Clean outside tenement building

This sounds like organic growth caused by prolonged water run-off rather than just surface dirt. The best approach is soft washing, not high-pressure washing, as pressure can damage masonry and mortar on tenement buildings. A biocide / fungicidal wash (such as a benzalkonium-chloride–based treatment) should be applied to kill mould, algae, and spores at the root. After allowing the solution to dwell, the surface can be gently rinsed. Any remaining staining may lighten over time once the biocide has fully worked. Preventing the issue long-term depends on fixing the guttering so water isn’t continually running down the walls. Avoid bleach or aggressive chemicals, as these can cause patchy results and damage the surface.

Hi Andrew, If going green then it’s algae, black will be mould. best 2 options: 1. instant results - Sodium Hypochlorite with a low pressure Wash 2. Results over time - Biocide Wash for effective result from 3 - 10 months Cheers

Best way to clean mould grouth on outside of a masonry building, is to use a jet wash spray/powerwash, results straight away 💯

This is quite common with leaking guttering. Once it’s replaced, the staining and mould can usually be cleaned with a mild exterior wall cleaner or fungicidal wash and a soft brush. Avoid strong chemicals or pressure washing, especially on older tenement walls. The staining should also fade over time once the wall dries out. Hope that helps.

Removing water stains and organic growth like mold or algae from a tenement building requires a careful approach, especially since these buildings are often constructed from porous sandstone or traditional masonry. ​Here is a breakdown of the best methods to clean the exterior without damaging the stone. 1. Safety First: The Golden Rule ​Before applying any cleaner, ensure the guttering is fully repaired. If water continues to leak onto the stone, any cleaning you do will be undone within weeks, and the moisture trapped in the masonry can lead to structural damage or dampness inside. ​2. Recommended Cleaning Methods ​A. Soft Washing (Highly Recommended) ​Unlike pressure washing, soft washing uses low-pressure water combined with specialized biological cleaners. This is the safest method for old tenement stone. ​The Cleaner: Use a biocide (like Patio Magic or professional-grade Benz Bio Clean). These are designed to kill mold, algae, and lichens at the root. ​The Process: Apply the solution, let it sit (often for several days), and the weather will naturally wash away the dead growth. It prevents regrowth much longer than scrubbing does. B. Steam Cleaning (Doff/Torc Systems) ​If the staining is heavy or includes carbon/soot from city pollution, professional steam cleaning is the gold standard. ​How it works: It uses high-temperature steam (150^{\circ}C) at very low pressure. ​Why it’s good: It kills spores and lifts staining without the "sandblasting" effect that can strip the protective "face" off the stone. ​C. The Manual Approach (DIY) ​If you are cleaning a small, reachable area yourself: ​Water and a Soft Brush: Start with just plain water and a natural bristle brush. Avoid wire brushes as they leave metal particles that rust and stain the stone. ​Non-Ionic Detergent: If water isn't enough, use a pH-neutral, non-ionic detergent (like vulpex or even a very dilute, mild soap). ​Rinse Thoroughly: Always rinse from the top down. ​3. What to Avoid ​High-Pressure Washers: Never use a standard "Karcher" style power washer on high settings. It can blast away the mortar joints and scar the surface of the stone, making it more porous and prone to future mold. ​Bleach: While it kills mold, household bleach (sodium hypochlorite) can leave salts behind in the stone that cause "efflorescence" (white powdery staining) and can lead to the stone crumbling over time. ​Acidic Cleaners: These can react with the minerals in sandstone or limestone, causing permanent discoloration.

A softwash by applying biocide treatment on the surface, followed by agitating with a brush and then finally a rinse down. Not the typical stuff that is available from the likes of B&Q, the biocide chemicals are the ones available to specialist cleaning contractors.