How to Remove Moss and Algae from a Flat Roof

Moss, algae, and lichen are common on flat roofs, particularly in areas with damp climates or surrounded by trees. Left untreated, they can cause significant damage to the roofing membrane and block drains.

Why Moss is a Problem

Moss retains moisture against the roof surface, accelerating the degradation of felt and other membranes. Its roots can penetrate micro-cracks and widen them. Moss and debris also block drains, leading to ponding water.

Safe Removal Methods

• Soft wash: applying a biocide solution that kills moss and algae without high-pressure water
• Manual removal: gentle brushing to remove loose moss (avoid wire brushes on membrane roofs)
• Low-pressure rinse: once biocide has worked, a gentle wash to remove residue
• NEVER use a pressure washer on a flat roof — it can force water under flashings and damage the membrane

Biocide Treatments

Professional roof cleaning products containing sodium hypochlorite or quaternary ammonium compounds are effective against moss, algae, and lichen. Allow the treatment to work for several weeks before rinsing. Follow the manufacturer's instructions carefully as some products are harmful to plants.

Prevention

The best prevention is regular maintenance — clear gutters and drains, remove debris promptly, and apply a biocide treatment every few years. Zinc or copper strips fitted at the ridge can also inhibit moss growth (the rainwater picks up trace metals which deter growth).