The Mortar Specification Library

Mortar is not a single material. It is a class of binder systems whose composition must match the brick or stone it bonds. Using the wrong mortar on the wrong masonry causes more damage than the deterioration it was meant to repair. This library documents the mortar specifications Delta uses on every project, mapped to construction era, brick hardness, and exposure conditions.

The Specification Principle

The fundamental rule of mortar selection: mortar should always be softer than the brick or stone it bonds. When freeze-thaw stress or thermal movement loads the wall, the joint should fail before the masonry unit. A failed mortar joint can be repointed for a few dollars per linear foot. A failed brick or stone face requires replacement at $25 to $50 per brick or hundreds per stone unit.

This principle is encoded in the ASTM C270 standard for unit masonry mortar, which defines four types (M, S, N, O) plus a discontinued historic type (K) and the lime-based formulations used in pre-1920 work. Compressive strength varies by an order of magnitude across the range.

Type M Mortar: 2,500 PSI

The strongest standard mortar, used for foundations, retaining walls below grade, masonry at or below grade, and applications where extreme structural strength is required. Type M is almost never appropriate for residential tuckpointing because it is harder than most residential brick and forces freeze-thaw stress into the brick face.

Delta specifies Type M only on structural masonry retaining walls and below-grade foundation work.

Type S Mortar: 1,800 PSI

High-strength mortar specified for structural applications, below-grade walls, and locations subject to high lateral or compressive loads. Type S is the correct mortar for new masonry construction at and above grade in most regions, including residential garden walls, chimney footings, and lintels.

Delta specifies Type S on:

• Foundation parging and waterproof coatings on block foundations
• Retaining wall construction
• Lintel reset and structural repair
• Stair stoop and porch rebuilds
• Chimney structural repair below the roofline

Type S is incorrect for tuckpointing residential brick walls above grade. The strength differential between Type S (1,800 PSI) and typical residential brick (2,000-3,500 PSI for face brick, less for soft historic brick) is too small. Stress concentrates at the brick rather than the joint.

Type N Mortar: 750 PSI

Medium-strength mortar and the standard specification for above-grade residential tuckpointing on post-1960 brick. Type N provides adequate bond and weather resistance while remaining softer than typical face brick, allowing thermal movement and freeze-thaw to fail at the joint rather than the brick face.

Standard batch ratio: 1 part portland cement, 1 part hydrated lime, 6 parts sand by volume. Color is calibrated to match the existing wall through sand sourcing and pigment additions.

Delta specifies Type N on:

• Post-1960 face brick residential walls (most postwar ranches, splits, Colonials)
• Chimney tuckpointing on standard residential stacks
• Above-grade veneer brick on contemporary construction
• Brick replacement on most postwar housing stock

For homes built between 1925 and 1960 with moderately soft brick, Delta typically modifies Type N with additional lime (a "Type N lime-blend" batch at 1:1.5:7 to 1:2:8 portland-lime-sand) to reduce hardness toward the brick. Read more in our guide to Type N vs Type S vs Type O mortar selection.

Type O Mortar: 350 PSI

Low-strength mortar specified for soft historic brick, interior partitions, and applications where minimal structural strength is required. Type O is the correct mortar for pre-1920 Chicago common brick, which has compressive strength typically below 2,000 PSI and is much softer than modern face brick.

Standard batch: 1 part portland cement, 2 parts hydrated lime, 9 parts sand. The high lime fraction increases plasticity and reduces hardness so the mortar yields before the soft brick face does.

Delta specifies Type O on:

• 1900-1925 Chicago common brick on bungalows, two-flats, and three-flats
• Soft brick on early Foursquare and Craftsman homes
• Interior non-load-bearing partition repair
• Applications where the original mortar tested below 600 PSI

Using Type N or Type S on soft historic brick is one of the most common contractor errors on the North Shore. The hardness mismatch concentrates freeze-thaw stress at the brick face, causing spalling within 3 to 8 years. We document this failure mode in our guide to brick spalling causes.

Pure Lime Mortar and NHL Blends: Pre-1920 Construction

For masonry built before approximately 1920, Portland cement mortar is almost always the wrong specification. The original mortar was lime-based, typically pure lime putty or natural cement (Rosendale cement from the Hudson Valley). These mortars cure by atmospheric carbonation over months to years rather than by hydration in days, and they are significantly softer than even Type O.

Delta uses three lime-based formulations on historic work:

NHL 2 (Natural Hydraulic Lime, 290 PSI)

The softest hydraulic lime mortar. Used on the softest pre-1900 brick and on limestone joints where the stone itself is significantly softer than typical Indiana limestone. NHL 2 cures more slowly than NHL 3.5 but produces the lowest compressive strength, matching the softest historic masonry.

NHL 3.5 (Natural Hydraulic Lime, 580 PSI)

Standard hydraulic lime mortar for pre-1920 residential restoration. Cures within months by combination of hydraulic set and atmospheric carbonation. Specified on most Tudor Revival, Georgian Colonial, and Edwardian masonry work where the original mortar was pure lime or a lime-pozzolan blend. Standard batch: 1 part NHL 3.5, 2.5 parts sand.

Rosendale-Style Lime Cement

Historic natural cement formulation used in the United States from approximately 1830 to 1900. Sourced from American Cement Company and used on the most authenticity-critical preservation work. Curing time is slower than NHL 3.5 but the material chemistry most closely matches the original mortar on pre-1900 Chicago masonry. Delta uses Rosendale-style cement on Late Victorian work where National Register status or historic preservation review requires authentic material specification.

For deeper guidance on historic mortar selection, see our lime mortar vs Portland cement guide.

Mortar by Construction Era: Quick Reference

Mortar Color Matching

Mortar color is determined by sand source, cement type, lime content, water ratio, and any added pigments. Color matching requires either lab analysis of an extracted mortar core sample or careful visual matching with a calibrated reference sand set. Delta maintains a regional sand reference library and develops a custom color match before any tuckpointing project begins. Read more in our guide to mortar color matching.

Why This Specification Library Matters

Most masonry contractors operating in Chicagoland do not vary mortar specification by construction era. They use Type N for everything because Type N is the most widely available, mixes consistently, and is forgiving on installation. On the wrong brick, that single-spec approach causes the freeze-thaw damage we document in our guide to Illinois weather and brick damage.

Delta develops a project-specific specification for every job. The mortar that goes into your wall is calibrated to the brick we found there, the exposure on each elevation, and the era of construction. This library is the public-facing reference for that practice.

References

• ASTM C270, Standard Specification for Mortar for Unit Masonry, American Society for Testing and Materials
• National Park Service Preservation Brief 2, Repointing Mortar Joints in Historic Masonry Buildings
• Brick Industry Association Technical Notes 8 and 8B, Mortars for Brickwork
• The Masonry Society TMS 402, Building Code Requirements for Masonry Structures
• International Lime Association technical guidance on natural hydraulic lime