Why Mortar Choice Matters More Than Most People Expect
In most construction discussions, mortar is background — the material between the bricks. In heritage restoration, it is often the most consequential decision on the project. The wrong mortar does not merely look incorrect; it can physically destroy the masonry it is intended to protect.
The mechanism is straightforward. Older bricks — particularly those made before the 1930s — were fired at lower temperatures and are comparatively soft and porous. The mortar used originally was lime-based and softer than the brick. When the wall moves (and all walls move, through thermal expansion and freeze-thaw cycles), stresses are absorbed by the mortar joint, which is the expendable and replaceable part of the assembly. The brick remains intact.
When a hard Portland cement mortar is used to repoint old brick, the softer brick becomes the weaker element. Stresses that the mortar no longer absorbs are instead transmitted directly into the face of the brick. Over time, the brick faces spall — the outer face delaminates and falls away. This is irreversible. Once a historic brick face is gone, it cannot be restored.
The Standard Mortar Types
Canadian masonry standards follow ASTM C270, which defines mortar types primarily by their compressive strength and the ratio of Portland cement to lime and sand.
Type S Mortar
Type S has a compressive strength of approximately 12.4 MPa (1800 psi). It is the standard choice for below-grade masonry, retaining walls, and applications subject to lateral loads. Its relatively high Portland cement content makes it durable under wet conditions.
Type S is not appropriate for repointing historic masonry. Its compressive strength typically exceeds that of pre-1940 brick. It is difficult to remove once set — a consideration in any restoration context where future work will be needed.
Type N Mortar
Type N has a compressive strength of approximately 5.2 MPa (750 psi). It is a general-purpose mortar appropriate for above-grade exterior masonry. The higher lime content (compared to Type S) makes it more flexible and workable. It is sometimes used in restoration contexts for post-1940 brick, but it is still harder than most lime mortars and should be used cautiously on historic structures.
Natural Hydraulic Lime (NHL)
NHL mortars are derived from limestone containing naturally occurring silicates and aluminates that allow the lime to set hydraulically — that is, through a chemical reaction rather than purely through carbonation in air. They come in three grades: NHL 2, NHL 3.5, and NHL 5, corresponding to increasing hydraulic strength.
For most above-grade historic masonry repair in Canada, NHL 3.5 is the baseline choice. It achieves adequate strength for wall repointing and external render repairs while remaining softer than most historic brick and stone. NHL 5 is reserved for applications requiring greater hydraulic resistance, such as foundation work or below-grade stonework.
NHL mortars are available from a small number of Canadian and imported suppliers. The supply chain is less straightforward than Portland cement products, and it requires some advance planning on larger projects.
Putty Lime Mortar
Putty lime (non-hydraulic lime) mortars are the softest option. They set purely by carbonation — absorbing CO2 from the air — and can take weeks or months to reach full strength, depending on temperature and humidity. Their compressive strength typically falls between 0.5 and 2 MPa, making them appropriate only for the most delicate historic masonry: very soft handmade brick, soft sandstone, and highly detailed carved stonework where any harder mortar would be destructive.
Working with putty lime in Canadian winters requires controlled conditions. Carbonation slows dramatically below 5°C and stops entirely in freezing temperatures. Fresh putty lime work requires frost protection for several weeks after application.
Assessing Existing Mortar Before Repair
Before selecting a replacement mortar, the existing joint should be assessed. A petrographic analysis — laboratory examination of a mortar sample — can determine the original binder type, aggregate size and type, and any additives. This is the standard approach for designated heritage properties and is increasingly expected by municipal heritage staff reviewing permit applications.
A simpler field test involves applying a few drops of dilute hydrochloric acid to the mortar joint. A strong fizz indicates high calcium content — consistent with a lime-rich mortar. A weak reaction suggests more Portland cement. This is diagnostic rather than definitive, but it provides useful initial guidance.
Repointing Technique
The mechanical process of repointing matters as much as mortar selection. Joints should be raked back to a minimum depth of 20 mm (3/4 inch) — enough to provide adequate mortar volume for bonding. Raking is best done by hand with a plugging chisel and mallet on soft historic masonry. Angle grinders and mechanical saw-cutting tools can easily damage brick faces and should be avoided on pre-1940 brick unless an experienced operator is working with a very narrow blade in a carefully controlled manner.
The joint profile — how the mortar is finished at the face of the wall — should match the original. Weathered joints, struck joints, and flush joints each have different water-shedding characteristics. On most 19th-century Canadian brick buildings, a slightly recessed flush joint or a simple weathered joint is appropriate. The tooled rope joint and V-joint commonly applied by renovation contractors do not match historical profiles on older Canadian masonry and are best avoided on heritage work.
Provincial Heritage Considerations
In Ontario, buildings designated under Part IV or Part V of the Ontario Heritage Act may require a Heritage Permit before any repair or alteration work. Municipal heritage staff in Toronto, Ottawa, Kingston, and other municipalities with significant historic building stocks have developed detailed standards for mortar specification and repointing practice. Consulting with heritage staff early in a project — before materials are purchased and before contractors are mobilized — avoids the costly situation of being required to redo work that was done with inappropriate materials.
Quebec's cultural heritage legislation (Loi sur le patrimoine culturel) similarly governs work on classified and inventoried heritage properties. The Direction du patrimoine of the Ministère de la Culture et des Communications maintains technical guidance documents on masonry repair that are aligned with international conservation standards.