Here is a fact that surprises most homeowners: the mortar holding your brick together should be weaker than the brick itself, not stronger. Choosing what type of mortar for brick comes down to matching the mix to the load, the location, and the brick’s hardness, and getting it wrong can crack or spall the very brick you are trying to protect. There are four standard mortar types graded by compressive strength, and each has a job it does well. This guide breaks down Type N, S, M, and O so you can specify the right mortar with confidence, whether you are laying a new garden wall or repointing a century-old facade.
How Mortar Is Graded
Mortar types are defined by ASTM C270 and are easy to remember through the old mason’s mnemonic from the word “MASON WORK,” where every other letter, M, S, N, O, names a type. Each blends portland cement, hydrated lime, and sand in different ratios. More portland cement means higher compressive strength but lower flexibility and breathability. More lime means a softer, more forgiving, more breathable mortar. The grades run from strongest to weakest:
- Type M: highest strength, roughly 2,500 psi
- Type S: high strength, roughly 1,800 psi
- Type N: medium strength, roughly 750 psi
- Type O: low strength, roughly 350 psi
The golden rule of masonry is that the mortar should be softer than the masonry units. Mortar is the sacrificial element; it is far cheaper and easier to repoint a joint than to replace cracked brick. When mortar is too hard, it cannot absorb the small movements and moisture expansion that brick experiences, so the stress transfers into the brick face and causes it to crack or spall.
Type N: The General-Purpose Choice
For most homeowners, Type N is the answer. It is the standard general-purpose mortar for above-grade exterior and interior walls, including chimneys, garden walls, and the typical veneer on a house. With a compressive strength around 750 psi, it offers a strong balance of bond, workability, and flexibility. Its medium strength makes it the most forgiving option, which is exactly why it is the go-to for softer modern brick and many older buildings.
If you are laying a new brick mailbox, a planter, a freestanding garden wall, or repointing standard exterior brick that sits above the ground line, Type N is almost always correct. A typical 70-pound bag of pre-blended Type N mortar mix costs about $6 to $12 and covers roughly 30 to 40 standard bricks depending on joint size.
Type S: When You Need Extra Strength
Type S steps up to roughly 1,800 psi and is built for situations with more structural demand. Use it below grade and for foundations, retaining walls, and any masonry that faces high lateral or wind loads. Because it has a higher tensile bond strength, it resists the sideways pressure that soil or strong winds put on a wall.
Good candidates for Type S include exterior walls at or below ground level, brick on a structure in a hurricane-prone region, masonry retaining walls holding back a hillside, and patios or pavement set in mortar. It is stronger than Type N but still workable enough for most projects. Avoid it on soft or historic brick, where its hardness can do more harm than good.
Type M: Maximum Strength for Heavy Loads
Type M is the strongest standard mortar at about 2,500 psi. Its high portland cement content makes it suitable for heavy loads, foundations, and below-grade masonry such as basement walls, driveways, and load-bearing structures in contact with the earth. It also stands up well to severe frost and groundwater.
That strength is also its limitation. Type M is rigid and unforgiving, so it should be reserved for genuinely demanding structural work. Using it on standard above-grade brick veneer is overkill and risks cracking the brick over time. Reach for Type M when an engineer or local code specifies it for a foundation or retaining structure, not as a default “stronger is better” upgrade.
Type O: Soft Mortar for Interior and Historic Work
Type O is the low-strength option at around 350 psi, and it has two clear roles. The first is interior, non-load-bearing work where strength simply is not needed. The second, and more important, is repointing soft, historic brick. Older homes, especially those built before the 1930s, were laid with soft, lime-rich brick and lime mortar. That brick needs an equally soft mortar that can flex and breathe with it.
Using a hard, high-portland mortar like Type M or S on soft historic brick is one of the most damaging mistakes in masonry. The brick expands and contracts with moisture and temperature; if the surrounding mortar is harder than the brick, the brick has nowhere to give and the face cracks, crumbles, or spalls off. Type O, or even a softer lime-based mortar, lets the wall move naturally and keeps the original brick intact.
Matching Mortar When Repointing and Tuckpointing
When you repoint or tuckpoint an existing wall, the priority shifts from raw strength to compatibility. The new mortar should match the original mortar in strength, composition, color, and joint profile. Pack a stronger mortar into joints surrounded by old, soft brick and you set the wall up to fail, because the new joints become harder than the units around them.
If you are unsure what the original mix was, have a small mortar sample analyzed by a masonry restoration specialist; a basic analysis often runs $75 to $200 and can save thousands in brick damage. As a safe default for older buildings, masons lean toward Type N or a custom lime mortar rather than anything harder. The principle never changes: mortar protects brick, so it must always be the weaker, more sacrificial partner in the wall.
Quick Reference for Picking Your Mortar
To summarize the decision in plain terms: choose Type N for everyday above-grade exterior and interior brick, the right call for the majority of homeowner projects. Step up to Type S for below-grade walls, foundations, retaining walls, and high-wind areas. Reserve Type M for the heaviest structural and below-grade loads where code or an engineer calls for it. Drop down to Type O for interior non-structural work and for repointing soft, historic brick. Above all, keep the mortar softer than the brick, and when repointing, match what was already there. Get that hierarchy right and your brickwork can last a hundred years.
