Summary

Mortar is the binding agent between masonry units — bricks, blocks, stone, or pavers. Getting the volume and mix ratio right at the start of a job prevents costly mid-job trips to the builders' merchant and ensures the mortar performs correctly for its intended application. Too strong a mix (e.g., 1:3 cement:sand) on older soft brickwork causes cracking as it is stiffer than the bricks and cannot flex; too weak a mix may not achieve adequate compressive strength for structural walling.

Mortar volume is calculated from joint volume — the space between masonry units. For standard UK brickwork (215mm × 102.5mm × 65mm bricks with 10mm joints), approximately 16 litres (0.016m³) of mortar is consumed per square metre of half-brick walling. This accounts for mortar in both bed joints and perpendicular (perp) joints, with a typical 10–15% wastage allowance already included.

Pre-mixed or bagged mortar products simplify site logistics but cost significantly more per m² than site-mixed equivalents. For any wall over approximately 15m², site mixing with bulk sand and bagged cement is almost always the more economical choice. For pointing, repointing, and small repairs, pre-mixed products offer convenience that outweighs the cost premium.

Key Facts

  • Half-brick wall mortar volume — 0.016m³ per m² (approximately 16 litres per m²)
  • One-brick wall mortar volume — 0.030m³ per m² (approximately 30 litres per m²)
  • One-and-a-half-brick wall — 0.045m³ per m²
  • Standard joint dimensions — 10mm bed joints, 10mm perpendicular joints for UK brickwork
  • General brickwork mix — 1:5 cement:sand by volume (BS 5628 / BS EN 998-2 Designation M4, 4 N/mm²)
  • Pointing and repointing — 1:3 cement:sand for exposed/weather-facing joints; 1:3 NHL for heritage brickwork
  • Gauged mortar — 1:1:6 (cement:lime:sand) — traditional mix, good workability and flexibility
  • Bags of cement per m³ at 1:6 mix — approximately 6–7 bags of 25kg Portland cement
  • Bags of cement per m³ at 1:5 mix — approximately 7–8 bags of 25kg Portland cement
  • Sand per m³ of mortar — approximately 1.0–1.1 tonnes of building sand
  • Water:cement ratio — approximately 0.5 by mass; excess water weakens mortar and causes shrinkage cracking
  • Pre-mixed mortar cost — typically £5–8 per 20kg bag (approximately 8 litres usable mortar per bag); site-mixed equivalent is 30–50% cheaper per m²
  • Building sand vs sharp sand — building (soft) sand for brickwork and pointing; sharp (concreting) sand for paving bedding and floor screeds
  • Paving bedding mortar — 40–50mm semi-dry 1:6 mix; approximately 0.040–0.050m³ per m²
  • Repointing rake depth — minimum 20mm; mortar volume approximately 0.002–0.003m³ per m² of wall face
  • Mortar shelf life (bagged) — 12 months dry in sealed bags; site-mixed mortar must be used within 1–2 hours of gauging
  • Frost protection — do not lay mortar below 3°C ambient and falling; protect fresh work if temperature drops below 3°C within 24 hours (BS 5628-3)
  • Retarder admixtures — extend workability to 2–3 hours; use with caution as they can delay strength gain

Quick Reference Table

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Application Mix Ratio (cement:sand) Mortar per m² Notes
Half-brick walling 1:5 or 1:1:6 0.016m³ BS EN 998-2 Designation M4
One-brick walling 1:5 or 1:1:6 0.030m³ Same mix, double volume
Block walling (100mm) 1:5 or 1:1:6 0.012m³ Fewer, larger units, less joint
Pointing (exposed modern) 1:3 to 1:4 0.002–0.003m³ Weather-facing joints
Heritage/lime pointing 1:3 NHL 3.5 or NHL 5 0.002–0.003m³ Do NOT use OPC on lime/soft brick
Paving bedding 1:6 semi-dry (sharp sand) 0.040–0.050m³ 40–50mm bed depth
Chimney stack 1:3 OPC:sand 0.016m³ High exposure zone
Below DPC / underground 1:3 to 1:4 sulphate-resisting 0.016m³ Use SRPC in sulphate ground

Detailed Guidance

Calculating Mortar Volume — Worked Example

For a new garden wall 10m long × 1.2m high, half-brick construction (102.5mm wide):

  1. Wall area = 10m × 1.2m = 12m²
  2. Mortar volume = 12m² × 0.016m³/m² = 0.192m³
  3. Add 10% for waste = 0.21m³

At a 1:5 mix, breakdown is:

  • Cement: 0.21m³ ÷ 6 = 0.035m³ ≈ 7 bags of 25kg Portland cement
  • Sand: 0.21m³ × (5/6) = 0.175m³ ≈ 0.28 tonnes building sand

For a one-brick extension leaf 8m² in area:

  • Mortar volume = 8 × 0.030 = 0.24m³
  • Cement: approximately 8 bags of 25kg
  • Sand: approximately 0.33 tonnes building sand

Mix Ratios Explained

Mix ratios are by volume (not weight). Key UK site designations:

1:3 (cement:sand) — strong, relatively brittle, high compressive strength (approximately M12). Used for chimney stacks, coping stones, below-DPC work, and exposed pointing. Not appropriate for flexible or heritage masonry — will crack the bricks rather than the joint.

1:4 (cement:sand) — a good compromise for external pointing on modern brickwork. Slightly more flexible than 1:3. Roughly M6.

1:5 (cement:sand) — standard general brickwork mix. Equivalent to BS EN 998-2 Designation M4 (4 N/mm² compressive strength). Appropriate for most above-DPC walling.

1:6 (cement:sand) — used for blockwork, internal walling, and semi-dry bedding mortar. Lower compressive strength, more flexible.

1:1:6 (cement:lime:sand) — the traditional gauged mortar. Lime improves workability, reduces cracking, and allows the wall to breathe. Correct choice for solid brick Victorian/Edwardian houses where both faces are exposed.

1:3 NHL (natural hydraulic lime:sand) — for heritage conservation, listed buildings, and pre-1920 construction. NHL sets by hydration but is weaker and more permeable than OPC. Do not mix NHL with Portland cement — the incompatibility causes rapid strength loss and cracking.

Building Sand vs Sharp Sand

Building sand (also called soft sand or bricklaying sand) is fine, rounded, and well-graded. It is the correct choice for all brickwork, blockwork, and pointing applications — produces smooth, workable mortar and a good joint finish.

Sharp sand (concreting sand or grit sand) has angular particles and a coarser grading. Use it for:

  • Paving bedding mortars (semi-dry 1:6 with sharp sand)
  • Floor screeds (mixed with OPC)
  • Concrete mixes (not mortar for brickwork)

Using sharp sand in brickwork mortar creates a harsh, unworkable mix that is prone to water ingress at the joint face.

Pre-Mixed vs Site-Mixed: Cost Comparison

For a 10m² half-brick wall requiring 0.16m³ of mortar:

Pre-mixed bagged mortar (e.g., Sika, Tarmac):

  • A 20kg bag yields approximately 8 litres usable mortar
  • Quantity needed: 0.16m³ ÷ 0.008m³/bag = 20 bags
  • Cost at £6/bag: approximately £120

Site-mixed (1:5):

  • 3 bags of 25kg cement at £8/bag = £24
  • 250kg building sand from bulk bag (£45/500kg) = £22.50
  • Total: approximately £46.50

Pre-mixed costs approximately 2.5× more per m² in materials. For small repairs, patches, and isolated pointing areas, the convenience justifies the cost. For any walling job over 5m², site-mixing is strongly recommended.

Paving Bedding Mortar Calculation

For natural stone, porcelain, or concrete block paving, the bedding layer is typically 40–50mm of semi-dry mortar at 1:6 cement:sharp sand. "Semi-dry" means just enough water to bind — squeeze a handful and it should hold shape without dripping.

Worked example: 20m² patio at 50mm bed depth:

  • Volume = 20m² × 0.05m = 1.0m³ + 10% waste = 1.1m³
  • Cement at 1:6 mix: 1.1m³ ÷ 7 ≈ 6–7 bags of 25kg
  • Sharp sand: approximately 1.1 tonnes

Repointing: Volume and Mix Selection

Rake existing joints to a minimum 20mm depth before applying new mortar. Required mortar volume is much less than for new walling — approximately 2–3 litres per m² of wall face (0.002–0.003m³/m²).

Mix selection rules for repointing:

  • Modern brickwork (post-1950, hard engineering bricks): 1:4 or 1:3 cement:sand
  • Interwar or Victorian brickwork (soft stocks, wire-cut bricks): 1:1:6 or 1:2:9 (cement:lime:sand)
  • Listed buildings and pre-1900 lime-pointed walls: NHL 3.5 or NHL 5 at 1:3 ratio
  • Never use a mix stronger than the brick itself — the joint must be the sacrificial element, not the brick face

Mixing Water Ratio

The water:cement ratio (W:C) has a significant effect on final strength and durability. A W:C of approximately 0.5 by mass is correct for typical site-mixed mortar. In practice, this means adding water gradually and mixing thoroughly — the mortar should be stiff enough to hold a brick in place without it sliding, but soft enough to spread and fully bed the unit. Never add extra water to a mortar that has begun to stiffen — discard and mix fresh.

Frequently Asked Questions

How many bags of mortar do I need for 100 bricks?

100 standard UK bricks in half-brick stretcher bond cover approximately 0.67m². At 0.016m³ per m², you need approximately 0.011m³ of mortar — roughly 11 litres. A 20kg bag of pre-mixed mortar yields approximately 8–10 litres of usable mortar, so two bags comfortably covers 100 bricks with some spare.

Can I use the same mortar for brickwork and pointing?

Not ideally. Brickwork bedding mortar (1:5 or 1:1:6) allows some flexibility across the full wall height. Pointing mortar for exposed joints is typically stronger (1:3 or 1:4) because those joints take direct weather exposure. For small DIY jobs, a single 1:4 mix works reasonably for both applications.

What happens if mortar is too wet?

Excess water weakens set mortar (reduces compressive strength), causes excessive shrinkage cracking as it dries, and increases the risk of efflorescence (white salt deposits) on the brick face. The correct W:C is approximately 0.5 by mass. Never add water to mortar that has started to stiffen on the spot board.

How long does mortar take to cure?

Initial set: 2–4 hours at 10–20°C. Adequate strength for light foot traffic on paving: 24–48 hours. Full compressive strength: approximately 28 days. Do not apply load or expose to frost within the first 24 hours if at all avoidable.

Can I add washing-up liquid to improve workability?

Not recommended. Washing-up liquid introduces air bubbles (uncontrolled air-entrainment), which reduces compressive strength unpredictably. The correct additive for improved workability is lime (in a gauged mortar) or a proprietary plasticiser — approximately one capful per 25kg of cement. Lime also improves frost resistance and reduces shrinkage cracking.

Regulations & Standards