Type N vs Type S vs Type M Mortar: Choosing the Right Mix for Chicago Brick

Type N vs Type S vs Type M Mortar: The Short Answer

Type N is the general-purpose mortar for most above-grade brick walls in Chicagoland. Type S is the stronger mix for below-grade masonry, foundations, and walls carrying structural or lateral load. Type M is the strongest and is reserved for heavy load-bearing and below-grade work. And for soft pre-1920 brick — the bungalows, two-flats and greystones that make up so much of this region — the right answer is usually neither, but a lime-rich Type O or custom lime mortar.

Behind all of it sits one rule: mortar must always be softer than the brick it holds. Emerald Masonry LLC matches mortar to the wall on every job across Chicago and the Chicagoland suburbs — family-owned, licensed and insured, 40+ years of experience, free on-site estimates. Call (708) 288-1696.

This is the most consequential decision in any masonry repair, and it is the one homeowners are almost never asked about.

Why Mortar Is Supposed to Be the Weak Link

A brick wall is not a solid object. It is thousands of individual units held in a matrix of mortar, and that matrix is engineered as the sacrificial element.

As the wall heats and cools, absorbs and releases moisture, and settles over decades, something has to absorb that movement. The design intent is that the mortar absorbs it and wears out — because replacing mortar is comparatively cheap and easy, and replacing brick is neither.

That is why deteriorating mortar joints are not a defect. They are the system working correctly, and repointing them on schedule is routine maintenance.

Now invert it. Fill those joints with a mix harder and stronger than the brick, and the mortar stops being the weak link. The brick becomes the weak link instead. Every freeze-thaw cycle now drives its stress into the brick faces, and within three to eight winters those faces begin to spall — flaking, popping and delaminating off the wall. That damage is permanent. You cannot repoint a brick back to having a face.

This single mechanism is behind an enormous share of the ruined masonry in Chicago's older neighborhoods, and it is almost always the result of a contractor using the strongest mix on hand rather than the correct one.

The Mortar Types, Plainly

Standard mortar types run from strongest to softest: M, S, N, O, K. The strength comes largely from the ratio of Portland cement to lime — more cement means harder, stiffer and less forgiving; more lime means softer, more flexible, more breathable, and better at self-healing hairline cracks.

| Type | Relative strength | Character | Where it belongs | |---|---|---|---| | M | Highest | Very hard, rigid, high compressive strength | Heavy load-bearing masonry, foundations, below grade, retaining structures | | S | High | Strong with decent flexural bond | Below grade, foundations, parapets in some cases, structural and lateral-load masonry, exterior at-grade work | | N | Medium | Balanced, general-purpose, forgiving | Most above-grade exterior brick walls, chimneys, veneer — the common default | | O | Low | Soft, high lime content, breathable | Soft historic brick, interior non-load-bearing, low-stress repointing on old walls | | K / custom lime | Lowest | Very soft, highly breathable | Very soft handmade historic brick, preservation and landmark work |

Two practical notes. First, higher is not better — this is a matching exercise, not a strength contest. Second, historic work often does not use a bagged standard type at all, but a custom-blended lime mortar formulated to match the original wall's composition, hardness and color.

Which Mix for Which Chicagoland Wall

Pre-1920 soft brick — bungalows, two-flats, greystones, worker cottages

Laid with lime mortar and softer, often locally made brick, including large quantities of porous Chicago common brick in the inner wythes and on side and rear elevations. These walls want Type O or a custom lime mix. Standard Type N is frequently too hard here, and Type S is a reliable way to destroy the brick.

Post-war brick — 1950s–70s ranches, Georgians, Cape Cods, split-levels

Laid with harder Portland-based mortar and denser machine-made face brick. Type N is the usual correct choice for above-grade repointing, with Type S where conditions genuinely warrant it.

Modern brick veneer

Dense contemporary units over a framed or block backup. Type N for typical above-grade veneer, Type S where structural or exposure conditions call for it.

Foundations, below grade and at-grade masonry

Moisture, soil pressure and salt exposure change the calculation. Type S — sometimes Type M — is appropriate here, though an old limestone or rubble foundation still needs a softer, more forgiving mix than a modern block wall.

Chimneys

The most exposed masonry on any building, wet on four sides with no interior heat drying it. The mortar must match the era of the stack: soft lime for a pre-1930 chimney, Type N for most post-war stacks.

Parapets and roof-edge masonry

Exposed on both faces and subject to thermal movement, so this is one of the places where Type S legitimately earns its place on above-grade work — but the wall's own brick still sets the ceiling on how hard the mix can be.

How a Mason Determines the Right Mix

1. Date the masonry. Construction era is the strongest single predictor of the original mortar.
2. Test the existing joint hardness. Scraping a joint tells an experienced mason a great deal — soft lime mortar gives readily, Portland-based mortar resists.
3. Assess the brick, not just the joint. Absorption and hardness of the units set the ceiling: the mortar must come in softer.
4. Consider exposure and location. Above grade or below, sheltered or weather-beaten, chimney or parapet, north-facing or south.
5. Analyze the mortar where the building's age or value justifies it — laboratory analysis identifies the original binder, aggregate and ratio.
6. Match the sand. Most of a mortar's color comes from its aggregate, so matching sand is central to matching appearance.
7. Mock up and cure a sample. Mortar dries lighter than it looks wet, so judging color on a cured sample rather than a fresh trowel is the only reliable way.

Materials and Technique Beyond the Mix

Mortar selection is necessary but not sufficient. Three other decisions determine whether repointing lasts.

Joint preparation depth. Failed mortar must be cut out to sound material — generally two to two and a half times the joint width — not skimmed over. Mortar smeared across the face of old joints has almost no bond and fails quickly, and it looks wrong immediately.

Grinding versus hand-raking. Power grinding is efficient and acceptable on modern masonry. On soft historic brick it widens joints permanently and chews the crisp edges of the units, so those joints are raked by hand. A crew that brings only a grinder to a greystone is the wrong crew.

Tooling and cure. Joints are struck to match the original profile — concave, weathered, raked, grapevine — at the right firmness, then protected and allowed to cure. Lime-rich mortars in particular need moisture and time; rushing the cure is a common cause of early failure.

What About Mortar for Stone, Block and Foundations?

Brick is not the only masonry on a Chicagoland building, and the same softer-than-the-unit rule applies across all of it.

Limestone and greystone. Quarried limestone is porous and comparatively soft, so it wants a soft, breathable, lime-based mortar. A hard mix at a stone joint concentrates stress at the stone face rather than in the joint, which is how carved sills, water tables and ornamental trim end up spalling along their bedding planes. Restoration work on limestone and sill repair almost always uses lime-modified mortar.

Rubble and quarried stone foundations. Common under pre-1940 homes across Joliet, Lemont and the older city neighborhoods. The stone itself is durable; the mortar between it is what washes out. A forgiving lime-modified mix serves these walls far better than a brittle high-strength mortar.

Concrete block (CMU). Modern block is dense and structural, and below-grade block walls resisting soil pressure legitimately call for Type S, sometimes Type M. This is one of the places where the stronger mixes are genuinely correct rather than merely convenient.

Glass block and specialty units. These are set with their own specified mortars and detailing, not whatever is left in the mixer.

How to Tell the Last Contractor Got It Wrong

• Hard gray joints standing proud of softer, warmer-toned old brick.
• A band of spalling brick corresponding exactly to where repointing was done a few years earlier.
• Smeared, sloppy joints rather than tooled ones, often overlapping the brick faces.
• New mortar noticeably harder than the original when scraped with a key or screwdriver.
• Wrong color — gray against tan, or bright white against weathered joints.
• Joints wider than the originals, a signature of aggressive grinding on soft brick.

Repointing done with too hard a mix frequently has to be removed and redone, which is more expensive than doing it correctly the first time — and any brick already spalled is simply gone.

Why This Matters More in Chicagoland

Freeze-thaw frequency is the reason. It is not extreme cold that breaks masonry but repeated crossings of the freezing point with water present, and this region delivers dozens each winter plus more in the shoulder seasons. Every crossing is a load cycle, and a mismatched mortar routes every one of those cycles into the brick.

Lake-effect moisture keeps walls wetter for longer, particularly on north elevations that dry slowly. Road salt drives salt crystallization damage into the lowest courses. And the age of the housing stock means a very large share of the region's masonry is soft, lime-laid and a century old — precisely the material that a hard modern mix destroys fastest.

Maintenance and Prevention

• Repoint on schedule with a matched mix rather than waiting for spalling.
• Manage water first — clear gutters, extended downspouts, grade sloping away, splash control. Most masonry damage is a water-management failure.
• Repair sills, crowns, coping and flashing promptly; they soak everything below them.
• Inspect annually, especially north and rear elevations, chimneys and anything above a walkway.
• Use breathable sealers only, and never paint soft brick — trapped moisture spalls the face from behind.
• Never sandblast or high-pressure wash older masonry.

Ask Your Contractor These Questions

1. What mortar type are you proposing, and why that one for this wall?
2. How did you determine what the original mortar was?
3. How deep will you cut the joints out?
4. Will you grind or hand-rake, and how did you decide?
5. How will you match the color and the sand?
6. What joint profile will you tool to?

A contractor who can answer these specifically is doing the work. One who answers "we always use the same mix" has told you what you need to know.

Related Services

Mortar selection drives nearly everything we do: tuckpointing and repointing, brick repair, brick replacement, chimney repair, historic masonry restoration, foundation masonry repair and parapet wall repair.

The Bottom Line

Type N for most above-grade Chicagoland brick. Type S below grade, at foundations, and where masonry carries real load. Type M for heavy load-bearing work. And for the region's soft century-old brick — a lime-rich mix, matched to the wall it is going into.

Get the mortar right and the repair blends in and lasts decades. Get it wrong and you will have paid a contractor to strip the faces off your brick, slowly, one winter at a time.

Emerald Masonry LLC is a family-owned, licensed and insured masonry contractor serving Chicago and the Chicagoland suburbs with 40+ years of experience in tuckpointing, chimney repair, brick repair and replacement, lintel and parapet repair, foundation and limestone/sill repair, caulking, sealing, and commercial, residential and historic masonry restoration. We match mortar to the wall, every time.

Free on-site estimates. Call (708) 288-1696 or request an estimate.