphs detail wood and asphalt shingles in more detail.

Wood Shingles

Wood shingles are made by splitting or cutting logs, usually from the redwood, western red cedar, southern yellow pine and cypress tree varieties into rectangular shapes. Technically, a shingle is sawn, whereas a shake is split from the log, which is the traditional manufacturing process. Industry terms for shingles dictate two main sizes – Royal (at 18” long) and Perfection (at 24” long). During the cutting/splitting process, the bottom end of the shingle/shake is left so that it is thicker than the top, allowing a taper so that the tiles can overlap. Due to the way that the shakes are split along the face of the wood, a natural uneven and rough finish is created on the surface which adds to the architectural aesthetic of the product. If wood shingles/shakes were installed on a roof without treatment, they are subject to two main risks – rot and fire. Therefore, wood shingles/shakes are commonly pressure-treated with preservatives and fire retardants as part of the manufacturing process prior to installation.

Shingles/shakes are installed on a roof using galvanized or stainless nails, or staples, with shingles/shakes installed onto a wood deck board installed off the roof’s rafters. Deck boards are often made from exterior grade plywood, oriented strand board (OSB), or more traditionally a plank deck formed from tongue and groove planks. Between the shingles/shakes and the deck, an underlayment is often provided which usually consists of a bitumen/asphalt impregnated felt, which is stapled to the roof deck. Where average January temperatures fall below 30° Fahrenheit, the National Roofing Contractors Association (NCRA) recommend the installation of an ice-dam membrane. This generally consists of a self-adhering modified bitumen membrane which is often polymer-based. This membrane runs from the eaves, up the line of the roof by 24”, or 36” if the roof has a pitch of less than 4:12 (18°).

Asphalt Shingles

Asphalt shingles consist of asphalt impregnated materials which are finished with an external granular face. Similar to wood shingles, asphalt singles consist of rectangular profile tiles which are typically cut to 12” x 36” sizes. Architectural shingles (also called “dimensional” or “laminate” shingles) vary in width and consist of various overlapping layers to provide variance across the roof; whereas traditional “3-tab” shingles are only produced in one uniform size and installed in a single layer. Both varieties of asphalt shingles are provided with adhesive strips on the rear of the shingles which activates from the heat of the sun after installation. When heated by the sun, the strips melt and adhere the surrounding shingles together, to create a homogenous single layer which is resistant to normal wind uplift forces. Traditionally, shingles have been made with felt materials to form a base mat, made from natural and man-made fibers which are then impregnated with a bitumen/asphalt compound. Historically, in many instances asbestos fibers were also used as a base mat material, although fiberglass or other organic materials such as waste paper and wood fiber products are now used. Similar to other asphalt coverings (see “Built-Up Roofing Systems” on this website), asphalt shingles are provided with a granular surface which protects the bitumen/asphalt compound from solar degradation.

Similar to wood shingles, asphalt shingles are installed on a roof using galvanized or stainless nails, with shingles installed onto a wood deck board installed off the roof’s rafters. Deck boards are often made from exterior grade plywood or wood plank decks; however, oriented strand board (OSB) decks are usually not favorable due to potential fastener instability and OSB’s movement tendencies when subjected to moisture. Underlayment is also commonly installed under the asphalt shingles and can either consist of asphalt impregnated felt, or a lightweight synthetic membrane alternative. Where average January temperatures fall below 30° Fahrenheit, the National Roofing Contractors Association (NCRA) also recommend an ice-dam membrane system, identical to the system described above for wood shingles, is installed.

Typical Flashing Details

Flashing details for wood and asphalt shingle systems are largely similar. Flashings at perimeter edges usually consist of perimeter edge metal flashings and counter flashings; valleys are usually manufactured from sheet metal; and penetrations are usually flashed with wood/asphalt shingles, other flat sheet materials, or generic proprietary pre-manufactured products. The following two diagrams show typical metal counterflashing details that may be used, for example where a shingle roof meets a parapet wall, or through-roof penetration such as a chimney.

Pros, Cons and Common Defects

Both wood and asphalt shingles have earned a reputation as popular roofing options for domestic buildings. However, like all roof systems they have their pros and cons. Table 1-1 summarizes the typical pros and cons of wood and asphalt shingle roof systems.

Table 1-1 – Pros and Cons of Shingle Roofing Systems

Material Type Pros Cons
Wood
  • Perform well in snow and hail conditions
  • High standard of finish and architectural appearance
  • Natural product
  • Long lifespan, if maintained correctly
  • Susceptible to fire and rot, unless treated
  • Can warp, distort and split
  • More expensive than asphalt shingles
  • Can attract wood-boring insects and pests
Asphalt
  • Virtually no maintenance required
  • Affordable
  • Various color and style variations available
  • Easy to install
  • Can degrade from weather conditions, particularly temperature and ultraviolet (UV) light
  • Do not perform well in extreme heat or temperature variations – cracks can form due to excessive heating and cooling
  • Not as durable and long-lasting compared to other systems (such as tile or slate)
  • Formed from oil/petroleum products – not environmentally friendly

Wood Shingles

Deterioration is advanced to wood shingles when debris is allowed to collect on the roof. Moss, lichen, leaf litter and debris collecting on the roof decreases the roof’s ability to discharge water. This further allows water to accumulate, allowing fungi to grow which can attack and destroy the wood shingles. Pressure washing of the roof is a simple and effective way of combatting this issue. Chemical treatments can also be added to the top surface of shakes/shingles following cleaning to limit fungi growth.

Asphalt Shingles

A benefit of asphalt shingles is that they are generally maintenance free once installed. However, asphalt shingles can retain debris and moss/lichen can grow on the textured surface of the roof. General cleaning of the roof to remove debris and moss/lichen growth is recommended where required; however, pressure washing is not advisable. Pressure washing is too aggressive and can wash the protective granular coating off the outer surface of the shingle. This exposes the shingle to ultraviolet (UV) degradation, which in time will deteriorate and break down the bitumen/asphalt compound of the tile resulting in premature failure of the shingle.

STOP – Safety Moment

Historically asbestos fibers have been used in asphalt shingle tiles. Although encapsulated within the bitumen and asphalt layers, if the material is cut or removed it can release asbestos fibers which are harmful to human health. Additionally, bitumen, asphalt and adhesive compounds are formed from petroleum/oil products which are harmful and can release organic compounds. Roofing works and maintenance/cleaning activities are completed at heights where injury or death from falling from height can occur. We therefore recommend that roofing systems should generally be inspected, tested, removed, installed and maintained by competent trained roofing professionals.

Roof Warranties

Two main warranties exist relating to roofs – a material warranty which covers the roofing product, and a contractor warranty which covers the installation by a roofing contractor/business. Usually, both warranties are provided on completion of a roofing project. Typically, roofing materials (the product which is installed) are guaranteed by a roofing manufacturer for a substantial period (say 20 to 25 years) against defects caused during the manufacturing process. A contractor warranty is typically much shorter (usually 1-2 years) and covers installation-related defects. The contractor warranty should detail what items are covered, and activities that will void them. It is important to ensure that copies of the warranties are obtained, and that building inspections and maintenance activities do not invalidate any warranties in place.

Related Websites

http://www.nrca.net/ – National Roofing Contractors Association

http://owic.oregonstate.edu/sites/default/files/pubs/shake_roof_maintenance.pdf – Shake Roof Maintenance

1 Comment

  1. Thanks for the detailed blog! I’m researching this topic and yours are the most well discuss an article, the photo illustrate makes the job easier to understand. Great job summing up all of this!

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