Mortar Mixes for Brickwork: Ratios, Strength Classes and When to Use Each

Summary

Mortar has one job: to bed and bond masonry units while allowing slight movement and distributing load. The critical principle is that mortar must be weaker than the masonry units it joins — not stronger. A mortar that is too strong will not accommodate thermal or moisture movement; the result is cracked or spalled bricks rather than cracked joints. Cracked joints are easy to repair; spalled brick faces are not.

Traditional UK mortars used hydraulic lime (NHL) or hot lime putty as the binder, producing flexible, breathable joints ideal for pre-1920 masonry. Modern mortar uses Portland cement (OPC), often with a plasticiser or a proportion of lime to improve workability. Masonry cement is OPC pre-blended with a plasticiser (often air-entrained) and sold ready to mix with sand.

Understanding the designation system (M1–M12) and the mix ratios behind each grade lets you match mortar to brick type, exposure, and location — protecting the brickwork for decades rather than years.

Key Facts

• BS EN 998-2:2016 — current UK standard for masonry mortar
• Designation system — M1 to M12 (compressive strength at 28 days in N/mm²): M1 = 1 N/mm², M2 = 2.5, M4 = 4, M6 = 6, M10 = 10, M12 = 12
• Mix by volume (cement:lime:sand) for common designations: M2 = 1:2:9, M4 = 1:1:5–6, M6 = 1:0.5:4.5, M12 = 1:0:3
• Masonry cement mixes: 1:5.5 (M4 equivalent), 1:4 (M6 equivalent)
• Lime-only mortars — NHL 2, 3.5, or 5 with sand; designations roughly M0.5–M3.5
• Never use OPC 1:3 (M12) for facing brickwork — causes cracking and spalling; reserve for manholes and engineering brick below DPC
• S2 soluble salt class — only relevant to bricks, not mortar; but sulphate-resistant Portland cement (SRPC) is needed where bricks have high soluble salt content
• Sulphate attack — OPC mortar attacked by sulphates in groundwater or brick leachate; use SRPC (BS EN 197-1) or SRC mortars for chimneys, below-DPC work, and retaining walls in high-sulphate ground
• Lime putty vs hydraulic lime — lime putty (air-curing) used for traditional pointing; NHL 2/3.5/5 (hydraulic) used for structural mortar and external pointing on pre-1920 buildings
• Plasticiser — improves workability; avoid overuse (reduces bond and frost resistance); 60–90ml per 25kg cement is typical
• Open time — conventional OPC mortar 1–2 hours; admixture-extended mortar up to 4 hours; never retemper mortar that has stiffened
• Bagged pre-mix mortars — convenient but check designation on bag; many cheap pre-mixes are M2 grade, unsuitable for external work
• Site mixing ratios — always mix by volume using gauging boxes (or bucket counts), not by shovelful; shovels of sand are not consistent

Quick Reference Table

Detailed Guidance

Matching Mortar Strength to Brick Type

The BDA (Brick Development Association) and BS EN 1996-2 provide guidance on compatible combinations:

Mortar for Period and Listed Buildings

For any pre-1920 building, and especially for listed buildings, traditional hydraulic lime mortar is required:

• Soft lime putty (non-hydraulic) — used for internally sheltered pointing, very fine jointing, and decorative work
• NHL 2 — weakest hydraulic lime; equivalent to M0.5–M1; used in softest, most vulnerable old brickwork
• NHL 3.5 — most commonly used for external pointing on Victorian/Edwardian brickwork; approximate M2–M3
• NHL 5 — strongest hydraulic lime; approximate M3.5; used for more exposed pre-1920 brickwork, natural stone, and engineering works

Why lime matters for old buildings: Traditional brickwork had no DPC; moisture evaporates through permeable joints. Cement mortar creates a barrier causing moisture to concentrate in the brick face — leading to spalling. This is especially destructive with reclaimed London stocks, flettons, and any pre-1900 soft brick. Using OPC mortar on a listed building without consent can constitute criminal damage under the Planning (Listed Buildings and Conservation Areas) Act 1990.

Mortar for Chimneys

Chimney mortar is subject to:

• Thermal cycling (cold to 200°C+ in flue)
• Sulphate attack from condensed flue gases
• Severe exposure (top courses fully exposed)

Correct specification for chimney pointing:

• Use sulphate-resistant Portland cement (SRPC) to BS EN 197-1
• Mix: 1:0.5:4.5 SRPC:lime:sharp sand (M6 grade)
• For chimneys with gas appliances (condensing): add NHL 3.5 lime component for flexibility
• Never use a bagged sand/cement pointing mix — check for SRPC content
• Flaunching (mortar cap around pot): 1:3 SRPC:sharp sand, feathered to shed water, minimum 45° slope

Colour Matching Mortar for Re-pointing

Getting the mortar colour right for repointing is as important as the mix. Factors:

1. Sand colour — sand dominates mortar colour. Silver sand produces light mortar; sharp yellow sand produces buff/yellow; pit sand produces brown tones
2. Pigments — iron oxide pigments mixed at max 10% by weight of cement (higher amounts weaken the mix)
3. Cement type — white Portland cement (WOPCC) produces cleaner colours with pigments; grey OPC tends to dull the mix
4. Lime content — more lime produces slightly lighter, more matte finish
5. Water/cement ratio — lower w/c ratio produces darker (denser) joints

Always mix a small test sample and let it cure for 3–4 days before comparing to existing work — fresh mortar is significantly darker than cured mortar.

Frequently Asked Questions

What mortar should I use for laying paving and flagstones?

For bedding paving, use a semi-dry mix: approximately 1:6 OPC:sharp sand mixed until the mortar holds its shape when squeezed but contains no free water. This semi-dry mix allows the slab to be adjusted for 20–30 minutes. For grouting joints after laying, use a 1:4 OPC:sharp sand slurry for natural stone or buff pointing mortar for block paving. For resin-bound surfaces, see resin bound gravel.

Can I use ready-mixed mortar from a bag for external brickwork?

Yes, but check the designation on the bag. Many retail bagged mortars are M2 or lower — suitable only for internal, sheltered work. For external brickwork, look for bags marked M4 minimum. Pre-mixed mortars with retarder have a limited open time (often stamped on the bag — typically 48 hours from opening). Always check and comply with the open time.

How do I know if existing mortar has sulphate attack?

Signs of sulphate attack include: horizontal cracks following the bed joints (often mid-way through joint depth), white crystalline deposits on the face of the mortar, and expanding/bowing brickwork. Confirm by having the mortar tested (sulfate content >4% SO₃ in cement indicates attack). Remediation requires complete raking out and repointing with SRPC mortar after addressing any water ingress source.

What mix should I use below the DPC?

Below the damp-proof course and in contact with ground or surface water, use M6 minimum with sulphate-resistant Portland cement (SRPC). Mix: 1:0.5:4.5 SRPC:lime:sharp sand, or 1:3 SRPC:sharp sand (M10) for engineering brick. The stronger mix is justified here because the bricks themselves (engineering Class B or above) are harder and can accommodate a stiffer joint without risk of spalling.

Regulations & Standards

• BS EN 998-2:2016 — Specification for mortar for masonry: Masonry mortar

• BS EN 1996-2 — Design of masonry structures: Design considerations, selection of materials and execution of masonry

• BS EN 197-1 — Cement: Composition, specifications and conformity criteria for common cements (includes SRPC specification)

• BS EN 459-1 — Building lime: definitions, specifications and conformity criteria (covers NHL grades)

• Approved Document A (Structure) — structural masonry design

• Historic England Practical Building Conservation — Mortars, Renders and Plasters — guidance for work on historic masonry

• Lime Green Products Technical Guidance — NHL lime mortar selection and mixing guides

• Tarmac Blue Circle Mortars Guide — cement and masonry cement technical data

• Brick Development Association — Mortars for Brickwork — free downloadable BDA guide

• Historic England — Mortars for Historic Masonry — guidance for pre-1920 buildings

• brick types — matching mortar strength to brick class

• pointing repointing — applying mortar in repointing situations

• efflorescence — sulphate attack and soluble salt issues

• mortar quantities — calculating mortar volume needed