UPVC and Fascia Cleaning: Why Bleach Makes It Worse

You've noticed grey or green streaks on your UPVC fascia or soffit. It looks like dirt, but it's not. It's algae and atmospheric pollution. Your instinct is probably to spray it with bleach. Stop. That's the worst thing you can do. Here's why bleach damages UPVC and what actually works.

What those green and grey streaks actually are

The streaks on your UPVC fascia aren't dirt or mould. They're a combination of:

• Algae growth: Algae thrives on damp, shaded surfaces. UPVC fascias, especially on north-facing sides, provide the perfect environment.
• Atmospheric pollution: Industrial particles, vehicle exhaust, and pollution from nearby roads settle on the surface and discolour the plastic over time.
• Lichen: In some areas, lichen establishes on UPVC, creating dark patches.

None of this is permanent damage to the material itself. It's surface contamination. But the way you remove it matters enormously.

Why bleach yellows UPVC permanently

UPVC is a plastic. Bleach is an oxidising agent. When bleach contacts UPVC, it chemically oxidises the plastic surface, breaking down the polymers. This causes:

• Yellowing: The plastic turns from white to a dull yellow or cream colour. This is irreversible.
• Brittleness: Repeated bleach exposure makes UPVC more brittle and prone to cracking.
• Hazing: The surface becomes cloudy instead of clear.

You remove the green algae, but you're left with a yellowed fascia that looks worse than it did before. And you can't fix it. You'd have to replace the fascia (£50–150 per metre) to restore the appearance.

Why domestic cleaners leave streaks

Most off-the-shelf UPVC cleaners are weak. They might loosen surface algae, but they don't penetrate the grime effectively. You end up with a partially cleaned surface that still looks streaky. Then you scrub harder, use more bleach, and the damage compounds.

Why high pressure alone doesn't work

Pressure washing UPVC fascia isn't the answer either. High pressure can:

• Crack the plastic if applied incorrectly
• Force water behind the fascia, causing internal water damage
• Loosen fixings or seals

And even if you blast it clean, you haven't killed the algae colonies. It regrows within months.

The correct UPVC and fascia cleaning process

This is what we do:

1. Low-pressure pre-treatment: Apply a specialist UPVC cleaner that's alkaline, not acidic or bleach-based. This breaks down the algae and pollution without damaging the plastic.
2. Dwell time: Let the chemical work for 10–20 minutes. You don't want to rush this.
3. Low-pressure rinse: Rinse at low pressure (under 600 PSI) to avoid forcing water into seals or damaging the plastic.
4. Optional biocide application: For added protection, apply a biocide that prevents algae regrowth for 6–12 months.

The result: genuinely clean, not damaged

This method removes algae, pollution, and lichen without damaging the UPVC. The plastic stays white, stays flexible, and stays protected. You get a clean fascia that actually stays clean because of the biocide residue.

How often should you clean UPVC fascias?

Most properties need a fascia clean every 2 years. If you live in a particularly damp area or have heavy tree coverage (which speeds up algae growth), you might need one every 18 months. North-facing fascias need more frequent attention than south-facing ones.

Signs it's time for a clean

You don't have to wait until it's heavily discoloured. Watch for:

• First appearance of green or grey streaks
• Overall dulling of the white colour
• Patches of darker discolouration that don't wash away with a hose

Cleaning it early, before heavy growth establishes, makes the job cheaper and the results faster.

What about gutters?

Gutters often have the same algae and pollution issues as fascias. We clean those at the same time using the same gentle, biocide-based approach. Gutters especially benefit from biocide treatment because it prevents moss and algae from establishing in the gutter itself, which slows blockage formation.