How to read your building: what the facade is telling you

Start with the material

Before you look at condition, identify what you are looking at. The material determines everything: which soiling types affect it, which cleaning methods are appropriate, which repair materials are compatible, and which planning framework applies.

Portland stone is recognisable by its off-white colour, the small oolites visible at close range, and the presence of shell fragments. It weathers to a silver-grey patina on exposed surfaces and accumulates a hard black sulphate crust on sheltered ledges. It is used extensively on London's civic and commercial buildings and appears as detailing on many domestic facades built from the Edwardian period onwards.

London stock brick is amber, cream, and grey, with a characteristic mottled appearance from the clamp-firing process. It is handmade, so individual bricks vary in size and texture. The colour changes slightly depending on which part of the brick was facing the fire during firing. Old stock brick is often darker than brick from the same period that has been protected from weather.

Limestone is softer and more uniform in colour than Portland stone. It ranges from pale cream to warm buff. It is used on ecclesiastical buildings and some Georgian domestic work. It weathers more quickly than Portland stone and is more susceptible to biological growth.

Sandstone is reddish-brown or buff, with a gritty texture and visible grain. It is less common in London than further north, but appears on some Victorian commercial buildings and on churches with northern English connections. It is permeable and can suffer severe freeze-thaw damage.

Reading the soiling

Soiling on historic masonry is not uniform. Different soiling types have different causes, different implications for the substrate, and different appropriate responses.

General atmospheric soiling is the grey-to-black discolouration caused by particulate matter — vehicle exhaust, dust, combustion products — adhering to the surface. On London buildings, this is almost universal. It is surface-level and responds well to DOFF steam cleaning. Its presence is not a sign of structural deterioration.

Sulphate crust is different. It is a hard, black, crystalline deposit that forms on sheltered areas of limestone and Portland stone where the material cannot be washed by rain. The sulphur dioxide in polluted urban air reacts with the calcium carbonate to form calcium sulphate (gypsum), which forms a hard layer over the stone surface. Below the crust, the stone is often deteriorating: the calcite crystals have been disrupted, the surface is becoming friable, and if the crust is removed by the wrong method, the weakened stone beneath can disaggregate. TORC vortex cleaning is generally more effective than DOFF alone on heavy sulphate crust.

Orange or rust-coloured staining on stone or brick usually indicates iron. Iron in the aggregate of the original masonry oxidises when exposed to water and air, producing rust-coloured streaks that run down from the source. Iron staining can also come from iron fixings — window frames, railings, cramps — that have corroded. Cleaning the stain without addressing the iron source will result in the staining recurring.

White crystalline deposits — efflorescence — are salts that have migrated to the surface with evaporating water and been deposited as the water evaporates. They are a sign of moisture movement through the masonry. Efflorescence itself is not harmful and usually brushes off dry. But the moisture movement that caused it should be investigated: is there a defective gutter or downpipe? A failed damp-proof course? An internal source of moisture? Cleaning the efflorescence without addressing the cause means it will return.

Green or black biological growth — algae, lichen, moss — thrives on damp masonry. Its presence indicates that the surface is retaining moisture. On north-facing elevations and sheltered areas, some biological growth is almost inevitable, but heavy colonisation suggests a moisture problem. DOFF removes biological growth effectively, but again: if the underlying moisture source is not addressed, the growth will return.

Reading the mortar joints

Look at the pointing. Flush, white, very hard mortar is OPC, applied at some point in the twentieth century. If it is sound — not cracked, not recessed, no visible gaps — it is causing slow damage to the masonry but may not require immediate attention. If it is cracked or loose, it needs to come out.

Recessed mortar that is soft and crumbly is failing original lime. It should be replaced. The question is how much: selectively in bad areas, or fully? A full repoint is disruptive and expensive. Selective repointing is appropriate where the majority of the mortar is sound and only localised areas have failed.

Look for mortar that has been applied over the face of the brick — ribbon pointing, or mortar smeared onto the brick surface at the edge of the joint. This was often done to cosmetically disguise defective joints or spalling brick edges. It traps moisture and accelerates deterioration. It needs to be removed.

Reading cracks

Cracks in masonry can be benign or serious. The key is to characterise the crack: is it in the mortar joint, in the masonry unit, or across both? Is it stepped, following the mortar joints in a staircase pattern? Is it vertical, horizontal, or diagonal? Is it recent or historic? Is it still moving?

Stepped diagonal cracks following mortar joints are usually differential settlement — one part of the building has moved relative to another. This is extremely common in Victorian terrace housing and is usually historic movement that has stabilised. A crack that has been filled and re-opened is still active. A crack with clean, unfaced edges is recent. A crack with rounded edges and visible biological growth is old.

Horizontal cracks in brick, particularly at the level of a damp-proof course or at the junction between a bay window and the main wall, may indicate lateral movement or moisture intrusion at that level. Vertical cracks at the junction between a return or extension and the main building are usually differential settlement between the two structures.

The important point is that a crack should be understood before it is repaired. Repointing a crack without knowing whether it is active and what is causing it simply fills the crack temporarily. If the movement continues, the repair will fail. If the movement is structural, filling the crack is not the right response at all.

What a good survey looks like

A proper condition survey of a historic facade takes time. It involves close inspection of the whole face — ideally from scaffold or a mobile elevated work platform, not from the pavement — and a systematic assessment of material type, soiling type, mortar condition, crack pattern, and any specific features of interest or concern. The output is a schedule of works that addresses the actual condition of the building rather than a generic specification based on what the building type usually needs.

We carry out condition surveys as part of every project. They are not an optional extra. The survey determines the specification, and the specification determines the outcome. A building that has been assessed properly and treated with the right methods in the right sequence will look better and deteriorate more slowly than one that has been cleaned and pointed according to a generic template. That is the point of doing it correctly.