Mortar is a workable paste made of a binder (portland cement, lime or masonry cement), sand and water, used to bed and join bricks, concrete blocks and stone. It fills the joints, bonds the units together, spreads loads evenly between them and seals the wall against wind and water.
Mortar is not the same as grout. Masonry grout is a more fluid mix poured into the cores of block walls, and tile grout fills the joints between tiles.
What mortar does
- Bonds masonry units into one wall.
- Takes up small differences in unit size so courses stay level.
- Seals joints against air and water.
- Bonds reinforcement and wall ties in the bed joints.
- Gives the wall its finished joint colour and profile.
Types of mortar
In North America, masonry mortar is specified by type under ASTM C270. The types are often remembered from the word MaSoNwOrK, in order of strength:
| Type | Minimum compressive strength (ASTM C270 property spec) | Typical use |
|---|---|---|
| M | 2,500 psi | Below grade, foundations, heavy loads |
| S | 1,800 psi | Below grade, structural and reinforced masonry, high wind or seismic areas |
| N | 750 psi | General above-grade exterior and interior walls, veneer |
| O | 350 psi | Interior non-load-bearing walls, some repointing |
Stronger is not always better. A mortar that is much harder than the units can make the wall crack through the bricks rather than the joints. In the UK and Europe, mortars are classed by compressive strength in N/mm² under EN 998-2 (for example M5 or M10), and UK specifications commonly call for designations such as M4 or M6. Thinset “mortar” for tile is a separate adhesive product.
Mortar vs grout vs concrete
- Mortar: cement or lime, sand and water; stiff and workable; bonds units.
- Masonry grout: similar ingredients, sometimes with pea gravel, but fluid; fills block cores and bond beams around rebar.
- Concrete: includes coarse aggregate; a structural material on its own.
On heritage buildings, softer lime mortar is often required so moisture and movement go through the joints rather than damaging historic brick or stone.
Site tips
- Batch consistently. Measure sand by volume with a gauge box rather than counting shovels.
- Use mortar within the time limit in the specification and do not retemper beyond what it allows.
- Tool joints when the mortar is thumbprint hard for a dense, weather-resistant joint.
- Protect fresh work from rain, freezing and fast drying.
- Match colour and type to the approved mockup.
Related terms
Related templates and guides
- Hot and cold weather masonry construction
- QA checklist: masonry works
- Masonry terms and definitions
- Safety meeting: concrete and masonry
Frequently asked questions
What is the difference between mortar and grout?
Masonry mortar is a stiff, workable paste used to bed and join bricks, blocks and stone. Masonry grout is a fluid mix poured into the hollow cores of block walls and bond beams, usually around reinforcing steel. Tile grout is a different product again, used to fill joints between tiles.
What is the difference between mortar and concrete?
Both use cement, water and aggregate, but mortar uses only fine aggregate (sand) and is made to bond masonry units together. Concrete includes coarse aggregate such as gravel or crushed stone and is made to be a structural material on its own.
Which mortar type should I use?
Use the type the drawings or specification call for. As a general guide, Type N is common for above-grade exterior and interior walls, Type S for below-grade and higher-strength structural work, Type M for heavy loads below grade, and Type O for interior non-load-bearing work and some repointing.
Why use lime mortar on old buildings?
Historic brick and stone are often softer than modern units. A softer, more breathable lime mortar lets moisture escape through the joints and takes up movement, so the mortar, not the masonry, cracks first. Hard cement mortar on soft historic masonry can cause spalling.
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