For both pitched and flat roofs, it is important to provide adequate ventilation of the roof space above the insulation. Even where air and vapour barriers are used, some moisture will leak around pipes and other openings and through the vapour barrier itself. If water vapour is allowed to accumulate in attic spaces and under flat roofs, during cold weather it is likely to condense in a cold spot in sufficient quantity to cause damage. Since most types of roof membranes are highly resistant to vapour transmission, the most practical way of removing vapour which enters the roof space is by ventilation.
During cold weather, heat loss through the ceiling insulation combined with exposure to sun may provide enough heat to melt the snow on the roof, but not on the eaves. Water from the melting snow can then freeze and form ice dams at the eave-strough and roof overhang. This may cause water to back up at the eaves, penetrate through the roof and leak into the walls and ceilings. Similar dams may form in roof valleys. A well-insulated ceiling and adequate ventilation will keep attic temperatures low and help prevent snow on the roof from melting. The installation of appropriate eave protection and valley flashings will also prevent potential water damage.
A common method of providing ventilation is to install louvered openings or continuous screened slots in the soffit at the eaves of gable and hip
HEALTHY HOUSING INSIGHT
wood roof truss with raised heel
A number of alternative roof framing systems are available which fulfill the Healthy Housing objective of resource efficiency. These manufactured alternatives use less material, and, due to their advanced engineering, can make use of waste material from less desirable, fast-growing tree species.
Parallel chord trusses provide longer spans than commonly available in dimensional lumber, and permit the placement of high levels of insulation while providing adequate ventilation of the roof cavity. This advantage is also available through conventional trusses when a raised heel is incorporated. This permits full depth placement of insulation over the exterior wall plates without restricting attic ventilation.
A novel approach to traditional post-and-beam construction involves using beams from timber, glulam cr structural composite lumber to support stress skin panels. These structuial panels comprise a sandwich of roof sheathing, insulation and an interior panel finish or nailing surface.
A common consideration for all of these alternatives is the need to plan accurately and to adhere to the final plan during construction. Unlike conventional framing approaches, these efficient alternatives are manufactured with precision, and there is much less flexibility for field changcs or adjustments.
wood roof truss with raised heel roofs (Fig. 64). Air movement through such openings depends primarily on wind. These are most effective when combined with vents located high on the roof such as ridge vents (Fig. 65A) or gable end vents (Fig. 65B).
Flat roofs insulated between the roof joists are difficult to ventilate unless there is clearance above the insulation, and the joist spaces are interconnected to permit free circulation of ventilating air (see Figs. 126 and 127). These common techniques are not applicable where fine snow can be wind-driven through such vent openings and deposited on the roof insulation. In such situations, local building practices should be followed.
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