Residential Roofs in the North
Technical Series 94-216
- Roofs have to perform exceptionally well in the North because of extreme weather conditions.
- Standard roof structures are used: trusses, joist systems and prefab panels. Post and beam construction is not common because of transportation.
- Because of continuing low temperatures and blowing snow, roof systems are not ventilated in the north.
- Because of high potential for moisture accumulation in the attic space it is extremely important to have an airtight air vapour barrier at the ceiling.
- It is good practice to
- locate the attic hatch outside the building or in an unheated vestibule,
- modify framing or use high heel trusses to allow for insulation at the eaves,
- to install a continuous layer of insulation over the joists in flat roofs
- Roofing materials: rolled roofing, shingles and metal are acceptable but not sheet membranes laid in hot asphalt. For flat roofs, prefab panels with a painted on rubber membrane are joined and sealed on site.
A roof for a northern dwelling must perform the same function as any other roof, that is it must be waterproof and strong enough to withstand wind, rain and snow loads; but it must do so in a harsh environment.
Factors that need to be considered in the design of northern roofs include:
- heavy snow and ice loads
- moisture and frost accumulation
- uplift due to strong winds
- the effect of extremely low temperatures
- availability of materials and skilled labour.
Several types of roof structures are used in northern housing.
Trusses are suitable for construction in the North since they allow a great deal of insulation to be placed in the attic space. However, there must be an effective, continuous air/vapour barrier over the full expanse of the ceiling. Trusses are typical for roofs with a slope of more than 1 in 3. In order to ensure a uniform insulation thickness over the eaves and exterior walls, the trusses are often modified as shown in Figure 1 to prevent thermal bridging at this location.
Figure 1: Retrofitted Roof. Enlarged Imaged
Joists are used in flat roofs, low sloped roofs, and conventional rafter ceilings with slopes of more than 1 in 3. In flat and low sloped roofs, the space between the joists is filled with insulation. In a roof with a slope of 1 in 3 or more, an attic is formed above the joists, allowing for the placement of more insulation, as with trusses. A modification similar to the one shown in Figure 1 can be made to prevent thermal bridging at the eaves. Alternatively, a dropped ceiling could be formed to create room for insulation. Again, a critical consideration is that there be an effective, continuous air/vapour barrier over the full expanse of the ceiling.
Prefabricated roof panels can be used in flat or sloped roofs. The double skins provide continuous lateral bracing against buckling of the webs and the cross-sectional rigidity is high enough to resist the imposed loads. For sloped roofs, special wedge shaped bearing members may be needed on top of supporting walls for sloped roofs. Special details are needed for anchorage.
Roofs carried by arched members or rigid frames are strong and stiff. The sheathing provides a strong diaphragm on which a membrane type roofing can be applied. It is necessary to provide fire stopping at the joints to prevent the spread of flame from the walls to the roof.
Large beams are not used extensively in the North due to the logistics of transportation. The maximum length of beam is determined by the transportation system. In the Western Arctic, transportation is often by barge or truck. In remote communities throughout the North, transportation of materials is by barge or ship, where the size of the beams is often restricted by the off-loading equipment. If there is only one crane, a large beam can be severely stressed under its own weight during off-loading unless special handling equipment is provided.
Ventilation of Roof Cavity
Ventilated roofs are designed to allow air to flow through the roof cavity (the space above the insulation) to remove moist air that migrates into that space. An unventilated roof system restricts or prevents air movement through the cavity.
In the North, ventilated roofs do not operate as effectively as they do in the South. The roofs seldom can vent the incoming moisture rapidly enough. The continuing low temperatures cause the water vapour to condense and turn into frost on cold surfaces before it can exit. This frost continues to build up until the spring.
In some regions of the North, if the roof is vented, ceiling finishes, ceiling joists, vapour barriers and insulation have a much shorter life span due to this moisture accumulation.
Moisture gets into the attic space in one of three ways (Figure 2).
Figure 2: Frost Accumulation in Attic Space. Enlarged Image
- If the air/vapour barrier (AVB) is absent, discontinuous, or damaged, water vapour that escapes from the house into the roof space condenses and freezes before it has time to dissipate. The frost can often reach thicknesses of 10 cm or more, discharging large quantities of water to the space when it melts in the spring.
- Frost can also be created by air entering the roof space from outside. As the temperature drops at night, this air cools and moisture condenses and freezes on the nearest cold surface.
- In the North, snow or ice crystals can be very fine and can be driven into some types of roof vents under wind conditions. Substantial quantities can accumulate under certain wind and snow conditions.
Unventilated roof systems have been used quite successfully in the North. Ventilated roofs have been retrofitted to unventilated roofs
by keeping the existing roof insulation, and adding more to fill the cavity. These retrofits have been found to eliminate many of the problems associated with moisture accumulation in ventilated roofs (see Figure 3).
Figure 3: Modified Truss End with Overhang. Enlarged Image
Other solutions to the problems of moisture accumulation in ventilated roofs include the following.
- Sealing the vents completely during the heating season reduces the amount of snow infiltration, but does not correct problems due to air leakage from the interior.
- Sealing the ceiling vapour barrier and air barrier completely to prevent the passage of water vapour into the space.
- Installing an insulated duct through the building envelope from beneath the house into an otherwise sealed attic to avoid the problems of powder snow and wind.
- Placing attic access hatches outside the building, or in a cold vestibule, so that warm, moist air doesn't leak into the attic space from the house.
For an unventilated roof system, it is recommended that the insulation be placed on the exterior of the envelope to ensure that there is no large unheated attic space where frost can accumulate. The building envelope must also be well sealed to prevent moisture penetration.
Whether the attic is ventilated or not, it is essential to have a well sealed air and vapour barrier between the insulated interior space and the exterior or attic space. All penetrations through this layer must be caulked and mechanically sealed to prevent the passage of moist air.
The performance of many roofing materials is different in northern applications than in the South, making some more suitable than others.
This type of roofing was used extensively in the past. It is inexpensive and installation does not require specialized labour or equipment. It does blister, however, and the blisters may break (when stepped on, for example) causing the roof to leak.
Shingles are extensively used. They should be firmly fixed to prevent them from being blown off (carefully nailed or stapled and tabs glued down). The other option is to use the type with adhesive tabs, which should be applied in warm weather (above 21°C). Regular weight shingles (i.e. 210 lb. per 100 sq. ft.) should not be used on roof slopes of less than 30%. An underlay of roll roofing is strongly recommended for the lower edges to prevent leaks that may be caused by ice-damming.
This type of roofing should be placed on a barrier such as rigid insulation with an air barrier or tongue and groove plywood in order to keep snow and moisture out of the roof assembly. Some sheet metal roofing can be used on very low sloped roofs. Because of the high winds in the North, the fastenings require special care, especially at joints and roof edges. On low slopes, joints, or laps, the sheets must be set in a durable mastic to avoid leaks, and other elements such as hip and ridge caps and valley gutters must be carefully sealed to the sheeting.
This type of roofing is expensive, but durable when properly applied. The paint-on type of membrane is built up by applying several layers of synthetic rubber paint, embedded into porous polyester fibre tape over the joints. It is tolerant of slight irregularities in the roofing deck as the paint fills them uniformly. A heavy coat is put over the entire roof to resist ultraviolet degradation. Pre-fabricated roofing panels, as well as some site applied plywood decks, are usually pre-coated with a penetrating undercoat and two heavy coats of rubber, then the joints are finished and final coat added at the site after installation. The top coating will have to be replaced every 10-15 years.
Various seam tapes are available for roofing, although self-adhesive roof seaming tapes are not recommended since they tend to lift, allowing leaks. Other synthetic rubber or plastic formulations should be used instead.
Another layer of defence against water penetration is to seal the plywood joints. The underside edges of the plywood are chamfered 3 mm. A bead of high quality synthetic caulking (silicone rubber) is put on the support. When the panels are nailed to the support, the triangular space formed will be filled with caulk providing a weather-tight seal.
Sheet membranes are used for built-up roofs and are comprised of several layers of roofing felt laid in hot asphalt. This type of roofing is not common in the North because of the difficult weather conditions and the need for special equipment. It is also expensive and more difficult to apply than the paint-on type. Vinyl membranes which are bonded to the roof deck can be used. They must be applied with great care: cracks and hollows in the roof deck must be filled in to avoid weak spots, all air bubbles must be removed, and the edges must be lapped about 5 cm.
Roofs in the North are often flat or low sloped, and in many cases do not have attic cavities. The insulation is placed between the joists. Solid lumber joists conduct heat many times faster than insulation and this can result in cool spots leading to condensation. Therefore it is recommended that a continuous layer of insulation be installed under the roofing materials to cover the wood members and the main insulation, eliminating much of the thermal bridging.
Examples of Housing Construction in the North; by Burdett-Moulton, Architects and Engineers, Northwest Territories, Don Jossa and Associates, Yellowknife, Northwest Territories, and Wayne Wilkinson, Consultant, Whitehorse, Yukon; for Canada Mortgage and Housing Corporation; April 1987.
Design Guidelines and Technology for Northern Housing Construction; Indian and Northern Affairs Canada, Technical Services and Contracts; April 1983.
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