Type N vs Type S Mortar: Strength, Uses & Which Should You Use?

Type N mortar has a minimum average compressive strength of 750 psi (5.2 MPa) and Type S has 1,800 psi (12.4 MPa) under the ASTM C270 property specification. Type N is the general-purpose mortar for above-grade masonry; Type S is used at and below grade, and where the wall has to resist real lateral load.

Assumptions: strengths are the minimum average 28-day values ASTM C270 sets for laboratory-prepared mortar. They are not pass or fail values for mortar sampled off a wall, and they are not the strength of the finished masonry.

The strength difference is real, but it is the wrong place to start a decision. This guide sets out both mortars in full, then covers the cases where the stronger one is the wrong answer.

The comparison in full

Assumptions: values from the ASTM C270 property and proportion specifications, and reproduced on the data sheets of the bagged products that meet them. Sand in the proportion specification is set at not less than 2¼ and not more than 3 times the sum of the cementitious volumes, which is why the familiar 1:1:6 and 2:1:9 ratios look the way they do.

Notice that both types have the same water retention requirement. The differences that matter on site are the cement-to-lime ratio inside the binder and, in bagged products, the air content.

What the strength numbers actually mean

Three things are often assumed about the psi figures and none of them are true.

They are not the strength of your wall. The compressive strength of masonry comes overwhelmingly from the units, not the mortar. Going from Type N to Type S raises the mortar’s own strength by 140 percent and raises the assembly’s compressive strength by a much smaller margin.

They are not a field test threshold. ASTM C270’s values apply to mortar mixed and cured in a laboratory. Mortar sampled from a board and tested under ASTM C780 usually reads lower, because the masonry units pull water out of the mortar in a way a laboratory mould does not. A field cube reading below 1,800 psi is not evidence that a Type S mortar failed.

They are minimums, not targets. A mortar that meets Type N and no more is a compliant Type N mortar. Chasing strength beyond the specified type has costs.

The case against always choosing the stronger one

Mortar is designed to be the weakest part of a masonry wall, and that is a feature. A wall moves: it expands in heat, contracts in cold, settles, and flexes in wind. Something has to give way when it does.

If the mortar is softer than the units, movement shows up as fine cracks in the joints, which are repointable at moderate cost. If the mortar is harder than the units, movement shows up in the units instead, as spalled faces and cracked brick, which is not repairable at all. Water then gets in through the damaged faces and the freeze-thaw cycle takes over.

This is why every serious selection guide, ASTM C270’s own selection appendix included, tells you to use the weakest mortar that satisfies the project’s requirements rather than the strongest available. It is also why the answer to “which is better” is “neither”.

Where Type N belongs

Type N is the default for above-grade masonry, and it is the right answer more often than not.

• Exterior above-grade walls and veneer. The everyday case, in brick or block.
• Interior partitions and non-load-bearing work. Type O is softer still, but Type N is the common bag.
• Chimneys above the roof line. Exposed but not carrying much load.
• Softer or older units. Where the brick is not a modern high-strength face brick, the extra lime in Type N protects it.
• Any job where workability matters more than strength. Type N spreads better, holds water better against absorbent units, and forgives a slower pace.

Type N is not appropriate below grade, and under TMS 402, the masonry code, Type N mortar and masonry cement mortars are not permitted in the seismic-force-resisting system in the higher seismic design categories. The Concrete Masonry and Hardscapes Association’s guidance on mortars sets that out for concrete masonry. If your project is in a high seismic region, that restriction can decide the mortar for you regardless of preference.

Where Type S belongs

Type S is the answer when the wall faces conditions Type N was not designed for.

• At and below grade. Foundation walls, basement walls, and the courses below the damp course.
• Retaining walls. Constant lateral earth pressure, often saturated.
• Paving, patios and steps set in mortar. Ground contact, water, and traffic.
• Manholes, sewers and other buried masonry.
• Walls with meaningful lateral load. High wind exposure, tall unreinforced walls, and engineered or reinforced masonry.
• Where the specification says so. On engineered work the mortar type is a design output, not a site choice.

Type S is stiffer under the trowel than Type N and holds water less well against a thirsty brick, so it needs a little more attention to consistency. If a batch is fighting you, the consistency field tests settle it faster than guessing.

Weather and exposure

Exposure changes the calculation more than most people account for.

Freeze-thaw. Saturated masonry that freezes is the single most destructive condition ordinary walls face. The mortar’s resistance depends more on air entrainment and on keeping water out of the wall than on the type letter. Well-tooled concave joints in either type outperform badly tooled joints in the stronger one.

Hot, dry, windy conditions. Water leaves the mortar fast, and the cement never fully hydrates. Type N’s higher lime content gives it a modest advantage in water retention. In either type, the fix is site practice: shade the board, dampen highly absorbent units, and keep batches small.

Persistent saturation. Below-grade and ground-contact masonry stays wet, and that is where Type S earns its place.

Repointing and older masonry

This is where choosing by psi does the most damage, and it deserves its own warning.

Old buildings were often built with lime mortars, or with soft-fired brick, or both. Repointing them with a modern Type S mortar creates a wall where the joints are several times harder than the brick around them. Moisture that used to evaporate through the joints is forced through the brick faces instead, and after a few winters the faces spall off.

The National Park Service guidance on repointing historic masonry is the reference here. The principle it works from is that the repointing mortar should be no harder than the original and no harder than the units, which on many historic buildings means a lime mortar or a Type O rather than anything from the N and S argument.

If the building matters and the original mortar is unknown, a laboratory mortar analysis costs less than one wall of ruined brick.

Choosing between them

Work down this list in order rather than starting at the strength column.

1. Is there a project specification, drawing note or engineer’s requirement? If so, that is the answer. Stop here.
2. Is the masonry structural, reinforced, or in a high seismic design category? Then the code and the design govern, and Type N may be excluded outright.
3. Is any of it at or below grade, in ground contact, or retaining soil? Type S or stronger.
4. Is it repointing or repair on older masonry? Match the existing mortar and the units, and read the historic guidance before ordering.
5. Are the units soft, historic, or low-strength? Stay at Type N or softer.
6. Otherwise, above-grade masonry in normal exposure? Type N.

None of the above is engineering approval for a specific wall. Load-bearing, reinforced and below-grade masonry is governed by the design documents and by local building regulations, and a guide cannot see your soil, your wind loads or your units.

Cement-lime, masonry cement or mortar cement

Both types come in three different cementitious systems, and the choice matters as much as the letter on the bag.

Cement-lime mortar is portland cement, hydrated lime and sand, proportioned on site or supplied pre-blended. It gives the highest bond strength of the three and the lowest air content, and it is what most specifications default to.

Masonry cement mortar uses a factory blend of cement and a plasticiser in place of the cement and lime. It is consistent, forgiving and widely stocked. The air-entraining agent that makes it work also lowers bond strength, and ASTM C270 allows it substantially more air: up to 20 percent for Type N against 14 percent for a cement-lime Type N.

Mortar cement mortar under ASTM C1329 is the middle route, with capped air content and a minimum flexural bond strength requirement. It exists because some projects need a bond strength number rather than a compressive strength number.

A Type S masonry cement mortar and a Type S cement-lime mortar both meet 1,800 psi in compression and behave differently in flexure. If the specification names one system, do not substitute the other because the type letter matches.

Common mistakes with N and S

Reading psi as a ranking. It is a requirement to be met, not a score. The right mortar is the softest one that satisfies the job.

Assuming Type S makes a stronger wall. Masonry compressive strength comes mostly from the units. Doubling the mortar strength moves the assembly strength by a far smaller margin.

Using a field cube test as a compliance check. ASTM C270’s values are for laboratory-prepared mortar. Field mortar sampled under ASTM C780 has no pass or fail strength requirement in C270.

Mixing up hydrated lime Type S with mortar Type S. They are unrelated classifications that happen to share a letter.

Repointing old brick with whatever is stocked. This is the mistake that causes real, permanent damage, and it is nearly always avoidable.

What each type costs you in materials

The two ratios use different amounts of cement for the same volume of mortar, and on a large job that shows up on the invoice.

Assumptions: proportions by volume of dry, loose material, matching the ratios the Mortar Calculator uses. Sand is 75 percent of the mix in both, so switching type changes the cement and lime split rather than the total volume.

For the same wall, Type S needs about a third more cement and a third less lime than Type N. The total mortar volume does not change, so the bag counts and coverage figures are the same either way if you buy pre-blended.

To get the quantities for your own wall in either type, set the mortar type in the Mortar Calculator and it splits the total by the ratio above.

The rest of the family

Type N and Type S are the two bags on most shelves, but ASTM C270 defines four. Type M at 2,500 psi (17.2 MPa) is for heavy load-bearing and below-grade work where Type S is not enough. Type O at 350 psi (2.4 MPa) is a soft interior and repointing mortar.

The ingredients that create those differences, and why lime does as much for a mortar as cement does, are covered in what mortar is made of. And if the underlying question is really whether to use mortar at all rather than concrete, the mortar, cement and concrete comparison sorts that out first.