When a masonry contractor shows up to repoint your building and you ask what mortar they're using, a lot of them will say "Type S" and move on. That answer is sometimes right and sometimes the beginning of a slow-motion problem. The mortar type used in any masonry repair is one of the most consequential decisions on the job — and it's also one of the decisions that property managers and building owners are least equipped to evaluate.

This post explains the practical difference between lime-based and Portland cement mortars, why the distinction matters for Chicago-area buildings, and what to ask a contractor before work begins.

The Basic Difference

Portland cement mortar is the modern standard. It's strong, sets quickly, and is what most contractors default to. The common types (from ASTM C270) run:

Type M: Very high strength (~2,500 psi), highly durable, used for below-grade and heavy-load applications
Type S: High strength (~1,800 psi), standard for exterior above-grade commercial masonry
Type N: Medium strength (~750 psi), appropriate for above-grade exterior work with moderate exposure
Type O: Low strength (~350 psi), for interior or protected exterior applications

Lime mortar uses hydrated lime as the primary binding agent, with much lower compressive strength (typically 50–500 psi depending on the mix). Lime mortar is flexible, breathable, and self-healing to a degree — small cracks can close through a process called autogenous healing as carbonation continues. Traditional lime mortar formulations used in buildings constructed before the 1920s are often nearly pure lime and sand, with no Portland cement at all.

Hybrid mortars mix Portland cement and lime together. This is actually the composition of most modern Type N and S mortars — they contain both Portland cement and lime in varying ratios.

Why This Matters: The Compatibility Principle

Here is the rule that every masonry contractor should know and most property managers have never heard: mortar should be softer than the masonry units it binds.

This isn't aesthetic preference — it's physics. Masonry walls experience thermal movement, moisture cycling, and minor settlement year-round. That movement has to be accommodated somewhere in the wall assembly. The design intent of traditional masonry construction is that movement occurs in the mortar joint — which is a thin, replaceable element — rather than in the brick unit, which is expensive to replace and, on historic buildings, irreplaceable.

When mortar is harder than the brick:

Seasonal movement and stress transfer into the brick face rather than the joint
Brick faces crack and spall along the joint lines
Water enters through the brick damage and accelerates deterioration
The repair that was supposed to protect the building is destroying it

Modern brick is fired hard enough to be compatible with Type S or M mortar. Pre-war brick — and especially pre-1920 brick — is softer and requires compatible soft mortar.

The Timeline Problem for Chicago Buildings

Most of Chicago's commercial and institutional building stock was constructed between 1880 and 1960. That's an enormous amount of masonry that requires mortar softer than what most modern contractors default to.

The situation is complicated by decades of repair history. Many older Chicago-area buildings have been repointed once or multiple times since original construction, often with progressively harder mortars as Portland cement became the industry standard. A building that was originally built with pure lime mortar may have been repointed in 1965 with an early Type N, again in 1990 with Type S, and now has layers of incompatible materials in its joints along with the spalling that resulted from each inappropriate repair.

For a building like this, proper restoration requires removing the incompatible mortar before installing new — a more involved process than standard repointing.

How to Identify What Your Building Needs

A few field indicators of what mortar is appropriate:

Building age: Pre-1920 construction almost certainly requires lime-based mortar. 1920–1950 requires assessment. Post-1960 commercial construction is generally compatible with Type N or S.

Mortar hardness test: Scratch existing mortar with a nail. If it scratches easily, the mortar is soft (lime-based). If it doesn't scratch, it's hard (Portland-dominant). New repair mortar should not be significantly harder than what it's joining.

Brick hardness: Try to scratch a brick face with a steel key. If the brick scratches (leaves a mark), it's soft and requires soft mortar. Modern face brick won't scratch.

Spalling patterns: If you're seeing spalling that follows joint lines — brick faces popping adjacent to repointed areas — that's a strong indicator that hard mortar was used on soft brick in a previous repair.

If you're not sure, a masonry contractor who specializes in historic work can assess the building and recommend a mortar specification. For high-value or formally designated historic properties, a laboratory mortar analysis (running $300–$800 typically) provides a definitive mix recommendation.

Practical Guidance for Property Managers

When hiring a contractor for masonry work on an older building:

Ask about mortar type before they start. "What mortar are you using and why?" is a reasonable question that any competent contractor can answer. The answer should reference the age and type of your masonry, not just a default preference.

Be skeptical of "we always use Type S." Type S is appropriate for many applications. It is not appropriate for pre-war brick. A contractor who applies the same mortar to every job hasn't assessed your building.

Ask whether they'll do a test patch. On buildings where color matching and mortar compatibility are both concerns, a good contractor will do a small test patch in an inconspicuous location, let it cure, and confirm both appearance and compatibility before proceeding.

Understand that soft mortar is not weak. Property managers sometimes worry that a lower-strength mortar is a compromise. It's not — it's the correct specification for the application. A lime mortar joint that lasts 50 years without damaging the brick is better than a hard mortar joint that lasts 15 years and destroys the brick in the process.

The Bottom Line

Mortar selection is one of the highest-stakes decisions in masonry repair, and it's one that's made by the contractor, not the building owner. Understanding the basic principle — mortar should be softer than brick, historic buildings need historic-compatible mortar — gives you the knowledge to ask the right questions and recognize when a contractor's proposal doesn't match the building.

The buildings that age well in Chicago's climate are the ones whose maintenance history respected this compatibility principle. The ones that develop accelerating spalling and require increasingly expensive repair are often the ones where it was ignored.

Emerald Masonry LLC specifies mortar based on building age and masonry composition, not convenience. We serve commercial and institutional properties throughout Chicagoland. Call (309) 323-9959 or request a free assessment.

Also see: Masonry Restoration | Tuckpointing | Brick Repair