Gloeocapsa Magma Taxonomy

The Black Streak Mystery

The dark streaks and dull black film that spread down a roof slope are almost universally taken for soot, traffic grime or the tile simply ageing. They are none of those things. They are alive. What you are looking at is a colony of cyanobacteria, single-celled organisms that photosynthesise like plants but are built like bacteria, and that have been colonising rock surfaces for something over two billion years.

Gloeocapsa is the best-known genus, and on British roofs it is generally found in mixed company with other cyanobacteria and green algae. The streaks run downhill because the colony follows the water: each rain washes cells from the top of the slope and seeds them further down.

A Prehistoric Survivor

Cyanobacteria were the first organisms to put oxygen into Earth's atmosphere, and they did it while living on bare, sun-scorched rock with no soil, no shade and no reliable water. A roof tile is an easy habitat by comparison. Concrete tiles offer a rough, mineral-rich, slightly alkaline surface; weathered clay is not far behind; and both hold the thin film of moisture after rain that a colony needs in order to grow.

The organism draws minerals from the tile surface itself, and the microscopic weathering that results is one reason a colonised concrete tile loses its finish faster than a clean one beside it.

The Natural Sunscreen

The cells are not black. They are surrounded by a gelatinous sheath, and it is the sheath that carries the pigment: a compound called scytonemin, which the organism manufactures specifically to absorb ultraviolet light. The more sun the colony receives, the darker it becomes. A black roof streak is, quite literally, sunburn protection, and it is why the growth is often heaviest on the slopes that receive the most light rather than the least.

The same sheath also holds water, which lets the colony survive weeks of dry weather that would kill an unprotected cell within hours.

Thermal Inefficiency

A dark surface absorbs more solar energy than a light one. On a pale concrete or buff clay roof, a heavy cyanobacterial film raises the surface temperature of the tiles measurably in direct sun. In the British climate the consequence is not an overworked air-conditioner but something slower: a wider daily temperature swing across the tile, more thermal movement, and more stress on the tile and on the mortar at ridges and verges over time.

It is a modest effect on its own. The thermal argument for treating a roof is real but secondary; the ecological argument in the next section is the one that matters.

The Precursor to Moss

Cyanobacteria are pioneers. They are the first living layer on a bare tile, and as the colony grows, dies back and regrows it leaves behind an accumulating film of organic matter that holds moisture and traps wind-blown dust. That film is soil, in miniature. Algae arrive next. Then moss, which needs the moisture the film retains. Then, given years, lichen.

This is ecological succession happening on your house, and the black streak is stage one. Every heavily mossed roof we treat was a streaked roof first. Treating the streaks is the cheapest point at which to interrupt the sequence.

Targeted Cellular Lysis

Roofs are not pressure washed. Water driven up under the tile laps ends up in the roof space, and the tile surface does not survive the jet. Instead the roof is soft washed: an approved biocide is applied at low pressure from safe access, allowed to dwell, and the colony is killed in place. The pigmented sheath breaks down over the following weeks and rain does the rinsing.

The roof will not look transformed on the day. It will look transformed a month later, which is how you can tell the organism has actually been killed rather than merely knocked off.

How to Identify It

Look for dark grey to black vertical streaks running down the roof from the ridge, or from beneath chimneys, vents and dormers. They start faint and patchy and gradually join into broad bands. When wet the surface looks dull and slightly slimy; when dry it looks crusty. Check it from the ground with binoculars, never from the roof itself.

What It Is Not

Soot is more even and concentrated around chimneys. Moss is three-dimensional, with cushions you can see in profile. Lichen forms separate round crusts, usually grey, yellow or orange. Tiles that are simply old are uniformly dull rather than streaked.

One giveaway: a clean stripe running down beneath old lead flashing or a metal ridge fitting. Metal washing down the roof inhibits the growth, which confirms that what you are looking at is alive.

Where to Look on Your Property

Concrete tile roofs, pale clay tiles, flat roofs and the tops of garden walls. Also check conservatory roof bars, porch canopies, bay roofs and dormer cheeks, which are easier to see from the ground and often colonise first. Gutters below a streaked roof usually hold dark sludge from the same colony.

What You Can Safely Do Yourself

Honestly, very little on the roof itself. Don't climb onto it, and don't use a pressure washer from a ladder. What you can do is keep the gutters clear, cut back branches that overhang the roof and drop debris onto it, and photograph each slope once a year from the same spot, so you can see whether the growth is spreading.

When to Call a Professional

When streaks cover a large part of a slope, when moss has started to appear within them, or before you sell, because a streaked roof reads as a neglected one. A professional soft wash from safe access kills the colony without the risks of walking on tiles. If you can also see cracked or slipped tiles, that is a roofer's job first and a clean second.

How to Keep It Away

Reduce the time the roof stays wet: trim overhanging trees and keep the gutters flowing. Some owners fit zinc or copper strips along the ridge. Rain washes metal ions down the slope and inhibits growth, although the effect weakens further from the strip and it will not remove growth that is already there. On shaded or rural roofs, retreatment every few years is typical.