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Researching the Fortress of Louisbourg National Historic Site of Canada
  Recherche sur la Forteresse-de-Louisbourg Lieu historique national du Canada

DOMESTIC BUILDING CONSTRUCTION 
AT THE FORTRESS OF LOUISBOURG, 1713 - 1758

By

Eric Krause

1996 Draft Report

(Fortress of Louisbourg
Report Number H G 10)

CHAPTER TWENTY

CHIMNEYS AND CHIMNEY OPENINGS -
CHIMNEY STACKS AND FIREPLACES

Chimney stacks and fireplaces were either freestanding or they were supported by or incorporated into a wall. A Royal Battery proposal, for example, stated that the brick stacks in its buildings were to be supported by 1 1/2 pieds thick interior bearing walls of rubblestone. A brick stack in the magasin des vivres, on the other hand, was set partially within a 2 pied thick perimeter wall of rubblestone.

As a chimney stack rose builders would diminish its width. In Block 34C, for example, a chimney base in the 1 1/2 storey house of François Vallée, was 3 pieds wide but only 2 pieds wide for the next 8 pieds 8 pouces of height. Off-setting, the setting back of the stack at various levels of height, was one means by which to achieve this reduction. A back-to-back fireplace in Louisbourg's second administration complex, for example, required one cubic toise of masonry for its foundation, 5 cubic pieds for its mantelpiece, but only 3 cubic pieds 4 pouces for up to its second off-set. One cubic pied more and the chimney was complete. On the other hand, the kitchen stack of the same building required only one off-set, and that stack rose 28 pieds above the fireplace.

It was a matter of good sense and some planning that ensured that a stack exited through a roof in the best possible location. A stack at a ridge was considered best; yet at Louisbourg some stacks exited within the pitch of a roof. There, up against the stack, water could stand and accumulate beneath roofing slates. That was the case with several chimneys of the King's Bastion barracks, and when the water froze it soon destroyed the pargings and allowed water to enter, causing considerable damage to the building. These chimneys should have instead been backed against the longitudinal bearing wall that ran down the middle of the building, which would have caused them to exit near the ridge.

Common sense and the Custom of Paris dictated how much further a stack rose once it reached roof level. According to the Custom of Paris chimneys should be at least 3 pieds above the roof ridge. In practice, 2 to 3 pieds was common on Isle Royale. A practical distance for obtaining a good draw, this height also lessened the danger of sparks or of a chimney fire igniting a roof.

Topping a stack was usually a cap of some type. Cut stone caps were preferred, particularly by the military, who included their repair or replacement in a 1736 general maintenance contract; but bricks were cheaper and were perhaps chosen more often by private builders. Even wooden caps were a possibility, as found on the stacks of a private masonry house in Block 34C. Some stacks, too, had a protruding masonry string circling it somewhat below the cap.

As a fire precaution some builders surfaced the entire exterior stack with a lime and sand mortar mixture known as a crépi. The 1736 contract for the maintenance of the roofs of king's buildings also directed that mortar be used for parging where a stack exited through a roof. In other cases, the parging was cement or gypsum plaster.

Lead for flashing around chimneys would have been superior to a mortar parging. Nonetheless, even the 1736 maintenance contract, which addressed the question of repairs to lead flashings elsewhere, such as the roof ridges and around dormers, did not specifically identify its use around chimneys. One but perhaps not the only factor for its absence as a flashing was possibly the high cost of lead relative to the mortar parging.