Identify the condition before treating it

The shape of a runoff mark often reveals its origin. A streak below a coping differs from a halo around a fixing, darkening beneath a cornice or biological growth in a damp, shaded area.

Map the staining and inspect the construction details before cleaning. A clean façade that remains poorly drained will soil again quickly, sometimes with even stronger contrasts.

Using the shape of a mark as a diagnostic tool

A streak beginning directly below a joint or fixing indicates a probable source. Diffuse wind-oriented soiling suggests exposure. A clear difference between rain-washed and sheltered bands shows the combined effect of rainfall and pollution.

Cleaning trials should recover an appropriate texture and colour without opening the stone surface or creating an isolated over-cleaned patch that cannot be blended.

Checks that will not make the condition worse

Step-by-step diagnosis

Treat the confirmed cause

What not to do

• Clean the entire façade before correcting water ingress.
• Use pressure that removes the stone’s surface layer.
• Hide a rust streak without inspecting the fixing.
• Apply a biocide without managing rinse water.
• Create isolated clean rectangles visible from ground level.

When to consult a specialist

• Work at height or above members of the public.
• Staining associated with a fixing, bulging, cracking or a moving element.
• A historic façade, black crust or fragile stone.
• Cleaning requiring containment, wastewater treatment or authorisation.

Preventing recurrence

• Design effective copings, upstands and drips.
• Select compatible metals and protect interfaces.
• Inspect joints and drainage to a planned maintenance schedule.
• Avoid permanent runoff from irrigation.
• Retain sample panels and cleaning records.

Frequently asked questions

Limits of remote diagnosis

This guide supports a methodical initial diagnosis. It does not replace an on-site investigation where stability, waterproofing, structure, public safety or professional liability may be involved.

Document the case with an overall view, a close-up with a scale, edges, joints, possible water paths and an unaffected comparison area. Record when it appeared, how it has changed, which products were used and any recent work.

Assess urgency before intervening

Read the pattern of soiling to distinguish run-off, pollution, corrosion, microorganisms, salts, coping defects and migration from joints or fixings.

The first objective is not to remove the visible mark immediately. Determine whether it is stable, progressive or hazardous. Historic dry soiling does not have the same priority as active ingress, a corroding fixing, moving stone or an element at risk of falling.

Begin by protecting people and the construction. Stop water sources where this can be done safely. Do not scrape, abrade or try a succession of cleaners. Dated elevation photographs and unaffected comparison areas usually provide better evidence than an aggressive trial.

Urgency increases when the mark changes rapidly, returns after cleaning, affects aligned elements, accompanies movement, occurs above the public or reveals active water ingress. Cosmetic cleaning must wait until safety, fixings and waterproofing have been assessed.

Diagnostic reasoning in typical cases

Vertical streaks below a coping suggest concentrated runoff or a missing drip. Inspect the coping, joints, falls and edges under controlled water exposure and correct the detail before cleaning.

An orange halo around a fixing point suggests corrosion of a fixing or adjacent metal. Prioritise a safety inspection before attempting to remove the staining.

Dark deposits beneath sheltered areas suggest pollution not washed by rain. Identify the crust and stone before selecting a cleaning method and avoid creating over-cleaned patches.

Green or brown growth in a damp area suggests algae, moss or biofilm. Reduce moisture and shade where possible and confirm a compatible treatment on a sample panel.

White deposits around joints suggest salts and moisture migration. Trace the path from behind the stone and correct the source before cleaning the surface.

Preparing an assessment request a specialist can use

A useful assessment request describes the building, suspected stone, façade system, age, orientation, height, date of appearance and recent work or cleaning. Include exposure to rain, wind, irrigation, pollution, frost and nearby metals.

Prepare an overall elevation, medium-distance views and close-ups with a scale. Photograph joints, edges, copings, sills, drainage, fixings and unaffected comparison areas under consistent lighting.

Attach cleaning-product references, the stone data sheet, façade and fixing drawings, water-management details and the repair history. State what has already been attempted and the result.

Choosing between monitoring, testing and treatment

Approve treatment only when it addresses the confirmed cause, respects the stone and can produce an acceptable appearance across the elevation. Sample panels verify texture, colour, drying and side effects and must be assessed from normal viewing distances.

Specialist involvement is essential for work at height or above the public; marks associated with fixings, bulging, cracks or movement; historic façades, black crusts or fragile stone; and cleaning requiring containment, wastewater treatment or authorisation. The diagnosis should state its limitations and any further investigation required.