Roofing Systems Explained

The purpose of the roof is multi-faceted. It protects the rest of the home and it's occupants from rain, snow, sun, and, wind. The roof acts as an umbrella shielding the home against rain and snow preventing moisture from entering the home and causing damage. When vented properly, the roof also acts as part of a system to ventilate your home. That is, its' underside (or attic space) provides a space to exchange air. Ventilation, insulation and vapour barriers in this area are key to maximizing comfort levels, energy efficiencies, and the life span of roof coverings (eg. shingles), underlay material (eg. plywood sheathing) and the structure itself (eg. rafters, trusses). The condition of the structure which supports the roof, its' underlay/sheathing and the covering are instrumental to a good roofing system. Often people concern themselves only with the age of the "roof" when buying a home. Specifically they're referring to the age of the shingles themselves. This is an important question but it is relative to the type of material used as they all have varying estimated useful lives.

Types of Roofs -- Generally there are two types of roofs: sloped and flat. The slope of the roof is often referred to as the pitch as well. The slope, or pitch, is determined by the rise and the run of the roof. We refer to the slope with the run always remaining constant at 12 feet (or inches). So, for every 12 feet (or inches) of run we can then measure the rise over that distance in feet (or inches). For example, a roof with a 7/12 pitch means that over a distance (run) of 12 inches it rises 7 inches. That means that a roof with a 12/12 pitch is steeper than a roof with an 8/12 pitch as the rise is higher.

Type of Roof - Slope/Pitch

Flat - 2/12 or less

Low Slope - 2/12 to 4/12

Conventional Slope - 4/12 or more

Types of Roof Coverings

Roof coverings come in wide variety of styles with varying characteristics, life spans, and costs. Here are some of the more common ones:

1) Asphalt Shingles -- these types of shingles are graded largely by their weight. The premise being that the heavier they are the more material there is and therefore the longer they will last. As such, asphalt shingles commonly have life spans of about 10 to 15 years (economy grade), 15 to 25 years (standard/conventional grade), and, 25 years and over (premium grade). Factors affecting the deterioration of the shingles include sunlight and the slope/pitch (the greater the slope the longer the shingles will last). The exposure or reveal of the overlapping shingles will also be factor in the life of the shingle.

What's A Square? -- Roofer's refer to "a square" as the amount of material required to cover a 100 square feet of the roof surface. Building supply houses sell these roof coverings/shingles in these lots as well.

2) Slate Tiles -- slate tiles are made from a quarried naturally occurring material. Although consistency and quality are hard to ensure these very heavy tiles nevertheless last a long time compared with other roof coverings and are known to last up to 100 years. Factors affecting the deterioration of the slate tiles are largely associated with the installation of the material (for example, exposure/reveal of tiles, types of hardware used to secure the tiles) although the tiles themselves can vary considerably too. The slope/pitch (the greater the slope the longer the shakes/shingles will last) is also a factor but to a lesser degree than asphalt shingles.

3) Wood Shingles/Shakes -- what's the difference? Wood shingles are cut or sawed whilst shakes are split from the log. Shakes are thicker and tend to last longer than wood shingles. Wood shingles last anywhere from about 20 to 40 years yet some are known to deteriorate much sooner. Shakes on the other hand tend to last about 25 to 50 years. Factors affecting the deterioration of the shakes and shingles are, their ability to dry out, sunlight and the slope/pitch (the greater the slope the longer the shakes/shingles will last). The exposure or reveal of the overlapping shakes/shingles will also be factor in their life.

4) Clay & Concrete Tiles -- these are gaining in popularity of late. Like slate tiles they are very heavy yet last anywhere from about 40 to 100 years. Like slate tiles, factors affecting the deterioration of the slate tiles are largely associated with the installation of the material (for example, exposure/reveal of tiles, types of hardware used to secure the tiles) although the tiles themselves can vary considerably too. The slope/pitch (the greater the slope the longer the shakes/shingles will last) is also a factor but to a lesser degree than asphalt shingles.

5) Built-up Roofs -- these are typically associated with flat and low pitched roofs. In the past, tar and gravel was the main stay but today torch-on and EPDM membranes are becoming more common.

a) Tar & Gravel Roofs -- these have multiple plies of roofing felts with asphalt applied between them. The term "tar" in this type of system comes from coal tar that was used on these roofs. Over the final layer of asphalt gravel is broadcast into it to provide for wear protection and reduce UV degradation. Although some roofers may use roll roofing for the final layer the life span (about 5 to 10 years) is far less than gravel (about 10 to 20 years for 4 ply and 5 to 10 years for 2 ply). Factors affecting the deterioration of the tar and gravel roofs are largely associated with the installation, too little gravel, and, sunlight.

b) Single Ply Membrane Roofs -- these include PVC (polyvinyl chloride), EPDM (Ethylene Propylene Diene Monomer) and EP (Ethylene Propylene). As the name implies these are the only "true" single ply membrane systems.
Although difficult to say, the estimated life span is about 25 plus years for PVC & EP, and 20 plus years for EPDM.

c) Multi-Ply Membrane Roofs -- these include Modified 2 Ply membranes where the felt (in a tar and gravel roof) is replaced with polyester/fibreglass incorporated with a "modified" asphalt. The modifiers are either:

SBS (Styrene butadiene styrene) that can have a granular finish (not unlike roll roofing) or a smooth finish, and, is torched welded, or,

APP (atactic polypropylene) which requires two layers, is thicker, not as pliable as SBS, and requires UV protection, and, is torch welded in place.

Although difficult to say, the estimated life span is about 25 plus years for SBS and 15 years for APP.

d) PMR (Protected Membrane Roof) & IRMA (Insulated Roof membrane Assembly) -- these have the roof membrane applied to the roof deck with insulation covering it. The membrane used varies from built-up roofing to PVC. The insulation must be weighted down to stop it from lifting in winds or floating when it rains.

6) Metal Roofs -- these can be made from a number of different materials including galvanized, copper, pre-coated steel, and, tin. Some metal roofs are installed in overlapping sheets or panels. Others are joined by crimping the sections of metal or soldering for example. Metal shingles are available as well. Life spans of these roofs vary but generally can last about 40 to 60 years. Factors affecting the deterioration of the metal roofs are largely associated with installation methods and maintenance.

Flashing -- flashing is used where two or more building materials come together. Its purpose is to keep water out. On a roof they are used around the chimney, skylights, at the intersection of a wall and a roof, cant strips (where a roof and wall meet), hips/ridges, parapet walls (where the exterior roof wall extends above the roof line), roof protrusions (plumbing vent stacks), and, roof valleys (where two roof planes meet). The material used for flashing can be from galvanized steel, copper, sheet lead, polyethylene and EPDM membrane.

Ice Damming -- is caused by the freeze-thaw action of snow built up on a roof. Commonly this occurs at the eavestroughs. As the snow/ice melts it is trapped by a lower ice dam and backing water underneath the shingles. This can wreak havoc not only with the shingles but the underlay/sheathing material. The cause for this can be heat escaping from through poorly insulated and ventilated attic spaces. Eave protection (waterproof membrane) under the shingles and extending about 3 feet into the insulated attic space will also reduce the risk of ice damming.

What are some different Roof Styles?

Curb -- a roof where the slope is in two or more parts on two or more sides creating a "curb" at the pane where the pitches changes

Gable -- a ridge roof ending in a gable

Gable Dormer -- same as a gable but with dormers (internal recess in the roof space) in the roof plane

Gambrel -- a roof slope that is in two parts with the lower part steeper than the upper

Hip -- a roof that has all of its sides slope up to a centre point or ridge

Mansard -- a type of curb roof with the slope of the upper portion not great and the lower portion steep

Monitors -- a type of gable roof often used with commercial buildings that is raised at the ridge to provide for windows

Pavilion -- a roof that forms a figure of more than four sides

Pent -- a single sloping surface that is not a lean-to roof

Polygonal -- a roof that forms a figure bound by more than four straight lines

Pyramid -- a hip roof usually with four equally pitched side meeting at a peak

Ridge -- two opposing roof slopes joined at the top with a gable at each end

Shed -- a roof with only one set of rafters that usually extends from a higher point on one wall to a lower part of another wall

Shed Dormer -- same as a shed roof but with dormers

Certain styles of homes have, for example, vaulted/sloped ceilings that can have little, or no, attic space to inspect. Generally speaking, these types of homes can have poorer ventilation underneath the roof covering, rafters and underlay material. That is, there is less of an "air wash" under the roof than homes with traditional attic spaces. This can reduce the life spans of these and other building related materials and cause concerns, such as, moisture, mildew, and fungus in these areas.

If you're considering an addition to your home, or buying a new home, scissor trusses can create a lesser vaulted/sloped ceiling yet still provide a smaller attic space. Alternatively, wood "I" beams which are deep enough can be used as rafters for these types of ceilings and still provide some "air wash" after the insulation has been installed.

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