Weathered Beauty: Restoring Centre Block’s century-old facade
In February 2019, the Senate moved to the Senate of Canada Building, a former train station built in 1912. The Senate will occupy this temporary location while Parliament’s Centre Block — the Senate’s permanent home — is rehabilitated.
Although Centre Block is shuttered for rehabilitation work, Canadians can still experience its art and architecture through the Senate’s immersive virtual tour.
Approximately 365,000 stones make up the 20,000-square metre facade of Parliament Hill’s Centre Block and every single one is going under the microscope.
Towering high above the Ottawa River, the building has been battered for decades by sleet, snow, ice and airborne pollutants — even earthquakes.
Restoring the century-old facade is an enormous project, said Jeff Meek, Project Manager with Public Services and Procurement, the government department supervising the Centre Block rehabilitation project.
“We’re looking at the exterior masonry in its entirety. That includes all the facades of Centre Block — north, east, west, south — three courtyards and four towers as well as the iconic Peace Tower.”
The building is surfaced entirely in sandstone — ancient sand compacted over millions of years to form a fine-grained, weather-resistant sedimentary rock. Composed mainly of quartz, the crystalline mineral left behind when bedrock erodes, sandstone makes a particularly durable building material.
Nepean sandstone — sourced from a quarry that operated on the western outskirts of Ottawa — forms the bulk of the facade and gives Centre Block its rich honey colour.
The stone began forming 500 million years ago, at the end of the Cambrian Era, when the ground on which Ottawa now stands was a desolate beach on the edge of an ancient protocontinent called Laurentia.
The occasional alien-looking creature may have ventured close to shore — a heavily armoured trilobite or centipede-like euthycarcinoid — but the land itself was utterly barren. There were no trees, no ferns, no flowers, not even a clump of moss. Terrestrial plants would not appear for another 50 million years.
Over half a billion years, that ancient beach slowly subsided, buried under successive deposits of sand as it compacted and fused to form a sandstone seam that runs through Eastern Ontario, Quebec, New York and Vermont.
The Nepean quarry closed in 1962, so masons working on Centre Block’s rehabilitation have had to find a replacement. Their search took them to the community of St. Canut near Mirabel, Quebec.
“St. Canut is a related geological formation,” Mr. Meek noted. “It’s as close as possible to the original Nepean sandstone.”
Work on Centre Block’s facade began in 2018 with drone surveys and exploratory probes into the wall behind the facade to assess its condition.
After that, scaffolds went up behind protective tarps and conservators began a stone-by-stone survey, examining the facade close-up to spot surface damage or tell-tale signs of deep structural deterioration.
Masons then did what they call a rake-out, removing the old, deteriorating mortar from the facade.
“The building has seen some patchwork mortaring over the years,” Mr. Meek said. “Now we’re doing a holistic rehabilitation, taking out the old stuff entirely so the new mortar that goes back in has continuity — the same strength and consistency — across the building.”
Where necessary, masons dismantle sections of the facade and shore up the brick support that forms the interior structural wall.
They then tackle the exterior sandstone blocks, replacing them outright or repairing them on site, depending on the extent of damage.
“A stone may be structurally compromised by a fracture right through the centre,” Mr. Meek said. “Our conservators and consultants often propose a replacement in that situation.”
Masons replace the bedding mortar, the matrix that forms a stable and resilient bond between the stones, then concentrate on pointing — filling the gaps between stones with fresh mortar.
The building has stood up to a century’s wear and tear surprisingly well.
“We were expecting to replace 15-20% of the stone,” Mr. Meek said. “In fact, less than 5% were replaced along the north side — and that’s a particularly exposed face.”
“It speaks to the condition of the stonework.”
When the team does step in, they follow a principle of minimal intervention.
It’s about being thorough while respecting the building’s history.
“We’re trying to retain as much heritage material as we can,” Mr. Meek said, “At the same time, we’re getting the building into the kind of shape where it can withstand at least another fifty years or more.”
