Why Do Gutters Get Black Streaks on the Outside, and How Do You Clean Them Without Damage?

Stand across the street from almost any Melbourne house that has been through a wet winter and look at the gutters. On a good number of them you will see dark vertical lines running down the outside face, sometimes faint, sometimes dense enough to look like a pattern. They are most obvious on light-coloured gutters, where they can make an otherwise clean house look neglected. And once people notice them they tend to try the hose, find it does nothing, and start wondering what they are.

Spring is when they are most visible and when they are worth dealing with, before the house is washed, painted or put on the market, and while the gutters are being cleared of whatever winter left in them.

What the stripes actually are

The stripes are not mould and they are not the gutter itself discolouring. They are a thin film of grime: fine soot and exhaust particles from traffic and wood heaters, general airborne dust, pollen, and the residue of moss and lichen from the roof. All of that settles on the roof between rains. When it rains, the water carries it down the tiles or sheeting and into the gutter.

In a gutter that is working, that dirty water goes down the downpipe. The stripes form when some of it goes over the front edge instead, and runs down the outside face in the same lines every time. As the water dries in the sun, it leaves its load of grime behind on the paint, and each rain adds another layer on top of the last.

It is worth ruling out two look-alikes before cleaning. Green or black growth that is fuzzy or blotchy rather than streaked, usually on the shaded side of the house, is organic growth such as algae or lichen, and it needs treating rather than just washing. Orange or brown runs, especially from joints, corners and fixing points, are rust or tannin staining, which can mean the metal underneath is starting to go. Crisp, dark, vertical lines that start at the front lip and fade towards the bottom edge are the grime film this post is about.

Why they get worse after winter

The stripes are a map of where water has been going over the edge of the gutter rather than into the downpipe, so anything that makes a gutter overflow makes them worse. Winter in Melbourne is when gutters fill with leaves, bark, seed pods and grit washed off the roof, and a gutter half full of wet debris cannot carry a heavy shower. Water backs up and spills over the front lip, taking the dirtiest of the run-off with it.

Sections of gutter that have sagged, lost their fall towards the downpipe, or sit below a roof valley that dumps a lot of water in one place are the usual hot spots. That is why the stripes are rarely even along a house. They cluster where the gutter is most often full.

Overhanging trees add to it in two ways: they fill the gutter faster, and the sap, pollen and leaf residue they drop darken the run-off. Houses on busy roads, or in streets where a lot of wood heaters run through winter, often show heavier striping for the same reason: there is simply more soot on the roof for the rain to carry.

Why a hose will not shift them

A fresh film washes off. An old one does not. Once the grime has dried onto the paint in the sun, repeatedly, over months, it holds on far more firmly than dust sitting on a surface. Running water from a garden hose will rinse loose dirt from around the stripes and leave the stripes themselves looking exactly as they did. Scrubbing harder is the usual next step, and that is where most of the damage to gutters comes from.

What damages a gutter face

Most modern Melbourne gutters are pre-painted steel, and the finish on them is what protects the metal underneath and gives the house its colour. It is tougher than house paint but it is not indestructible, and three common approaches harm it.

A pressure washer close enough to shift a bonded film is close enough to strip, etch or dull the coating, and once the finish is compromised the gutter picks up dirt faster and weathers sooner. Abrasive pads, scourers and stiff wire brushes remove the stripes by scratching the surface, leaving it dull and patchy in sunlight. And strong solvents, degreasers not meant for painted metal, and some household chemicals can soften or discolour the finish, or run down onto the paintwork and garden below.

There is also the ladder. Gutter faces are cleaned from height, often reaching sideways, and anything that involves force or kickback makes that more dangerous than it needs to be.

How it is done properly

The approach that works is chemistry and patience rather than force. The gutter is cleared first, so there is no debris to wash over the face mid-job. A cleaner suitable for painted metal is applied to the face and given time to loosen the film, then worked with a soft brush and rinsed at low pressure, section by section, from one end of the gutter to the other. The run-off is managed so it does not streak the walls, windows and fascia below, and surrounding garden beds are protected.

On a house that is also being washed, the gutters are done before the walls, because the rinse from the gutters runs down over everything underneath. Heavily marked sections sometimes need a second application rather than more scrubbing.

Stopping them coming back

Cleaning removes the stripes, but only the gutter decides whether they return. A gutter that is kept clear and drains freely carries the run-off down the downpipe, and the face stays clean far longer. Keeping gutters clear before the heavy rain seasons, checking that downpipes are open all the way to the drain, and having sagging sections re-hung so they fall towards the outlet all help.

The roof matters too. A roof carrying moss, lichen and a build-up of grime sends more of it into the gutter with every rain, so a roof that is kept clean produces fainter stripes, more slowly. For a house going on the market or about to be painted, doing the roof, gutters and walls together in the right order, top to bottom, gives the cleanest result and means each surface is only washed once.

Frequently asked questions

Can I use a pressure washer to get black streaks off gutters?

It is not a good idea. A pressure washer close enough to shift a bonded film is also close enough to strip or etch the painted finish on the gutter, and once the coating is damaged the gutter marks and weathers faster than before. It also tends to drive water up under the tiles and into the eaves, and it is being used from a ladder, which is a poor combination with a tool that kicks back. A soft brush with a suitable cleaner and a low-pressure rinse does the job without the risk.

Are the black streaks a sign that something is wrong with the gutter?

Often, yes. The stripes form where dirty water runs over the outside edge of the gutter, and in a gutter that is working properly the water goes the other way, down the downpipe. Heavy striping, or striping concentrated in one section, usually means that part of the gutter is blocked, overflowing, sagging or falling the wrong way. It is worth having the gutter cleared and its fall checked, not just the face cleaned.

Will the black streaks damage the gutter if I leave them?

The film itself is mainly a cosmetic problem. What it points to is not: a gutter that overflows regularly is sending water down the fascia, into the eaves and against the walls and footings, and the debris that causes the overflow holds moisture against the gutter itself. So the stripes are best treated as a symptom. Clean the face for the look of the house, and clear the gutter for the house itself.

Do black streaks on gutters mean the roof needs cleaning too?

Not necessarily, but they are related. The grime in the stripes starts on the roof, and a roof carrying a lot of moss, lichen and dirt sends more of it into the gutter every time it rains. If the roof is visibly green or dark, or the gutters fill with grit and moss rather than leaves, cleaning the roof and gutters together tends to slow the stripes down more than doing either one alone.