Mortar Types M, S, N, O and K Explained

Where the letters come from

The mortar types are M, S, N, O and K, and the letters are not initials of anything. They are every second letter of the phrase MASON WORK, taken in order - a mnemonic that has outlived whatever reasoning produced it. The sequence runs strongest to weakest in exactly that order.

All five are proportioned the same way: portland cement, hydrated lime and sand, by volume, in that order. What changes down the list is the balance between cement and lime. Cement brings compressive strength and stiffness; lime brings workability, water retention, bond and - crucially - the ability to tolerate small movements without cracking.

So the ladder is not simply "more or less strong". It is a trade between two different kinds of good behaviour, and each type sits at a point on that trade chosen for a particular kind of masonry.

Why the strongest mortar is usually wrong

In a masonry wall the units - brick or block - are the strong, expensive, permanent part. The mortar is the sacrificial part. When the wall moves, and every wall moves with temperature, moisture and settlement, something has to give. A wall built with soft mortar cracks along its joints, which is cheap to rake out and re-point. A wall built with mortar harder than its bricks cracks through the bricks instead.

That is the whole reasoning behind the default choice. The general-purpose type sits well down the strength ladder because ordinary above-grade brickwork does not need compressive strength from its mortar - it needs bond, workability and the tolerance for movement that lime provides. Specifying a stronger type "to be safe" makes the wall less safe, not more.

The failure is at its most expensive in historic work. Soft, old, handmade brick re-pointed in a modern high-cement mortar traps moisture and concentrates stress at the brick face, and the brick faces spall off over a few freeze-thaw cycles. This is why the softest types exist at all, and why conservation work reaches for them rather than for anything modern and strong.

Choosing a type

The decision is driven by three questions, in this order: is the masonry structural, is it in contact with ground or persistently wet, and how hard are the units themselves.

Ground contact and load push you up the ladder. Foundations, retaining walls and anything below grade have both high compressive demand and constant moisture, which is what the strongest type is proportioned for - and it is proportioned for that at the cost of workability, which bricklayers notice immediately. Exterior work at or below grade and structural veneer sit one step down, which is the sweet spot where good bond and real strength overlap.

Above grade and out of the ground, the default type is right for the overwhelming majority of brick and block. Interior non-load-bearing work and re-pointing can drop further still, and conservation of fragile masonry drops off the standard table entirely.

One practical note on procurement: bagged mortar mixes are usually sold as a named type, and masonry cement products reach the same classification by a different route than the cement-lime proportions above. Match the type to the job and read the bag; do not assume that two products labelled with the same letter were proportioned identically.

These are typical reference proportions and strengths for planning and general work. Structural masonry must follow your local building code and a qualified engineer's specification.

• Below grade, retaining, high load → the strongest type, accepting poor workability.
• Exterior at or below grade, structural veneer → one step down; the best balance of bond and strength.
• Ordinary above-grade brick and block → the general-purpose type. This covers most jobs.
• Interior non-load-bearing, tuck-pointing → a softer type.
• Soft historic masonry → softest available, chosen to be weaker than the brick.

Proportion specification versus property specification

The strengths in the table are minimums from the property specification - what a laboratory-prepared sample of that type must achieve. They are not what you should expect to measure out of a mixer on site, and they are not a target to design a wall around.

There are two legitimate ways to specify a mortar. The proportion specification names the volumes of cement, lime and sand, which is the route the ratios above describe and the one ordinary site work follows. The property specification instead requires the mixed mortar to meet stated laboratory properties, and lets the supplier arrive there however they choose. You specify one or the other, not both - asking for a proportion and then testing it against the property table is a common way to fail a perfectly good mortar.

Either way, the mortar's strength is a means to an end. What actually holds masonry together is bond: the mechanical grip between mortar and unit, which depends on the unit's absorption, the mortar's water retention, and how the bricklayer works. A high-cement mortar with poor bond is worse in a wall than a softer mortar with good bond, whatever the cube test says.

Mortar, grout and render are not the same material

Three cement-based materials get used on the same wall on the same day, and their ratios look confusingly similar. Mortar is the bedding between units, mixed stiff enough to hold a brick where you put it. Grout is poured into block cores and cavities at a high slump so it flows around reinforcement, and it is proportioned from cement and sand - with pea gravel added for wider spaces - rather than from cement and lime. Render is the coating applied to the finished face.

Render brings its own version of the same weakest-link principle. A render system is built in coats, each softer than the one beneath it, so shrinkage stresses point inward toward the wall rather than outward at the surface. The undercoat proportion is the same cement : lime : sand as the general-purpose mortar type, which is a nice illustration of how the same proportion can be doing a completely different job.

If you are choosing between these three rather than between mortar types, the comparison guide on this site covers the distinction directly.

Frequently asked questions