Building Restoration

Lime Mortar — Why It Matters

Every building constructed before roughly 1920 used lime mortar. Portland cement mortar only became standard in the mid-20th century. The difference matters more than most people realise.

Lime mortar is softer than the bricks or stones it bonds. When a building moves (and all buildings move — thermal expansion, ground settlement, wind loading), the lime mortar flexes and accommodates the movement. Cement mortar is harder than most old bricks. When a cement-pointed building moves, the cement doesn't flex — instead, the bricks crack and spall. Over time, you end up with sound mortar joints and shattered brickwork. Exactly backwards.

Lime mortar is also breathable. It allows moisture to evaporate through the joints, which is how traditional buildings manage damp. Cement is impermeable — it traps moisture in the masonry, leading to damp walls, frost damage, and salt crystallisation that destroys the brick faces from inside.

We use three types of lime mortar depending on the building and exposure:

Lime putty mortar (non-hydraulic): The most traditional. Made from slaked lime and sand. Sets by carbonation (absorbing CO₂ from the air). Very soft and breathable. Used for internal work and sheltered external locations. Slow setting — needs protection from rain and frost during curing.

Natural hydraulic lime (NHL): Sets by both hydraulic action (reaction with water) and carbonation. Stronger than lime putty and more weather-resistant. Available in three strengths: NHL 2 (feebly hydraulic — for most repointing), NHL 3.5 (moderately hydraulic — for exposed locations), and NHL 5 (eminently hydraulic — for severe exposure, chimneys, below-ground work). Our standard choice for external repointing.

Hot lime mortar: Made by mixing quicklime with sand and water on site. Produces a mortar with excellent workability and strong adhesion. Increasingly popular for restoration work because it closely replicates the original mortar used in many pre-Victorian buildings. More labour-intensive to produce but gives a superior result.

Like-for-Like Replacement

The principle of like-for-like is central to listed building restoration. If the original building has Welsh slate on the roof, the replacement should be Welsh slate — not Spanish slate, not concrete tiles, not synthetic slate. If the windows are painted softwood sashes, the replacements should be painted softwood sashes — not hardwood, not uPVC, not aluminium.

This doesn't mean you can never improve anything. Modern slim-profile double glazing can be fitted into traditional sash window frames. Discreet secondary glazing can be added behind original single-glazed sashes. Insulation can be added to loft spaces without affecting the visible character. Heating systems can be run in concealed routes. But every improvement needs to be done with the building's character in mind, and most will need Listed Building Consent.

Timber Repair Techniques

When structural timbers are damaged — typically at bearing points where they sit in damp walls — there are several repair options depending on the severity:

Resin repair: For decay limited to the surface or end of a timber. The rotten wood is cut away, the remaining timber is treated, and the missing section is rebuilt using a structural epoxy resin (brands like Repair Care, Rotafix). This preserves as much original timber as possible and is the preferred approach for listed buildings where conservation of original fabric is paramount.

Timber splice: For more extensive decay. A section of new timber (same species) is scarfed or bolted onto the surviving sound timber. The joint is designed by a structural engineer to transfer the full load. Oak pegs or stainless steel bolts are used depending on the building's original jointing method.

Full replacement: When the timber is beyond repair. The entire member is replaced with new timber of the same species, section size, and finish. For listed buildings, the conservation officer may require evidence that repair wasn't feasible before approving full replacement.

Cement Repointing on Lime-Mortar Buildings

This is the single most common and most damaging mistake made on pre-1920s buildings. A well-meaning builder (or homeowner) repoints the walls with ordinary Portland cement mortar. Within 5–10 years, the bricks start spalling (the faces pop off), damp appears on the internal walls, and the cement itself cracks because it can't flex with the building. We rake out the cement, allow the masonry to dry out, and repoint with appropriate lime mortar. On a typical terraced house elevation, removing cement and repointing in lime costs £3,000–£6,000 per elevation.

Injected Chemical DPC

The damp-proofing industry has been injecting silicone into old walls for decades, claiming it cures "rising damp." The evidence for rising damp as a widespread phenomenon in solid-walled buildings is thin at best. Most damp in period properties is caused by external factors — high ground levels, cement pointing, failed drainage, missing lime render. Injecting chemicals into a solid stone or lime-mortar wall does nothing useful and can make things worse by creating a horizontal barrier that traps moisture above the injection line. We fix the actual causes of damp rather than treating symptoms with chemicals.

External Cement Render on Breathable Walls

Rendering a solid stone or brick wall with cement-based render seals the surface and prevents moisture from evaporating outwards — the way these buildings were designed to work. The trapped moisture migrates inwards, causing damp internal walls, mould, and decay of embedded timbers. If a solid-walled building needs external rendering, it should be lime render — breathable, flexible, and compatible with the underlying masonry. Removing cement render and re-rendering in lime is one of our most common restoration projects.

uPVC Windows on Listed or Period Buildings

uPVC replacement windows on a listed building are almost always a consent violation and will need to be replaced with appropriate timber windows. On unlisted period properties, uPVC windows may be legally permissible but they look wrong, they don't match the proportions of the original openings, and they reduce the property's value and character. Restoring the original timber windows — or fitting new timber windows that accurately replicate the originals — is always the better approach.

Concrete Floors in Place of Suspended Timber

Many Victorian houses originally had suspended timber ground floors — joists on sleeper walls with air bricks providing ventilation underneath. A common "improvement" was to dig up the timber floor and pour a concrete slab, often without a damp-proof membrane. This eliminates the sub-floor ventilation that the building was designed to rely on, traps ground moisture under the slab, and causes damp problems in the walls that previously had airflow at their base. Where this has been done, we either reinstate the suspended floor or install a properly detailed concrete floor with DPM and insulation, along with measures to restore wall ventilation.