Mortar Mix Ratio for Bricks: Type N, S, M and Which Mix to Use

Mortar types: N, S, M and O

In the US, mortar is specified by type under ASTM C270 (current edition C270-26). The standard can be applied in two ways: a proportion specification, which sets the volumes of cement, lime and sand, and a property specification, which sets a minimum strength, water retention and air content for mortar tested in a laboratory. If a project names neither, the proportion specification governs. Bagged mortar mix and masonry cement are sold labelled with these letters, so this is the first thing to decide before you buy or mix anything.

The mixes shown are typical, not the only valid ones. ASTM C270's proportion specification keeps cement at 1 part and lets the lime vary within a band for each type — ¼ part for Type M, over ¼ to ½ for Type S, over ½ to 1¼ for Type N and over 1¼ to 2½ for Type O — with sand measured damp and loose. The strengths are minimums from the property specification for laboratory-prepared mortar; mortar sampled on site is not tested against them.

Stronger isn't always better

If you're looking for a "strong mortar mix", it's worth knowing that the Brick Industry Association's basic rule is to choose the mortar type with the lowest compressive strength that meets the project's requirements. In most brickwork, bond strength, workability and water retention matter more than compressive strength, and higher-lime mortars show more plastic flow, which lets the masonry accommodate slight movement.

Type M in particular is a high-strength mortar but not very workable, so keep it for the foundation and retaining walls where it is recommended. When repointing, the new mortar should be no stronger than the existing mortar; a stronger repointing mortar can cause spalling in older brickwork.

Bagged mortar: mortar mix vs masonry cement

Most people don't batch mortar from separate cement, lime and sand. There are two common bagged options, and they are easy to confuse:

• Mortar mix (pre-blended) — cementitious materials and sand already blended. Just add water. Sold by type: Quikrete Mortar Mix, for example, is formulated as Type N and Quikrete Mason Mix as Type S, in bag sizes that include 60 and 80 lb.
• Masonry cement (US, ASTM C91)— a proprietary blend of cement and plasticising materials such as limestone or hydrated lime, with no sand. You add sand on site but no extra lime: under ASTM C270, masonry cement is used alone with sand, at no more than 3 parts sand per part masonry cement by volume. Quikrete's Type N masonry cement, for example, is mixed one 70 lb bag to 240 lb of mason's sand.

UK mortar designations: cement, lime and sand

In the UK, prescribed (site-proportioned) mortars are described by designation (i) to (iv), with an indicative strength class given by PD 6678 and the National Annex to BS EN 998-2. The Mineral Products Association's table below gives the traditional proportions by volume. Each designation can be made with lime, or as a cement:sand mix with or without air entrainment. The higher the sand figure per part cement, the lower the mortar class.

The sand itself should be a mortar sand: ASTM C144 in the US, or fine aggregate to BS EN 13139, which UK factory-produced mortars use.

Adding lime

Hydrated lime contributes workability, water retention, elasticity and extent of bond. Because lime hardens slowly on contact with carbon dioxide, water and air that get into small hairline cracks react with it and can seal them — a process known as autogenous healing.

Lime does not replace the cement. In both the ASTM C270 types and the UK designations, cement stays at 1 part, and the lime and sand go up together as the mix gets weaker. The trade-off is strength: compressive strength rises with cement content and falls as the lime, water or sand content increases, which is why the high-lime mixes — Type O and designation (iv) — are the weakest. For a cement-lime mortar mixed to the ASTM C270 proportion specification, the sand limit is:

Air entrainment as a lime alternative

Instead of lime, a mortar can get its workability from air entrainment — the UK designation table lists cement:sand mixes "with or without air entrainment" alongside the cement:lime:sand mixes. Air comes at a cost: as air content increases, compressive strength and bond strength fall. ASTM C270 therefore limits air content, and the Brick Industry Association does not recommend air-entraining admixtures in cement-lime mortar or combining two air-entraining materials in one mix. If you use an admixture, use one that complies with ASTM C1384 and whose effect on the mortar is known.

How much water to add

Mortar is not like concrete. The Brick Industry Association advises mixing it with the maximum amount of water consistent with workability, because a wetter mortar bonds better to absorbent brick; the mason judges this by how the mortar responds to the trowel. Excessive water, though, reduces both workability and bond strength. For bagged mortar, follow the bag: Quikrete, for example, starts at about 5 quarts (4.7 L) of water per 80 lb bag of Mortar Mix and expects the maximum to stay below 6 quarts (5.7 L).

In a mechanical mixer, BIA's sequence is about half the water, about half the sand and all the lime, then the cement and the rest of the sand, then the rest of the water, mixing for 3 to 5 minutes. Water lost to evaporation can be replaced by retempering, but all mortar should be used within 2½ hours of mixing (2 hours in hot weather), and mortar that has begun to set should not be used.

How much mortar per square metre

In a single-leaf wall of standard bricks with 10mm joints, the bed joints (horizontal) and perpend joints (vertical) hold about 17.6 litres of mortar per square metre. That is the joint volume only. Mortar used to fill frogs and mortar lost while laying come on top, so allow a waste margin, and for bagged mortar check the manufacturer's stated coverage — Quikrete, for example, rates an 80 lb bag of its Mortar Mix at up to 37 standard US bricks.

The mortar calculator works out the wet mortar volume from the joint geometry plus your waste allowance, then converts it to cement, lime and sand (applying a 1.3 dry-volume factor) and to a pre-mixed bag count. The bag count allows about 10.5 litres of joint volume per 80 lb bag, which leaves headroom for frogs and droppings; the cement, lime and sand figures do not include that extra, so raise the waste allowance if your bricks have frogs to fill.

Common mistakes

• Choosing a stronger mix than the job needs— the Brick Industry Association recommends the type with the lowest compressive strength that meets the project's requirements
• Too much water — mortar should be as wet as workability allows, but excessive water reduces both workability and bond strength
• Inconsistent batching — measure sand with a box or bucket rather than by eye, and add the same quantities in the same order every batch
• Using mortar too late — retempering is fine, but use all mortar within 2½ hours of mixing and never once it has begun to set
• Working in cold conditions without precautions — in the US, TMS 602 requires cold-weather protection when the temperature is below 40°F (4.4°C); in the UK, the Mineral Products Association advises against building masonry while it is below 3°C and falling. Discard any mortar that has frozen

Putting it together

Before mixing mortar, it helps to know exactly how many bricks you're laying and how much material you'll need. Our brick calculator gives brick count and pallet estimates for any wall size, our mortar calculator works out cement, lime, sand and bag counts for Type N, S, M or O, and the bricks per square metre guide covers how coverage varies by brick type.

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