Removing Moss and Lichen: Why You Should Never Pressure Wash Old Tiles

You look up at your roof. It looks like a green carpet. Big clumps of moss are rolling into the gutters, and crusty yellow lichen is spreading across the tiles. You have a high-powered pressure washer in the garage. It seems like the perfect solution: climb up, blast the dirt off, and make the roof look new in an afternoon.

This is the single worst thing you can do to your home.

While pressure washing gives you an instant "wow" factor, it causes invisible, irreversible damage to the tiles. You are trading a clean roof today for a leaking roof next winter. Professional roof cleaners have moved away from jet washing for a reason. Here is why you should too, and how to clean your roof safely using the "Scrape and Spray" method.

The Surface Damage: Stripping the Skin

Roof tiles are not solid blocks of stone. They are engineered components with protective coatings.

• Concrete/Granular Tiles: These have a sandy, granular finish designed to protect the concrete core from UV light and acid rain. A high-pressure jet (3000 PSI) blasts this sand off instantly, leaving the tile "bald." This exposes the porous concrete, which then soaks up more water, attracts more moss, and cracks in the frost.

• Slate: Slate is formed in layers. High pressure forces water between these layers, causing the slate to delaminate (flake apart).

• Clay: Old clay tiles have a "fire skin" from the kiln. If you blast this off, the tile becomes soft and crumbly.

2. The Structural Risk: The Upward Spray

Roofs are designed to handle water falling down from the sky. They rely on gravity and overlaps to keep water out. They are not designed to handle water travelling upwards at 100mph.

When you stand on a ladder and spray up the roof:

1. You drive water under the tile laps.

2. You saturate the timber battens (causing rot).

3. You soak the loft insulation (destroying its thermal value).

4. You can even blow the tiles clean off the roof if they aren't nailed down (which many older tiles aren't).

The Scrape and Spray Method

If you can't blast it, how do you clean it? You use a combination of Manual Removal and Chemistry. This is known as Soft Washing.

Step 1: The Dry Scrape

You must remove the heavy moss manually.

• The Tool: Use a Roof Scraper with a specialized head profile. You can buy blades shaped exactly like your tiles (Marley, Redland, Roman, etc.).

• The Method: Gently scrape the moss loose. It falls down the roof into the gutter (make sure you block the downpipe first!).

• The Benefit: This removes 90% of the biomass without using a single drop of water or pressure.

Step 2: The Chemical Treat (Biocide)

Once the big lumps are gone, the tiles will still look dirty and may have lichen spots (black/white/yellow crusts). You need a Biocide (usually Benzalkonium Chloride or DDAC).

• Application: Spray the liquid onto the dry tiles using a low-pressure garden pump sprayer.

• The Kill: The biocide soaks into the pores and kills the tiny spores and lichen roots that the scraper missed.

• The Wait: This is not an instant fix. You spray it and leave it. Over the next 3–6 months, the weather will wash the dead decay off the roof. The roof will get cleaner every time it rains.

The Self-Cleaning Effect

The beauty of a biocide treatment is that it acts as a preventative shield.

• Because you haven't stripped the surface of the tile (like pressure washing does), the tile stays waterproof.

• The chemical residue stays active for 12–24 months, meaning moss cannot grow back.

• A pressure-washed roof usually turns green again in 12 months. A soft-washed roof stays clean for 3–5 years.

A Note on Safety

Falls from roofs are a major cause of serious injury.

• Never walk on old tiles: They are brittle and will snap under your weight.

• Work from the gutter line: Use a scaffold tower or a secure ladder.

• Use extending poles: Telescopic scraping poles allow you to reach the ridge from the safety of the eaves.

Conclusion

Pressure washing is a harsh, destructive "short cut." Soft washing is a gentle, long-term cure. Don't age your roof by 20 years just to get it clean for the weekend.

• Scrape the moss dry

• Soak with Biocide

• Let the weather finish the job