Choosing mortar and joint profiles for durable brickwork

Mortar gets ordered by the bag and mixed by eye more often than almost any other material on a residential site. The brickies have their usual mix, the supplier has the usual product, and the wall goes up. Most of the time that works. The walls that come back to the builder years later, with eroded joints, crumbling faces and white salt staining across the brick, are usually the ones where the mortar was chosen for convenience rather than for the units it was laid against.

Mortar has two jobs that pull in different directions. It has to bond the units together and carry load, and it has to stay in the joint without breaking down under the weather. The mix that does the first job best is not always the mix that does the second, and the joint profile the mason finishes with has a large effect on how water behaves on the face of the wall. Both decisions are made once, early, and they are very hard to change afterwards.

This article is about the choice of mortar relative to the brick or block, and about how the joint profile affects water shedding and weathering. It does not cover movement joints or tie detailing, which are dealt with in the guide to brickwork movement joints and ties. The aim is to give a builder enough of the mechanism to judge a mortar specification, to question a substitution and to spot a joint that is going to fail before it does.

The principle: mortar should be weaker than the units

The starting point is a principle that many builders have heard but not fully absorbed. The mortar should be no stronger than the masonry units it joins, and in many cases it should be weaker. The reason is what happens when the wall moves, which all walls do through temperature change, moisture change and settlement. Movement has to go somewhere. If the mortar is the weakest element, the cracks form in the joints, where they are narrow, cheap to repair and largely invisible from the street. If the mortar is stronger than the brick, the cracks run through the units themselves, and a cracked brick is far harder to replace than a joint is to repoint.

This is why a cement-rich mortar placed in a soft, low-strength brick wall is such a common source of trouble. The brick cannot take the stress the mortar transfers to it, so it spalls at the face, and the damage is permanent. The builder is left with a wall that looks wrong, and the cost of replacing the affected units is far higher than the cost of the mortar would ever have been.

The reverse applies too. A very weak, lime-rich mortar used with a hard, dense brick may give a wall that performs well structurally but loses its joints quickly in the weather. The aim is balance, not the highest possible number on the bag. The right mortar for a given unit is the one that bonds well, carries the load and stays intact under the conditions the wall will face.

How the units change the answer

Units vary widely in strength, porosity and surface. Dense, high-strength clay bricks, softer common bricks, concrete bricks and blocks, and autoclaved aerated concrete all behave differently, and they call for different mortar. Porosity matters as much as strength. A highly porous unit draws water out of the mortar as it is laid, which weakens the bond and can leave the joint chalky. A very dense unit does not take water in at all, so the bond depends on the mortar’s own workability and on careful wetting of the surface.

Autoclaved aerated concrete is a special case, because it is usually laid with a thin-bed adhesive rather than a conventional mortar. The guide to autoclaved aerated concrete block walls covers those systems. The point here is that a builder should not substitute a general-purpose mortar into a wall built on a different system without checking the manufacturer’s requirements for that product.

The face of the unit matters as well. Face bricks are often made to be seen, and the mortar has to sit neatly against them without staining or leaving a film that shows. Some mortars contain pigments or additives that affect the colour of the joint, and a mix that looks right in the bag can look very different once it has cured on the wall. Test a sample panel before the full wall is built, and leave it through a week of weather before deciding.

What each ingredient does in the mix

Cement gives strength and speed of set. It is the reason a cement-rich mix goes off quickly and carries high load, and it is also the reason a cement-rich mix is stiff and less forgiving of movement. Lime gives workability and flexibility. A mix with a good proportion of lime spreads easily, holds water well in the joint, and accommodates small movements without cracking as quickly. The sand is the bulk of the mix, and its grading and cleanliness affect how the mortar spreads and how much water it needs.

Water is the ingredient most often adjusted on site, and it is the one that does the most damage when it is adjusted carelessly. Adding water to a mortar that has begun to stiffen weakens the bond and makes the joint more likely to shrink and crack as it dries. A builder should mix only what can be used within the working time, keep the consistency steady, and avoid retempering a batch that has already started to set. A mortar that is too wet slumps and stains; a mortar that is too dry bonds poorly. The target is a consistent, workable mix from the first trowel to the last.

Admixtures and proprietary mortars can change these characteristics, sometimes usefully. A water-retaining admixture can help a mortar hold moisture when laid on very absorbent units in hot weather, and a colourant can give a uniform joint. Each product has its own requirements for mixing, working time and curing, and the manufacturer’s instructions govern. Do not mix a product into a traditional mix on the site unless the manufacturer says it is compatible.

Joint profile and water shedding

The joint profile is the shape the mason leaves on the face of the joint once the mortar has stiffened. It decides how water that reaches the face of the wall will behave. A joint that is recessed, with its surface set well back from the face of the brick, holds water at its top edge and lets it sit against the mortar. Over time the joint erodes, the water finds its way into the wall, and the efflorescence and salt damage that follow can be seen clearly on the face.

A joint that is finished so the mortar slopes away from the brick above, and is compacted to a dense surface, sheds water rather than holding it. The weathered profile does this deliberately, with the top of the joint sloping inwards so that rain runs off. A concave finish, such as a bucket-handled or struck joint, also compacts the mortar and presents a dense surface to the weather, which tends to perform well on exposed walls. A flush joint sits level with the face of the brick and is simple to finish, but it can trap water at the edges if the joint is not carefully packed.

The compare visual below sets out the common profiles and how they behave on an exposed wall. It is a guide to the mechanism, not a specification. The specification for a given wall should come from the designer and the mortar manufacturer, and the joint profile should be agreed before the work starts so the mason finishes to the same standard across the whole job.

Timing the finish matters as much as the profile. A joint tooled too early smears and tears the mortar, and one tooled too late is hard to shape and may leave the surface crumbling. The mason should tool each joint when the mortar has stiffened enough to hold its shape under a thumb press without marking, and should brush the face clean of droppings at the right moment. Those decisions are made by feel, and they are the difference between a joint that lasts and one that needs repointing sooner than it should.

Choosing a mortar for the job, step by step

A sensible selection process starts with the units and works outwards. The steps below are the order a builder should work through before the mortar is ordered, and the order matters, because each step narrows the choices left for the next. Skipping ahead to the bag is the usual shortcut, and it is the usual cause of the problems described above.

Worked example: a garden wall and a brick veneer on the same block

Consider a builder in regional NSW finishing a brick veneer house on a sloping block, with a low garden wall of a different brick along the boundary. The builder ordered a single general-purpose mortar for both jobs, because it was simpler to keep one bag on site, and the bricklayer used the same mix for each. The garden wall used a softer, more porous brick, and the veneer used a dense face brick.

Within the first year the garden wall began to show efflorescence and some surface loss on the upper courses. The cause was not the mortar alone. The mix was stronger than the soft brick it was laid against, so the brick took the stress at the face, and the wall, which was exposed on its top and both sides, held water in its joints for much of the year. The veneer, with its dense brick and a weathered profile, was in good condition.

The builder did two things. The garden wall was repointed with a lime-rich mortar and a weathered profile, which matched its exposure and the brick behaviour, and the veneer was left alone. The builder then changed the purchasing practice, so that each brick type on a job is paired with a specified mortar and profile before the first order. The extra work was small. The earlier mistake had cost more in remedial labour and client goodwill than the difference in materials would ever have been.

Keeping water out of the joint once the wall is up

The mortar and profile do the most work in the first few years, but a wall is only as durable as its detailing. Water that reaches the face of the wall at the top, at the base or around openings will find the joints regardless of how well the mortar was chosen. Flashings, sills and cappings are the first line of defence, and a detail that channels water onto the joints will undo a good mortar specification within a short time.

Protect fresh work from rain while the mortar cures, and avoid laying in freezing conditions, because the water in the mix can freeze before it has bonded. Cover the top of the wall overnight, and do not allow the mortar to dry out too quickly on hot, windy days. These are simple disciplines, but they are often the difference between a joint that bonds properly and one that is weak from the start.

Efflorescence is the visible sign that water has moved through the wall and left salts behind on the face. It is not always a mortar problem, but the mortar and the joint profile are where it can most often be prevented. The guide to preventing efflorescence on face brick covers the causes and the controls in more detail.

Where the specification comes from and what to check

The mortar and joint specification for a particular wall should come from the designer, the unit manufacturer and the mortar manufacturer, and it should be recorded on the job. The builder’s job is to confirm that the specification has been followed, to question any substitution, and to make sure the bricklayer understands why the mix and the profile matter. A verbal instruction to use the usual mix is not a specification.

If the specification is unclear, ask before the wall is built. A designer who has specified a joint profile will expect it to be built, and a mason who has been left to choose will choose the profile that is quickest to finish. The cost of clarifying the specification is small, and the cost of an exposed wall that weathers badly is large.

For a builder who wants to keep the technical side of the licence current, the guide to how NSW builder CPD works sets out the framework, and the course catalogue carries topics on materials and construction practice. The principle that runs through all of it is the same: choose the material for the unit and the exposure, not for the convenience of the bag on the pallet.

Questions builders ask

• Masonry
• Materials
• Mortar

Published 30 September 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.