It is very common for homeowners to look up at their roof, see unsightly moss, lichen, or dark streaks, and assume a quick blast with a power washer is the fastest way to clean it.
While power washing works great for concrete driveways and stone patios, using high-pressure water on roof tiles or slate causes severe, irreversible damage.
The 4 Main Reasons You Should Never Power Wash a Roof
1. It Destroys the Protective Coating on Roof Tiles
Concrete and clay roof tiles are manufactured with a granular, protective surface coating designed to shed water and resist weather. High-pressure water blasts this granular layer off, instantly degrading the lifespan of the tile and leaving it porous.
2. It Forces Water Upward Under the Tiles
Roof tiles are designed to overlap so rain glides downward with gravity. High-pressure power washers blast water upward at an angle. This forces high-volume water under the tiles, soaking the underlayment felt, saturating the timber roof battens, and leading to ceiling leaks and timber rot inside your home.
3. It Causes Severe Moss Regrowth
Power washing knocks off the visible moss clumps, but the high pressure leaves behind microscopic spores deep inside the porous tile. Furthermore, because power washing roughs up the smooth surface of the tile, it creates tiny pits and grooves. This gives future moss spores an even easier surface to anchor to, causing moss to return twice as thick within 12 to 18 months.
4. It Voids Roof Guarantees
Most major roof tile manufacturers explicitly state in their warranty terms that high-pressure power washing voids the product warranty.
What Is the Correct Way to Clean a Roof?
Instead of high-pressure washing, roofs should always be cleaned using a two-step low-pressure soft wash system:
1.Manual Moss Scraping:No high pressure required.
Heavy moss clumps are carefully scraped off the roof using custom-shaped blades that match the contour of your specific roof tiles, keeping tile surfaces intact.
2.Low-Pressure Bio-Cleanser Application:Kill growth at the root.
A specialized, breathable biocide treatment is sprayed onto the roof at garden-hose pressure. This solution penetrates deep into the tile pores, killing all remaining moss, lichen, and black spot spores at the root.
3.Self-Cleaning Action:Long-term self-cleaning.
Over the following weeks, natural rainfall gently washes away dead organic residue, leaving the roof completely clean without damaging a single tile or driving water into your attic.
Key Takeaway for Homeowners
Cleaning a roof is not about force; it is about using the right treatment to kill organic growth at the root. A low-pressure scrape and soft treatment protects your roof tiles, prevents attic leaks, and keeps moss from coming back for years.