What this guide covers: How to clean shed and outbuilding roofs made from roofing felt, corrugated plastic and corrugated metal — including why felt roofs must only be treated with softwash, how to work safely from a ladder, and why clearing the gutters afterwards matters as much as the clean itself.
Moss and algae on a shed or outbuilding roof is one of those things that gets ignored until it becomes a problem. By the time a felt roof is visibly green and heavily contaminated, the biological growth has usually been holding moisture against the felt long enough to accelerate deterioration. Cleaning it properly and treating it extends the life of the roof. The approach differs significantly between felt, corrugated plastic and corrugated metal.
What type of roof do you have?
Roofing felt — softwash only: biocidal moss treatment by pump sprayer, long dwell time (30 minutes+), gentle rinse at lowest pressure setting. Never use standard lance pressure on roofing felt. Corrugated plastic — more tolerant of gentle pressure washing; avoid directing pressure at the laps between sheets and fixing points. Corrugated metal — most pressure-tolerant; standard fan nozzle at standard pressure; check fixing points and surface coating damage after washing.
What you need
- Pressure washer — V-TUF V3; use at lowest pressure setting for felt, medium for plastic, standard for metal
- Garden pump sprayer — for applying biocidal cleaner to felt roofs
- Biocidal roof and moss treatment
- Telescopic lance extension — for reaching across larger roofs without climbing on them
- Safety ladder, properly footed and secured — never stand on a felt or corrugated plastic roof
Key rules
Work down the slope so water and loosened contamination runs toward the eaves. Use a telescopic lance to reach across the surface safely from a ladder. Clear gutters after cleaning — moss and debris washed from the roof goes into the gutters. Apply a biocidal maintenance treatment after cleaning — a cleaned but untreated felt roof re-grows moss quickly.
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