A New Roof for St. Ambrose Church

Located on a quiet residential street in Redcliff, Alberta, St. Ambrose Anglican Church is distinguished by its buttressed brick masonry exterior, steeply-pitched gable roof and pointed arch windows. These characteristics strongly identify the 1914 church with the Gothic Revival style popular in the Victorian era for ecclesiastical architecture in England, a style also eagerly adopted by Anglican, Presbyterian, Methodist and Congregational churches across Canada. Unlike many churches, though, St. Ambrose was modelled on small Anglican parish churches in England and is a variant of the Gothic Revival style seldom found in Alberta.

St. Ambrose’s architecture hearkens back to England but the building’s local roots are evident in the “clinker brick” masonry exterior, an overfired brick with distinctive irregular or lumpy shapes and striking colour variations. Clinker brick resulted from high firing temperatures in the kiln which caused the clay to partly vitrify or melt, sometimes to the point where clumps of bricks would fuse together and had to be broken apart. This lack of uniformity was appreciated for its decorative qualities and the clinker brick at St. Ambrose was produced at the Redcliff Brick and Coal Company just blocks away. The combination of far-reaching colonial stylistic influences and distinctive local materials contributed to the church’s designation as a Provincial Historic Resource in 2008.

Another regional trait is that, unlike slate-roofed prototypes of rural England, St. Ambrose Church historically had a roof of Western red cedar shingles, a material sourced from the forests of British Columbia and distributed by rail across the prairies during the pre-World War boom of settlement and urbanization. Cedar shingles, which differ from thicker split shakes, were produced in abundance and noted for their durability due to natural oils or “extractives” in the wood that protected against rot. Many cedar roofs have since been replaced with less costly asphalt shingles but St. Ambrose has retained the traditional roofing material as a character-defining element over the years.

In southern Alberta, exposure to wind and intense sun are important factors in the deterioration of cedar roofs, with outright rot – if it occurs at all – as a final stage in a chemical and physical process lasting decades. Weathering begins immediately, with intense sun (ultraviolet light) weakening wood cells and causing new shingles to turn from cedar’s characteristic red hue to a more familiar silver or gray. Exposed wood is further weathered by wind and grit that, like sandblasting, erodes wood’s cellulose components and eventually leaves ridged surfaces with greater susceptibility to absorbing moisture. Moisture breeds lichens, mildew and even moss, which slowly digest the wood and weaken it further. Wood shingles naturally swell with each rain and subsequently shrink as they dry from the sun and wind, creating tiny cracks that absorb moisture and promote yet more shrinkage and cracking. Because drying occurs fastest on exposed upper surfaces while protected undersides remain damp, shingles have a long-term tendency to curl or cup upward. Years of such wetting and drying cycles eventually cause severe splitting such that shingles may be torn loose by strong winds. At St. Ambrose Church, splinters of roof shingles scattered across the lawn after storms were a clear sign that the roof installed in the 1990s needed to be replaced.

The new roof at St. Ambrose consists of No. 1 grade “blue label” cedar shingles, a premium grade material comprised almost entirely of quarter-sawn or “edge grain” hardwood. No. 1 shingles are recommended for durability. Wood shrinks across the grain, and in premium edge-grain material, swelling and shrinkage with wetting and drying cycles will occur across the face of the shingle – i.e. from edge to edge, in the plane of the roof. Narrow gaps between shingles allow for such movement and also facilitate drying. Lower-grade shingles include some face grain material recognizable as a telltale flame-like grain pattern on shingle faces. This grain orientation, in combination with the tendency for upper surfaces to dry fastest, promotes cupping and more rapid deterioration of lower-grade roof shingles.

The new roof received a few minor tweaks based on historic photographs and current roofing practices. The 1914 photograph above shows the original roof with a distinctive series of horizontal lines or bands created by doubling every sixth row of shingles to create a subtle shadow line. Both the new roof and its immediate predecessor replicated this treatment, but the new roof uses an approximately one to two-inch offset to create the shadow line based on the detail revealed in a high-resolution reproduction of the historic photograph.

Perhaps a more noticeable change is the new roof shingles are laid on a five-inch exposure or row height rather than the wider eight-inch exposure of the previous roof.  Narrow rows were common historic practice and also appear in the early photograph. The narrower row spacing results in 10 bands of doubled (or slightly offset) shingles over the main roof and reproduces the original roof treatment. The extra shingles also provide greater protection, since any given portion of the roof is effectively covered with three layers of material, and helps resist shingles’ tendency to cup. The extra durability of a narrower exposure is welcome, since cedar shingles are a natural product and even a premium grade material is subject to some grain variation. Shingles sourced from old-growth forests, where biodiversity and competition slows growth and results in more closely spaced growth rings, make for a tighter or closer grain that contributes to dimensional stability.

Historic buildings often have surprises up their sleeve and St. Ambrose was no exception. Traditional flashing details were to be replicated as part of the church’s architectural character. Where a roof meets a parapet or sidewall, original galvanized steel flashings had been set into the mortar joints of the brick masonry then built up the wall in a series of steps – hence the term “step flashing”.  However, the original flashings cleared the roof by as little as an inch, whereas modern roofing guidelines require a minimum four inch clearance. An attempt to extend the flashings up the walls to achieve this minimum was thwarted in part by the irregular, projecting outer faces of the clinker brick. It’s hard to be sure but it seems that the church’s original builders were careful to avoid clinker bricks and used only uniformly flat-faced bricks where the roof and walls met and flashings would be required. The masons of 1914 could never have anticipated a code requirement for wider flashings more than a century later. The new raked flashings differ from the historic detail but St. Ambrose Church retains its strong architectural character and, with the benefit of a durable new roof, the building will maintain its historic integrity for decades to come.

Written By: Fraser Shaw, Heritage Conservation Adviser