Lime Mortar and Repointing for London Victorian Brickwork: Why It Matters and How to Get It Right

**The fundamental principle: the mortar must be softer than the brick**

The most important rule in specifying mortar for historical brickwork is that the mortar must always be weaker (softer) than the brick it surrounds. This is not a matter of tradition or aesthetics — it is a physical requirement based on how masonry accommodates thermal and moisture movement.

*Why mortar must be softer than the brick it points*:

All masonry expands and contracts with temperature changes and moisture variation. In a masonry wall, the mortar joints are designed to accommodate this movement. If the mortar is weaker than the brick, movement causes the mortar to crack and deteriorate over decades — and eventually the mortar can be raked out and replaced. The bricks themselves are preserved.

If the mortar is harder and stronger than the brick (as OPC mortar is relative to soft Victorian stock brick), movement is forced through the brick rather than the joint. The face of the brick spalls (the surface delaminates and falls off). Once a brick face has spalled, the brick cannot be repointed — it must be replaced, which is expensive and difficult to match on Victorian London stock brick.

**The physical properties of London Stock Brick that require lime mortar**:

• London stock brick is a fired clay brick with the following physical characteristics relevant to mortar selection:
• •*Compressive strength*: typically 10–30 N/mm² (much lower than modern engineering brick at 50–150 N/mm²; and lower than typical hard-fired Staffordshire bricks at 30–60 N/mm²)
• •*Water absorption*: high — 15–25% by weight. London stocks are very permeable and absorb significant moisture
• •*Vapour permeability*: very high — London stock brick walls 'breathe' by absorbing moisture and releasing it as vapour. This is the fundamental mechanism by which Victorian terraced houses manage moisture — the wall absorbs rain, drives moisture inward through the brick, and then the moisture evaporates from the internal and external surfaces. This requires a highly breathable wall system throughout — including the mortar

**What OPC mortar does to London Stock Brick**:

• Ordinary Portland Cement mortar (a 3:1 or 4:1 sand:OPC mix as typically used in modern brick construction) has the following properties that make it incompatible with Victorian stock brick:
• •*Compressive strength*: typically 10–25 N/mm² — similar to or stronger than London stock brick. This violates the fundamental rule that mortar must be weaker than the brick
• •*Vapour impermeability*: OPC mortar is almost vapour-impermeable — it blocks the moisture movement through the wall that the wall was designed to accommodate. Moisture that would previously have distributed evenly through the breathable brick wall and evaporated now concentrates at the OPC joint/brick interface
• •*Sulphate attack on OPC mortar*: Victorian brick often contains sulphates absorbed during firing or from atmospheric pollution. When sulphates contact OPC mortar in the presence of moisture, they react to form ettringite — expanding crystalline salts that push the OPC mortar out of the joint (a common cause of the distinctive horizontal cracking and joint deterioration seen on Victorian brickwork that has been repointed with OPC)
• •*Result*: OPC-repointed Victorian stock brick walls develop a cycle of moisture concentration, frost damage at the restrained interface, spalling of the brick face, and eventually structural deterioration of the brickwork

**The specification solution: lime mortar in two forms**:

*1. Lime putty mortar (non-hydraulic lime, CL 90S to BS EN 459-1)*:

• Lime putty is made from burning and slaking pure limestone (calcium carbonate → calcium oxide + CO₂; then CaO + H₂O → Ca(OH)₂ — calcium hydroxide lime putty). Lime putty mortar hardens by carbonation — absorbing CO₂ from the atmosphere, reverting to calcium carbonate. This process is slow (full hardness achieved in months to years) and produces a mortar with:
• •Very high vapour permeability (MVTR: moisture vapour transmission rate similar to the brick)
• •Low compressive strength (typically 1–5 N/mm² after full carbonation — significantly weaker than Victorian stock brick)
• •High flexibility — lime putty mortar can accommodate small movements without cracking
• •Self-healing — hairline cracks in lime putty mortar can carbonate closed

• *Lime putty mortar mix for London stock brick repointing*:
• •Binder: mature lime putty (CL 90S; minimum 3 months maturation after slaking; 'hot lime' mortars from fresh-burned lime on site are a specialist technique and not appropriate for routine repointing work)
• •Aggregate: sharp washed sand (river sand or pit sand with angular grains); ideally colour-matched to the original joint colour. Common sand choices for London stock brick: buff or yellow sharp sand; silver sand (paler); local Thames river sand (buff-brown)
• •Mix ratio: 1 part lime putty : 2.5–3 parts sand by volume
• •No cement addition — pure lime putty + sand. Even a small OPC addition (10%) significantly increases the hardness and impermeability beyond what is appropriate for stock brick

*When lime putty mortar is used*: for pre-1900 London stock brick buildings; listed buildings and conservation area buildings where conservation principles require it; any wall where the existing mortar is a traditional lime mix and repointing must match in hardness and permeability.

*2. Hydraulic lime mortar (NHL — Natural Hydraulic Lime)*:

Natural Hydraulic Limes (NHLs) are made from burning impure limestone containing clay minerals (silicates) that produce hydraulic compounds. Unlike lime putty, NHL mortars harden by both carbonation AND hydraulic setting — giving faster initial strength gain and higher final compressive strength than lime putty, while retaining much better vapour permeability and flexibility than OPC.

• *NHL grades*:
• •NHL 2 (Feebly Hydraulic): slowest setting; lowest final strength (2–7 N/mm² at 28 days); highest permeability; highest flexibility. For very soft brick and internal applications
• •NHL 3.5 (Moderately Hydraulic): moderate setting; moderate strength (3.5–10 N/mm²); widely used for London Victorian brick repointing — a good balance of workability, flexibility, permeability, and durability
• •NHL 5 (Eminently Hydraulic): fastest setting; highest NHL strength (5–15 N/mm²); less flexible than NHL 2 or 3.5; for harder brick types and exposed high-frost-risk positions (copings; cappings; below DPC)

• *NHL 3.5 mix for London Victorian brick repointing*:
• •1 part NHL 3.5 : 2.5 parts sharp washed sand (by volume; measured dry)
• •No cement addition
• •Can be used in cooler weather than lime putty (hockey setting allows some use down to +3°C) — lime putty mortars should not be used below +5°C or when frost is expected within 48 hours of application

*When NHL mortar is used*: NHL 3.5 is the most widely used mortar for London Victorian brick repointing on properties that are not listed and are post-1880 (late Victorian and Edwardian red brick). NHL 5 for exposed positions (copings; chimney stacks; below DPC). NHL 2 for the softest, most permeable stock brick on pre-1860 London buildings.

**Why the joint profile matters**:

The pointing profile — the shape of the mortar joint at the brick face — affects both the weatherproofing performance and the visual appearance of the brickwork. Victorian London brickwork was typically pointed in one of a small number of profiles, and repointing in an inappropriate profile (a common error when OPC was used from the 1960s–1990s) produces a visually jarring result that is particularly noticeable in conservation areas.

**Common original pointing profiles on London Victorian and Edwardian brickwork**:

*1. Flush pointing*: The mortar is finished flush with the brick face — no projection; no recession. The mortar is slightly textured by the scraper/trowel used to finish it. Common on much of London's Georgian and early Victorian stock brickwork, and on quality Victorian red brick buildings. Provides good weatherproofing as there is no ledge for water to sit on and penetrate.

*2. Weathered ('struck') pointing*: The mortar is cut with a slightly angled profile — the top of the joint is set slightly back from the face and the bottom is flush or slightly projecting. Water runs off the sloped face away from the joint. Common on London Victorian terraces. The classic 'pointing' profile when Victorian joints are described as struck or weathered.

*3. Recessed pointing*: The mortar is set back 3–5mm from the brick face, producing a defined shadow line at each joint. Common on higher-quality Georgian and Regency London brickwork (where it emphasised the precision of the brickwork) and sometimes used on Edwardian red brick buildings for visual effect. Should only be used where evidence of original recessed pointing exists — not appropriate for typical mid-Victorian stock brick terraces.

*4. Bucket-handle ('Rodded' or 'Ironed') pointing*: The mortar is finished with a curved metal tool producing a slightly concave profile, like the cross-section of a bucket handle. Common on late Victorian and Edwardian red brick brickwork, and the standard pointing profile for inter-war London brick (Fletton). Very common and widely recognised as appropriate for Edwardian red brick terraces.

*5. Ribbon or 'strap' pointing*: Mortar applied as a wide projecting band over the brick face, covering a significant proportion of the brick surface. This is a modern application (1970s–1990s style, typically using OPC mortar) that has no historical precedent on Victorian or Edwardian London brickwork. It is universally regarded as inappropriate in conservation areas and should not be used on historical brickwork under any circumstances.

**How to identify the original pointing profile before repointing**:

1. Look for sheltered areas of the wall where the original pointing has survived undisturbed — typically under window sills; behind downpipes; under canopies; or on sheltered rear walls 2. Scrape carefully to expose unweathered mortar — the original profile will be visible in sheltered sections 3. Take photographs of the original profile for the contractor's reference before any raking out begins 4. In conservation areas, discuss the original joint profile with the LPA conservation officer before agreeing the repointing specification

**The repointing process — step by step**:

*1. Raking out*: the deteriorated mortar is raked out to a minimum depth of 15–25mm using hand raking tools or an oscillating multi-tool with a narrow blade. Power angle grinders should never be used on Victorian stock brick — they damage the brick arris (edge) and produce a visually and structurally poor result. Raking must not damage the brick face or arris.

*2. Cleaning the joint*: after raking, the joint is brushed with a stiff natural-fibre brush to remove dust and loose material. The wall is damped down before the new mortar is applied — damping prevents the dry brick from drawing water too rapidly from the mortar and prevents premature drying (a particular issue with absorbent London stock brick on hot, sunny walls).

*3. Mortar mixing*: the mortar is gauged carefully (consistent ratios) and thoroughly mixed. NHL mortar: mix dry materials first; add water gradually. Lime putty mortar: lime putty is pre-mixed with aggregate and gauged; add water only as needed. Do not over-water — soft mortar runs out of joints.

*4. Application*: mortar is packed firmly into the joint using a pointing trowel (small 'Tuck' or 'Frenchman' tool for accurate joint work). Work from the top of the wall downward. Pack each joint firmly — no voids or air pockets.

*5. Finishing*: the joint is worked to the specified profile while the mortar is in the green (firm but not set) state — typically 1–4 hours after application depending on temperature and NHL grade. A Frenchman tool and straight edge for weathered joints; a bucket-handle tool for rodded joints; a scraper/brush finish for flush pointing.

*6. Protection and curing*: newly pointed brickwork must be protected from rain (for 24–48 hours), frost (for 2–7 days depending on NHL grade), and direct sun (avoid rapid drying on south-facing walls in summer). Hessian or polythene sheeting protection during curing significantly improves the durability of the set mortar.

**Cost of lime mortar repointing in London (2025)**:

Repointing costs depend on the area to be repointed, the height and access requirements, the condition of the existing joints (depth of deterioration; whether any brick replacement is needed), and the specification (lime putty vs. NHL; joint profile complexity).

*Typical repointing rates (London 2025)*:

| Scope | Typical cost per m² | |---|---| | NHL 3.5 repointing, ground level, flush or weathered joint | £35–£55/m² | | NHL 3.5 repointing, scaffold required | £50–£80/m² (inc. scaffold contribution) | | Lime putty mortar repointing | £45–£70/m² | | Conservation area specialist repointing (putty lime; traditional profile; heritage brick) | £60–£100/m² | | Brick replacement + repointing (reclaimed matching brick) | £80–£150/m² |

• *Scaffold costs for multi-storey repointing*:
• •A full scaffold to a standard 3-storey London terraced house (front elevation): approximately £1,500–£2,500 for the scaffold erect/strike. Scaffold costs should be included in the total quotation.

• *Typical areas for a standard London terraced house (front elevation only)*:
• •2-storey front elevation: approximately 25–35m² brickwork area
• •3-storey front elevation: approximately 40–55m² brickwork area

• *Total repointing cost examples*:
• •2-storey London terraced house front elevation, NHL 3.5, scaffold: £2,000–£3,500 total
• •3-storey Victorian terraced house front elevation, lime putty, scaffold: £3,500–£6,000 total
• •Full external repointing (front + rear + gables, 3-storey): £6,000–£12,000 total

**How to find and brief a competent repointing contractor in London**:

• *What to look for in a repointing contractor*:
• •Experience with lime mortar — not just OPC masonry. Ask explicitly: 'Do you use NHL or lime putty mortar for Victorian stock brick? What ratio do you use?' A contractor who insists on OPC mortar, or who cannot explain the difference between NHL and OPC, should not be entrusted with Victorian brickwork
• •Traditional Mortars Group (TMG) membership, or membership of the Society for the Protection of Ancient Buildings (SPAB) contractor register, indicates awareness of conservation principles
• •References from previous repointing work on similar Victorian properties in London — ask for addresses that can be inspected
• •In conservation areas: confirm the contractor is familiar with the LPA's materials requirements and can submit or support a materials sample panel if required

*Key questions to ask before awarding a repointing contract*: 1. What mortar specification will you use? (Answer should be: NHL 3.5 or lime putty for Victorian stock brick; specify sand colour) 2. How deep will you rake out? (Answer should be: minimum 15–20mm; by hand or oscillating multi-tool; no angle grinder) 3. What pointing profile? (Should match the original — ask to see their photographs of the original profile on the wall) 4. Will you damp the wall before pointing? (Answer should be: yes) 5. How will you protect the work during curing? (Answer should be: hessian or polythene protection from rain and frost for minimum 48 hours) 6. Is scaffold included in the quotation? (Get clarity on this before signing)

• *Building Regulations and planning requirements for repointing*:
• •Repointing of existing walls like-for-like does not normally require Building Regulations approval or planning permission
• •In a conservation area, repointing with a significantly different mortar type or colour may be considered a material change to the appearance of the property — technically requiring planning permission, though enforcement of this for repointing (rather than for OPC introduction on listed buildings) is relatively uncommon. In practice, using the correct lime mortar matches the appearance more closely than OPC mortar, so the LPA is unlikely to object
• •For listed buildings: any works affecting the external appearance, including repointing, require Listed Building Consent. The specification must be agreed with the LPA conservation officer before any raking out begins