Is the black on my gutters mould?

Usually not. It is mostly dirt carried by water spilling over the lip, sometimes with algae mixed in on shaded runs.

Do you clean the outside of gutters as well as clearing them?

Yes. We clear the inside first so the result lasts, then clean the outside face, fascia and soffit in the same visit.

Will the gutters look new afterwards?

The dirt and stripes come off. Plastic that has yellowed from sunlight is ageing of the material, which cleaning cannot reverse.

Yes, before and after photographs of every section we clear.

What Causes Black Streaks on Gutters?

White gutters rarely stay white for long. First comes a grey tinge, then neat black stripes running down the outside face, often lined up with each other a few inches apart. People tend to assume it is mould or that the plastic is failing. In most cases it is neither. Those stripes are a record of water going the wrong way, and they tell you something about what is happening inside the gutter as well as outside it.

Water coming over the front lip

When a gutter is partly full of silt, moss and leaves, rain cannot run along it freely. The water level rises and spills over the front edge, carrying fine dirt with it. Every time that happens the same drips follow the same path down the face, and the dirt they leave builds into a stripe. The regular spacing people notice is usually the ribs or high points on the gutter lip, where water finds it easiest to escape. So the streaks are often the first visible sign that a gutter is not flowing as it should.

This guide sits under our Gutter Cleaning & Clearing work, covering 20-mile radius of Plymouth.

Airborne dirt and the roof above

Even a clear gutter collects airborne dirt, traffic film and pollen on its face, and rain washing off the roof edge concentrates it. On the fascia behind, black streaking is airborne dirt washed down and concentrated by run-off from the gutter lip. Houses near busy roads or under trees streak faster because there is more to wash down. The surface of older uPVC also becomes slightly porous with age and sunlight, which gives that grime somewhere to cling.

Why wiping them does not last

You can scrub a stripe off from a ladder, but if the gutter behind it is still full, the next heavy shower brings dirty water over the lip and draws the stripe straight back. Cleaning fascias without clearing the gutter behind them is a short-lived result. That is why we treat the inside and outside as one job: clear the run with the gutter vacuum, check it with the camera so we know it now flows, then clean the outside face, fascia and soffit while the access is set up.

How the outside face is cleaned

The gutter exterior, the fascia and the soffit should be done as one item, and it is far more efficient than booking them separately. We use low pressure and a suitable cleaner on uPVC rather than force, because a jet can drive water into the soffit and does nothing that a gentle clean will not. Green algae in the shaded soffit and on the north side is treated at the same time. Expect the stripes to come off; do not expect heavily sun-yellowed plastic to return to showroom white, because yellowing on south-facing uPVC is degradation of the plastic itself and is not dirt at all.

When the streaks are telling you more

If stripes appear at one point only, rather than evenly along the run, look for a sag between brackets or a joint that is dripping. Water pooling in a low spot overflows in the same place every storm. We flag loose brackets and fascia issues we see during the clear; replacing rotten fascia board is a builder's job rather than ours, but knowing about it early saves a lot of damage to the wall below.

Keeping them away

A yearly gutter clear, or twice a year under heavy tree cover, removes the main cause. Cleaning the outside faces every one to two years alongside that clear keeps them consistently white. Left for many years, streaking becomes ingrained and never comes up quite as well.

Reader note: What causes black streaks on gutters?

Owner with white uPVC gutters marked by black vertical stripes wanting to know why.

Water coming over the front lip: When a gutter is partly full of silt, moss and leaves, rain cannot run along it freely. The water level rises and spills over the front edge, carrying fine dirt with it. Every time that happens the same drips follow the same path down the face, and the dirt they leave builds into a stripe. The regular spacing people notice is usually the ribs or high points on the gutter lip, where water finds it easiest to escape. So the streaks are often the first visible sign that a gutter is not flowing as it should. Airborne dirt and the roof above: Even a clear gutter collects airborne dirt, traffic film and pollen on its face, and rain washing off the roof edge concentrates it. On the fascia behind, black streaking is airborne dirt washed down and concentrated by run-off from the gutter lip. Houses near busy roads or under trees streak faster because there is more to wash down. The surface of older uPVC also becomes slightly porous with age and sunlight, which gives that grime somewhere to cling. Why wiping them does not last: You can scrub a stripe off from a ladder, but if the gutter behind it is still full, the next heavy shower brings dirty water over the lip and draws the stripe straight back. Cleaning fascias without clearing the gutter behind them is a short-lived result. That is why we treat the inside and outside as one job: clear the run with the gutter vacuum, check it with the camera so we know it now flows, then clean the outside face, fascia and soffit while the access is set up. How the outside face is cleaned: The gutter exterior, the fascia and the soffit should be done as one item, and it is far more efficient than booking them separately. We use low pressure and a suitable cleaner on uPVC rather than force, because a jet can drive water into the soffit and does nothing that a gentle clean will not. Green algae in the shaded soffit and on the north side is treated at the same time. Expect the stripes to come off; do not expect heavily sun-yellowed plastic to return to showroom white, because yellowing on south-facing uPVC is degradation of the plastic itself and is not dirt at all. When the streaks are telling you more: If stripes appear at one point only, rather than evenly along the run, look for a sag between brackets or a joint that is dripping. Water pooling in a low spot overflows in the same place every storm. We flag loose brackets and fascia issues we see during the clear; replacing rotten fascia board is a builder's job rather than ours, but knowing about it early saves a lot of damage to the wall below. Keeping them away: A yearly gutter clear, or twice a year under heavy tree cover, removes the main cause. Cleaning the outside faces every one to two years alongside that clear keeps them consistently white. Left for many years, streaking becomes ingrained and never comes up quite as well.

Questions we get on this

Is the black on my gutters mould?: Usually not. It is mostly dirt carried by water spilling over the lip, sometimes with algae mixed in on shaded runs.
Do you clean the outside of gutters as well as clearing them?: Yes. We clear the inside first so the result lasts, then clean the outside face, fascia and soffit in the same visit.
Will the gutters look new afterwards?: The dirt and stripes come off. Plastic that has yellowed from sunlight is ageing of the material, which cleaning cannot reverse.
Do you send photos?: Yes, before and after photographs of every section we clear.

If this describes your situation

Send a photo of the streaked run and we will quote clearing the inside and cleaning the outside together.

Send photos for a quote Gutter Cleaning & Clearing Call 07830 242403

What causes black streaks on gutters?