Which Mortar Type Should You Use for Stone Veneer?

For new adhered stone veneer, choose a setting mortar approved for the stone and substrate—using a polymer-modified mortar meeting ANSI A118.4 or A118.15 where specified. Type S is suitable for some manufactured-stone installations, but it is not a universal bonding mortar. Type N or S may be appropriate for the scratch coat and visible joints even when a different mortar is required behind the stone. CMHA’s manufactured-stone installation guide separates these uses explicitly.

First establish what you have: cement-based manufactured stone, natural thin stone, or existing traditional stonework needing repair. This guidance concerns adhered veneer, not the mortar specification for a mechanically anchored stone facing. A recommendation for one system does not automatically suit another.

Three mortar jobs, potentially three choices

In an adhered veneer assembly, the scratch coat provides a prepared base, the setting bed bonds the stone, and the pointing mortar fills the visible joints. Where the specified system uses cement board instead of lath and a scratch coat, that first mortar layer is omitted.

For manufactured stone, CMHA distinguishes the layers as follows:

Mortar’s job	What to look for
Scratch coat over lath	Type N or S meeting ASTM C270 for site-mixed mortar, or ASTM C1714 for preblended mortar, as specified for the assembly
Setting bed behind the stone	A mortar approved for the particular veneer, backing and exposure; ANSI A118.4 or A118.15 modified mortar where specified
Pointing or joint filling	Type N or S, or an approved veneer pointing mortar, with the required color and joint finish

These distinctions follow CMHA’s installation guidance. Its guide specification, revised August 2025, specifies ANSI A118.4 or A118.15 modified setting mortar for bonding manufactured stone to a scratch coat or bondable concrete/masonry, while retaining Type N or S options for pointing. It is a project-editable specification, not a universal building-code requirement. CMHA guide specification

The ASTM and ANSI references here come from North American guidance. Elsewhere, use the locally applicable specification and the veneer system’s installation requirements rather than treating these designations as local rules.

When is Type S enough—and when is it not?

For manufactured stone, CMHA’s April 2024 installation guide recommends conventional Type S setting mortar for several applications, including exterior single-family residential work. However, its recommendations change with height, unit size, substrate and orientation. It recommends modified mortar for direct bonding to cement board and requires ANSI A118.4 or A118.15 mortar when a unit exceeds 1 square foot (about 0.1 m²) in face area or 24 inches (610 mm) in any dimension. Conventional Type N has a much narrower setting-bed recommendation: interior applications below 10 feet (3 m) above the finished floor. These older application recommendations do not override the project specification, manufacturer’s instructions or applicable code. CMHA application table and cement-board guidance

For natural thin stone, check the stone producer’s instructions rather than borrowing a manufactured-stone recommendation. Old World Stone Veneer, for example, requires ANSI A118.4 or A118.15 polymer-modified bonding mortar for its products. It specifically rejects conventional Type S or N for bonding its smooth-backed stone to the scratch coat, but permits them for joint filling. That is a product-specific requirement, not proof that every natural stone needs the same mortar. Old World installation guidelines

“Type S” and “polymer-modified” are not mutually exclusive labels. One describes a masonry mortar classification; the other describes modification of the mix. QUIKRETE’s polymer-modified veneer mortar, for example, also meets ASTM C270 and C1714 Type S property requirements. Its data sheet illustrates why the full product specification matters—not just the large letter on the bag. Neither label alone demonstrates compliance with an ANSI requirement specified for your assembly. Product data sheet

Check the backing before buying mortar

A stronger mix cannot compensate for an unsuitable surface or poorly detailed wall. Before ordering, confirm:

• The approved substrate: scratch coat, concrete, concrete block or a particular cement board.
• Surface condition: sound and free of dust, paint, sealers, release agents or other bond-inhibiting material.
• Exterior moisture management: the required water-resistive barrier, drainage, flashing and weep details.
• Application limits: stone dimensions, installation height and any required mechanical support.

CMHA requires a structurally sound, bondable backing and gives separate requirements for these assembly details. Surface preparation and installation guide

Ask the supplier for written confirmation that the proposed mortar suits your stone, backing, location and mortar layer. Follow that product’s water dosage, mixing, application and curing instructions; do not assume all veneer mortars share one recipe.

Existing old stonework is a different decision

Do not use new-veneer bonding guidance to select repointing mortar for a historic stone wall. The National Park Service advises matching the historic mortar’s appearance and choosing replacement mortar softer and more vapor-permeable than the masonry units, and no harder or less permeable than the historic mortar. Dense, incompatible mortar can redirect moisture and deterioration into the stone. NPS Preservation Brief 2

For that task, start with a historic masonry assessment and compatible repair specification. If adhered veneer is loose, filling its visible joints is not a substitute for investigating the bond and backing.