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The healthy roof: Staying on top of heritage preservation

Photo: This decorative cupola adorns McMartin House, a Foundation property in Perth

By

Ontario Heritage Trust

Buildings and architecture, Tools for conservation

Published Date: Sep 08, 2005

The following excerpt appears in Well-Preserved: The Ontario Heritage Foundationʼs Manual of Principles and Practice for Architectural Conservation (Third Revised Edition), by Mark Fram (Boston Mills Press, 2003). Well-Preserved can be downloaded here.

We started this series from the ground up – foundations and then walls and superstructures. Now we cap things off with the healthy roof. In this issue, we examine a building’s roof and the significance of this key structural element in keeping a building intact.

The roof is the most exposed part of a building; it often dominates a building’s visual character, but is also the single element most vulnerable to weathering and thus to periodic change. Even when well maintained . . . roofing materials do not last as long as other parts of the exterior. Much deterioration throughout a building is caused by too much moisture . . . and much of this moisture gets in through gaps or weaknesses in the roofing . . . At some points in a building’s life an owner will face a crucial decision whether to continue repairs or to replace the roofing entirely. In these cases . . . the craft, durability and visual impact of the old must be recalled very carefully by the new.

Roofing materials in 19th-century Ontario included shingles in wood, slate, and metal, as well as continuous seamed sheets of metal. Metals for roofing included copper, tin-plated iron, terne-plated iron (terne is a lead/tin alloy), and (very rarely) lead. The early 20th century added asphalt shingles and clay and concrete tiles to the repertoire . . .

In Ontario, flat and shallow roofs are usually covered with continuous sealed membranes of tar or bitumen . . . on a built-up base of paper and felt over a wooden substructure of joists and roof boards. Much rarer for low-pitch roofs is sheet metal . . . with interlocking flat seams – extremes of temperature make metal roofs especially vulnerable to creeping, curling and punctures.

A flat roof must retain its integrity despite accumulations of rain and snow as well as tremendous variations in temperature . . . Many tar-and-gravel roofs on quite old buildings sit on top of worn-out metal. Few built-up roofs last long without leaking, though the effective life of a well-maintained flat roof ranges from 10 years to perhaps 30.

On sloping roofs, metal roll or sheet roofing provides a smooth, relatively impervious surface, but can fail at seams and joists as well as at punctures. Thermal expansion and contraction tax every part of a metal roof . . . The use of standing seams or even wooden battens at seams gives metal roofs a characteristic vertical emphasis and also offers the metal considerable room to expand and contract.

Repairing metal roofing is expensive and requires experienced experts; poor short-term repairs will accelerate deterioration. Shingle roofing in any material is more vulnerable to leaks between units and at flashings, but it is more amenable to bit-by-bit repairs; a roof’s overall life can be extended by those repairs, but only to a point.

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