How to Get Rid of Black Streaks on Fascias

The quick answer: The black streaks on your fascias are "tide-lines" of rainwater and algae dripping off the bottom lip of your gutter and running down the boards. They need a dilute biocide / soft-wash to kill and lift the growth — not scrubbing, and definitely not a pressure washer, which damages uPVC. Done properly from the ground, the boards come back to near-white. To get yours sorted, call 01737 652 515.

If your house otherwise looks tidy but those dark drip marks down the fascias make it look neglected, you're not alone — it's the single most common complaint we hear. This guide explains what actually causes them, why the obvious tool (a pressure washer) is the wrong one, the method we use, and what you can safely tackle yourself. For the full service, see our fascia, soffit & cladding cleaning page.

What Actually Causes the Black Drip Streaks?

Rainwater landing on your roof picks up dirt, pollution and microscopic algae spores as it runs down the tiles. When it reaches the gutter, some of it overshoots or clings to the underside of the front lip, then drips off and runs straight down the face of the fascia. Each drip leaves a thin trace behind. Repeat that over a wet British year or two and you get the tell-tale vertical black streaks — "tide-lines" — that follow the line of the gutter.

Three things make it worse in Surrey:

• North-facing damp: Boards that never get much sun stay wet longer, so algae thrives.
• Overhanging trees: Oak, sycamore and lime along Surrey roads drop debris and shade the roofline, feeding the green growth.
• Traffic film: Roadside and Gatwick/M23 traffic lays a fine grey film over everything, which the algae grabs onto.

Green algae on soffits is the same story from the underside — trapped moisture and shade. None of it means your boards are failing; it's surface growth sitting on otherwise sound uPVC.

Why a Pressure Washer Is the Wrong Tool

This is the mistake we get called out to fix most. uPVC fascias and soffits should never be high-pressure washed^[1]. Here's what goes wrong:

• The jet can crack or distort thin uPVC panels, especially older, sun-brittled ones.
• It breaches the seals and joints between boards.
• Worst of all, it forces water up behind the fascia and soffit, into the roof void — where it has no business being and can sit against your roof timbers.

Blasting at the streaks also tends to just spread the algae around rather than killing it, so it comes straight back. If a quote you've been given mentions a pressure washer on uPVC, treat it as a red flag. The same caution applies to render and cladding — our render cleaning guide covers the soft-wash principle in more depth.

The Method That Actually Works

The professional approach is a soft-wash, done from the ground:

1. A dilute biocide / cleaning solution is applied low-pressure to the fascias, soffits and gutter exterior. The biocide kills and lifts the algae at the root rather than smearing it.
2. It's given a little dwell time to break down the staining.
3. A water-fed pole with a soft brush head agitates the surface and rinses with purified (deionised) water, so the boards dry spot-free and streak-free with no detergent residue^[2].

Because it's all done from the ground with a telescopic pole, there are no ladders leaning on your guttering — which is itself a common cause of gutter and fascia damage — and no scaffold cost on a normal 2-3 storey house. We're a Redhill 2-man crew and we'll always tell you straight if an awkward, very high roofline needs extra access before we start, never after.

Careful DIY for Low-Level Boards

If you've got low boards you can reach with both feet flat on the ground — a porch, a single-storey extension, a bungalow — you can safely have a go yourself. Do it the right way:

• Use a dedicated uPVC cleaner or warm, mild soapy water and a soft cloth or sponge.
• Don't use abrasive pads, scouring cream or wire wool — they scratch and permanently dull uPVC^[3].
• Don't use neat bleach or harsh solvents — they discolour the surface and degrade the rubber seals over time.
• Work gently, rinse well, and dry off watermarks with a microfibre cloth.

Where DIY runs out is the moment you'd need a ladder. Anything at roofline means working at height, and a ladder against a plastic gutter is a recipe for a fall or a cracked gutter. That's the point to call a pro.

How to Stop Them Coming Back

You can't stop algae forever in a wet climate, but you can keep on top of it:

• Keep gutters clear. Overflowing gutters dump water straight down the fascia, accelerating the streaking. A yearly gutter clear is the single best prevention.
• Clean the roofline about once a year — twice if you're near heavy tree cover — the same schedule as your gutters. Our guide on how often to clean fascias, soffits & gutters has the full schedule.
• Add an optional anti-green biocide treatment. Applied after cleaning, it slows algae regrowth and re-activates each time it rains, keeping the boards cleaner for longer between visits.

When to Call a Pro vs DIY

Call a pro when: the boards are at roofline, you'd need a ladder, the staining is heavy and ground-in, or you also want the gutters and soffits done in one go. DIY only when: the boards are within safe ground reach and lightly marked. There's no shame in the former — working at height is the genuine risk here, and a proper soft-wash gets a better, longer-lasting result than scrubbing ever will.

Frequently Asked Questions

What causes black streaks on fascias?

Rainwater carrying algae and dirt drips off the bottom lip of the gutter and runs down the fascia, leaving dark "tide-line" streaks that build up over a year or two. North-facing damp, overhanging trees and traffic film all make it worse.

Can I pressure wash black streaks off uPVC fascias?

No. uPVC should never be high-pressure washed — it can crack panels, breach seals and force water behind the boards into the roof void. The correct method is a low-pressure soft-wash with a dilute biocide and a pure-water rinse.

How do professionals remove black streaks from fascias?

With a water-fed pole and a dilute soft-wash/biocide that kills and lifts the algae, then a purified-water rinse that leaves the boards spot-free and near-white — all done from the ground with no ladders on your gutters.

Can I clean fascia black streaks myself?

You can safely clean low boards within ground reach using a dedicated uPVC cleaner or mild soapy water and a soft cloth. Avoid abrasive pads and neat bleach. Anything at roofline is a working-at-height job for a pro.

How do I stop black streaks coming back?

Keep gutters clear, clean the roofline about once a year (twice near trees), and ask for an optional anti-green biocide treatment that slows regrowth and re-activates with rain.

Get Your Free Quote

We provide free, no-obligation quotes for fascia, soffit and gutter cleaning across Surrey. Local Redhill 2-man crew, fixed price up front. If it's not right, we redo it free.

We serve Redhill, Reigate, Crawley, Horley, Dorking, Banstead and all areas within a 20-mile radius of RH1. Get My Free Quote See the service

Related guides: Fascia & Soffit Cleaning Cost UK | How Often to Clean Fascias, Soffits & Gutters | Gutter Cleaning Guide | Render Cleaning Guide | Pressure Washing Myths Debunked

Sources

Technique and safety points in this guide are drawn from published uPVC-cleaning and soft-wash references, not invented claims.

1. Pureseal Services UK — Think Twice Before Pressure Washing Your Soffits and Fascia. Why high pressure damages uPVC and forces water behind boards. puresealservices.co.uk — pressure washing fascias. Accessed 16 June 2026.
2. Cardiff Window Cleaner Ltd — How to Clean uPVC Fascia Boards. Water-fed-pole and pure-water method for a spot-free finish. cardiffwindowcleanerltd.co.uk — clean uPVC fascias. Accessed 16 June 2026.
3. Calder Windows — Cleaning Soffits and Fascias: How to Do It and How Often. Avoid abrasive pads and harsh chemicals on uPVC. calderwindows.co.uk — cleaning soffits and fascias. Accessed 16 June 2026.