Components of a sloped roof system

This article discusses steep slope roof systems found in Canada.

Schematic showing main roof components

Schematic showing main roof components

A.                 Insulation

Insulation keeps the heat in and the cold out.  In Quebec, the current recommendation is to have a minimum of R-40 in the roof.  Most insulation works by holding air in a stationary state.  The less air the insulation contains, the less it insulates.  If there is air movement through the insulation, its insulating value diminishes.  The insulating values for traditional insulation (fiberglass bats, cellulose, rock wool) are determined under lab conditions where air movement is not an issue.  However, in the real world, air movement, improper packing, and condensation all work to diminish the insulating value of the installed insulation.  In a later article we will look at the types of insulation available and how to evaluate its effectiveness.

B.                 Vapour Barrier

The vapour barrier is usually a thin sheet of polyethelyne film located on inside of the gyproc or drywall, between the insulation and the drywall.  Its main purpose is to prevent moisture migration from warm areas (ie, the living space) into the cold areas (the attic or wall spaces).  Its secondary purpose is to prevent air movement through the walls or ceiling.  Careful installation usually requires that the vapour barrier joints be sealed by sealing tape, and effort is made to ensure that there are no openings or pin-holes which would allow air to move through.  However, vapour barriers often fail to do their job because they are penetrated by pot lights, power outlets, switches, and poor finishing detail around doors and windows.

C.                 Ventilation

Ventilation in roof systems is used for three purposes:  to reduce the heat buildup in summer, to dissipate the residual heat loss in winter, and to dry out any residual condensation in the attic.  In a northern climate, passive ventilation in preferred to power ventilation, as there is always danger that power drafts will pull moisture from living spaces into the cold spaces if the vapour barrier is not effective.  Passive ventilation is often expressed in terms of ventilation area to attic area.  The (Canadian) National Building code minimum is 1:300 (1 unit of ventilation to 300 units of attic area).  The recommended ratio is usually double that (ie, 1:150).

Another aspect of ventilation is how evenly the ventilation flows through the roof system.  Ideally, cold dry air enters low in the roof system, at the soffits, flows linearly towards the ridge, and is exhausted out of the structure at the highest point.  However, given the complexity of designs of many roofs, the flow is not linear, and there are often unventilated or poorly ventilated areas.  Under those circumstances, there are techniques that can be used to reduce the potential damage.

D.                Decking or Sheathing

Decking is the wood structure supporting the roof covering.  Its main functions are to support the roof covering, resist wind forces, keep the precipitation out of the house, and provide the physical structure that give the roof its appearance.  The decking is usually supported by joists or rafters.  Decking can be made from planks, plywood, or oriented strand board.  The integrity of the roof system depends critically on the integrity of the deck.  Things that damage the deck are moisture (either infiltration or condensation), shrinkage (caused primarily by heat drying out the wood), and physical force (falling objects).  The first two causes can be minimized by intelligent roof design.

E.                 Underlayment

The underlayment is the material between the roof covering and the decking that functions to keep the external moisture out of the roof system.  15 lb. tar paper is a commonly used underlayment.  Various grades of waterproofing membrane (knows also as Ice-and-Water shield) are used to resist water infiltration under pressure, as can occur behind an ice dam.

Typical flashing locations

Typical flashing locations

F.                  Flashings

Flashings are metal components that are used to reinforce specific weak points.  For instance, end-wall flashings are commonly used along the roof/wall edge to prevent moisture from moving from the roof into the wall.  Valley flashings are used in valleys to diminish the damage that concentrated water flow (or ice for that matter) can cause in the valley joints.  The flashings can be simple (sections of L-shaped metal), or very complex, depending on their intended purpose and design.  The metals used in making flashings include galvanized steel, painted aluminum, copper, galvalume (steel coated with an alloy of aluminum and zinc), and more exotic combinations.  In a well-designed roof system, specific flashings are used in key locations to counter potential weaknesses.

G.                 Roof covering

The roof covering is the external skin of the roof.  It faces the elements (rain, snow, ice), the wind, the sun (with its heat and UV rays), extremes of temperature, and also had to resist physical damage (falling objects like branches, hail, foot traffic, ice movement during winter).  It provides the primary esthetic component of the roof system.  It may be the primary water-resistant layer, and in some cases is the ONLY water-resistant layer.

Many different materials have been used to make the roof covering, ranging from straw (in thatch roofs), to asphalt shingles (the most common covering in North America), to clay and ceramic products, to plastic products, to metal products.  There are also wood roofs, glass roofs, slate roofs and hybrid roof coverings.  Each type of material has its strengths and weaknesses, and requires intelligent choice and design to function properly.  Factors that affect roof choice include cost, maintenance requirements, durability, longevity, appearance, ease of use, and reliability.

In addition to the roof covering, the fasteners that are used to hold the roof covering on the roof, and their method of use, often either contribute to the effectiveness of the roof, or cause premature failure.

When investigating the state of the existing roof covering, it is important to know the typical pattern of deterioration of the roof covering.  Each type of roof covering has its own pattern of deterioration and eventual failure.

H.                Roof design

Roof design has an important role to play in how well the roof system plays.  The simpler the roof system, the easier it is to determine how well it works.  The more complex designs that are popular with modern builders, are unfortunately not straightforward to implement in an effective and durable way, and there are often conflicting effects.  This may influence the durability and the longevity of the roof system.  While it is usually not possible to do much to a roof design after it is built, it is useful to understand how it behaves channeling water, resisting wind, and supporting ice and snow loads.  The specific design may place loads on the roof coverings that would (or should) eliminate some of them from consideration.

I.                   Accessories

Roofs support a bewildering array of various accessories, from weather vanes, to antennas, to snow control devices.  These are usually installed without much thought about their effect on the roofing system.  These must be examined periodically to determine if they are causing, or contributing to damage to the roof system.

J.                   Roof penetrations

Roofs have many penetrations, including plumbing vents, ventilation vents, electrical poles, skylights, chimneys, air conditioning fixtures, outlets from bathrooms, kitchens, and laundry rooms, and so forth.  Each penetration has specific characteristics that have to be taken into account when building an effective roof system.  As with roof coverings, roof penetrations have characteristic patterns of deterioration over time.  The roof penetrations should be inspected at least once a year to determine if they are still serviceable, and whether they are approaching the end of their useful life.

(c) 2013 Paul Grizenko

Roof components, roof penetrations, insulation, decking, sheathing, underlayment, waterproofing membrane, ventilation, roof covering, asphalt shingles, vapour barrier, flashings, metal roofs, roofing materials

One thought on “Components of a sloped roof system

  1. Pingback: Planning for a new (re-) roof? A checklist of things to consider. | PRS Consulting

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