Most people obsess over the brick and treat mortar as filler. In reality the mortar often decides whether a wall lasts decades or cracks in a few winters. This guide explains what mortar has to do, how to pick a mix strength, and how to mix it so the joints stay sound. You will leave able to match the mortar to the job instead of defaulting to the strongest bag on the shelf.

What mortar is actually doing

Mortar is not glue. Its jobs are to bed the bricks so loads spread evenly, to bond the units together, to seal the wall against water, and to absorb small movements without cracking. Because it must do all four, the goal is a balanced mortar, not simply the hardest one.

The key principle: mortar should be weaker than the brick

This surprises people. If the mortar is harder than the brick, stresses from movement and moisture get forced into the bricks, and the brick faces crack or spall. If the mortar is the softer element, any cracking happens in the joints instead, and joints can be raked out and repointed. Sacrificing the joint protects the expensive brick. So the mix should be strong enough for the exposure, and no stronger.

The ingredients and what each contributes

• Cement gives strength and early set. More cement means a harder, less forgiving mortar.
• Lime adds workability, flexibility, and breathability, and helps the mortar heal fine cracks over time. It makes the mix easier to lay and less brittle.
• Sand is the bulk and structure. It must be clean, well-graded builder’s sand. Dirty or single-sized sand weakens the mortar and harms workability.
• Water makes it workable and lets the cement hydrate. Too much water weakens the mix and causes shrinkage.

Choosing a mix strength

Recognised mortar types run from stronger to softer, and each suits different exposure. Under the ASTM C270 system used in North America, the common types from harder to softer are M, S, N, and O. Type N, roughly one part cement to one part lime to six parts sand, is a good general-purpose mortar for most above-grade walls. Stronger types such as S suit higher-load or more exposed work; softer types suit sheltered or historic soft brick.

Exposure	Typical choice	Why
General above-grade walls	Medium (e.g. Type N)	Balanced strength and flexibility
High wind, load, or below grade	Stronger (e.g. Type S)	More strength where needed
Soft or historic brick	Softer, lime-rich	Protects fragile units, breathes

Always match the mortar to the brick and the exposure. Putting a hard modern cement mortar on soft old brick is a classic way to destroy the brick.

Mixing it properly

• Measure by volume with a consistent gauge, such as a bucket, so every batch matches. Guessing ratios by the shovel gives inconsistent joints.
• Mix the dry sand and cement (and lime) until the colour is uniform before adding water.
• Add water gradually to reach a workable, cohesive consistency that holds a shape on the trowel and does not slump off.
• Mix only what you can lay before it starts to stiffen. Do not re-temper with extra water long after the set has begun, as that weakens it.

A real scenario

Consider a repointing job on an older house where a builder used a strong cement-only mortar because it looked tidy and set fast. Two winters later the soft old bricks had blown faces along the repointed courses, while the original lime-mortar joints elsewhere were fine. The hard new mortar had trapped moisture and forced it to escape through the brick instead of the joint. The repair had to be raked out and redone with a softer, lime-based mortar matched to the brick. The strongest mix was the wrong mix.

Common mistakes and how to fix them

• Always using the strongest mortar. Fix: match strength to the brick and exposure; keep mortar weaker than the units.
• Adding too much water for easier laying. Fix: aim for cohesive, not soupy; add water in small amounts.
• Inconsistent ratios batch to batch. Fix: gauge every mix by volume with the same bucket.
• Dirty or wrong sand. Fix: use clean, well-graded building sand; reject sand with clay or silt.
• Laying in the wrong weather. Fix: avoid mortar in frost or scorching heat, both of which disrupt the set.

Action checklist

• Identify how strong and hard the brick is.
• Assess the exposure: sheltered, general, or severe.
• Pick a mortar type that is strong enough but softer than the brick.
• Use clean, well-graded sand and clean water.
• Gauge ingredients by volume every batch.
• Mix only what you can lay before it stiffens, and protect fresh work from frost and rapid drying.

Conclusion and next step

Good mortar is a balance of strength, flexibility, and breathability, tuned to the brick it serves. Your next step: before the next batch, confirm your brick type and exposure, then choose a mortar that is strong enough for the job and deliberately softer than the brick, so the joints, not the bricks, take the wear.

Frequently asked questions

Should mortar be stronger or weaker than the brick?

Weaker. A mortar softer than the brick lets movement and cracking concentrate in the joints, which can be repointed, rather than in the bricks, which cannot easily be repaired.

What is a good general-purpose mix ratio?

A cement-lime-sand mortar around one to one to six (a Type N in the ASTM system) suits most ordinary above-grade brickwork. Increase cement for more exposed or loaded work, and lean toward lime for soft or older brick.

Why add lime instead of using cement only?

Lime improves workability, adds flexibility so the mortar tolerates small movements, lets the wall breathe, and helps seal fine cracks over time. Cement-only mortars are harder but more brittle and less forgiving.

Can I add extra water to old mortar to reuse it?

Re-tempering mortar that has begun to set with fresh water weakens it and is best avoided. Mix smaller batches you can use before the set starts instead.

References

• ASTM C270, Standard Specification for Mortar for Unit Masonry, which defines mortar types M, S, N, and O.
• Brick Industry Association (BIA) technical notes on mortars for brickwork.
• Brick Development Association (BDA) guidance on mortar selection and lime mortars.