Lime mortar for repointing a stone house in rural Ottawa?

Lime mortar is a reasonable choice for repointing a natural stone house, and for historic walls the Centretown Heritage Conservation District plan notes that "lime-based mortar should be used when repointing historic brick and masonry as cement mortar prevents moisture from escaping through the mortar and causes brick damage." That guidance applies within its district.

The Ontario Building Code (9.20.3.1.(1)) requires mortar to conform to CSA A179, and 9.20.3.1.(3) says lime used in mortar shall be hydrated. The Code sets mortar types by location: for exterior above-grade non-loadbearing walls, Type N or S; for loadbearing walls, Type S. The Code has no rule requiring lime mortar, requiring mortar softer than the stone, or requiring that new mortar match old mortar. Those expectations come from masonry practice and, where applicable, heritage district plans.

A few things are worth sorting out before work starts:

• Whether the property is designated under the Ontario Heritage Act or sits in a heritage conservation district. If it is, alterations that affect heritage attributes can require council's written consent or a heritage permit, depending on the designation by-law or district plan. The City can confirm whether the property is designated.
• What the existing mortar is. A mason can assess the joint profile, colour and approximate composition so the new mortar can be matched in appearance. The Centretown plan says "care should be taken to match the colour and joint profile of the mortar."
• How joints are cut out. Mortar removal from stone joints involves grinding or chiselling, which generates silica dust. Ask how the crew will control dust -- wet cutting, dust extraction and respirators are the methods to ask about.
• What the written quote covers: the area of wall, the mortar type specified by name, colour matching, joint profile, and cleanup and disposal.

Start with the mason: ask them to look at the existing joints, name the mortar type they propose and explain how they arrived at it. That conversation will tell you whether you need to go further with the City or a heritage specialist.