Black streaks on a Cobham roof: what causes them and how to remove them safely

What the streaks actually are

The dark vertical stains running down Cobham roofs — most obvious on north-facing pitches on Between Streets, Fairmile and the Sandy Lane estates — are not dirt, tar wash-off or shingle failure. They are colonies of gloeocapsa magma, an airborne cyanobacteria that feeds on the limestone filler baked into modern concrete and asphalt tiles.

Spores travel miles on the wind, land on a damp tile, germinate within 24 hours, and slowly work downhill under gravity as the colony grows — which is why the marks always run vertically and are heaviest under ridges and near shaded gables.

Why Cobham gets it worse than nearby towns

Three local factors combine: the Mole valley traps humidity 4–6% above the Surrey average; mature garden trees shade many roofs for most of the day; and the popular Marley Modern / Redland Regent concrete tile used on 1970s–2000s Cobham estates has a lime-rich surface that gloeocapsa loves.

Why you must not pressure wash it off

High-pressure washing strips the protective factory coating from concrete tile, exposing the porous body underneath. Within 12–18 months the algae returns worse than before, and the tile itself absorbs water — accelerating freeze-thaw damage every Surrey winter.

Every major UK tile manufacturer (Marley, Redland, Sandtoft, Russell) publishes technical bulletins warning against pressure washing pitched tile roofs. Doing so also voids most residual tile warranties.

The correct removal method

A low-pressure soft wash using a DEFRA-approved sodium-hypochlorite biocide is the only manufacturer-approved treatment. The chemistry kills the colony to the root; UK rainfall then lifts the dead residue over 2–4 weeks. There is no mechanical action on the tile face at all.

• Pre-wet surrounding planting and hardscaping
• Apply biocide at <100 psi across every affected pitch, valley and ridge
• Allow 20–30 minutes dwell time
• Light rinse and full gutter flush
• Optional zinc or copper ridge strips for long-term prevention

Typical Cobham cost

For streak-only treatment (no heavy moss), expect £295–£495 on a semi and £495–£795 on a detached KT11 home. Adding zinc ridge strips is £120–£220 extra and typically pushes recolonisation from 4 years to 8+ years.

Prevention that actually works

Zinc and copper strips fixed under the ridge tile release trace ions with every rainfall — enough to inhibit algae germination on the pitches below. Combined with a biocide top-up every 24 months this is the most cost-effective long-term regime for shaded Cobham roofs.