The short answer
Cleaning removes the moss, algae and grime that have been masking the covering, so the roof often looks several shades lighter and the original colour comes back. It does not repaint the roof. Concrete tiles that have faded, or slates with uneven weathering, show the difference more. Any patchiness evens out over a few weeks as the treatment works and the roof weathers.
What you are really seeing when the colour changes
A roof that has been streaked or mossy for years has a layer of growth and dirt over the covering. The eye gets used to that layer, and the roof comes to look dark red-brown, grey or black. When the growth is removed, the true surface appears. That is why a cleaned roof can seem to have changed colour, when what has changed is the layer on top.
The effect is strongest where the growth was thickest. A shaded north slope may have been dark green and black, while the sunny side stayed clean. After a clean, the two sides are closer, and sometimes the shaded one is briefly the lighter. This is normal, and it tends to settle as the roof dries and weathers. Our answer on why one side of a roof gets more moss explains the cause.
It is worth knowing that no cleaning method adds colour. If a tile has faded, a clean will reveal the faded tile.
Different coverings, different results
Concrete tiles get their colour from a pigmented face, and on an older roof that face may be worn or sun-bleached. The clean shows the tile as it now is, which can be paler than when it was fitted. Clay tiles are coloured right through and tend to come up well, with a warm tone returning after the growth is lifted.
Natural slate varies from slate to slate, so a cleaned roof can look more mottled than the uniform dark mass you remembered. That is the stone showing its natural range, and many owners find it handsome. Some slate also has mineral content that streaks brown if it weathers, which cleaning will not prevent. Stone slates, common on older Cotswold-style buildings, carry a patina of lichen that gives them part of their character, and our page on cleaning stone roofs explains why a gentler approach, and sometimes a partial clean, suits them.
On flat roofs, the colour change is typically less dramatic, though a cleaned membrane or a fresh stone chipping finish will look brighter for a while.
Why it can look patchy at first
Straight after a visit, some owners see blotches. The biocide is still working, and the dead growth releases its grip over the following weeks. Rain washes the loosened material off, and the surface lightens bit by bit. A roof looks its best a month or two after treatment, not the day we pack up.
Differences in exposure make the patchiness more visible. Areas under overhanging eaves get little rain and clear more slowly, so they may look streaky for longer. Areas that were bare of growth beforehand will look unchanged next to freshly cleaned slopes. If the roof still looks uneven after a couple of months, get in touch and we will come back to it.
A blasting jet, by contrast, tends to give an instant and uneven result, with clean stripes where the nozzle passed, and it can leave the covering rougher and quicker to regrow. We cover that in our answer on bleach and in our guidance on harsh methods generally.
How a cleaned roof weathers afterwards
A clean roof is not a sealed roof. From the day the growth comes off, the roof starts to weather again. Dust, pollen and airborne spores settle, and on a shaded or damp slope they will in time be colonised once more. How quickly depends on the same things that caused the growth in the first place: shade, trees, damp air and a rough surface.
A fresh clean usually gives several seasons of good looks before a light film returns, though we cannot promise a figure, because it depends on the position of the house. Cutting back overhanging branches and keeping the gutters clear helps a great deal.
Daylight matters as well. A roof seen in low winter sun looks darker and flatter than the same roof in bright summer light, and wet tiles always look deeper in tone than dry ones. Judge the finished colour on a dry day, at the same time of day as the photographs you took before.
Some owners ask about a coating to hold the colour. It is a separate decision with its own pros and cons, including the effect on a mortgage or a survey, and we set that out on our page about roof coatings.
Setting expectations before the visit
Neighbours can help your own judgement. If the houses either side have similar tiles and have not been cleaned, look at their roofs: that is roughly what yours looks like to passers-by now, and the contrast after a clean is worth bearing in mind.
At the survey we photograph your roof so that you can see its condition before any work starts, and so we can compare afterwards. If the covering is faded, worn or patched with mismatched tiles, we tell you that a clean will show it clearly. In those cases a repair, or a recover, may do more for the look of the house than a clean alone, and our roof cleaning cost page helps you weigh it up.
Our roof cleaning service starts with a free survey, and the quote that follows is a fixed price in writing. If you can find an old photograph of the house, bring it out, since it shows the colour the tiles had before moss and lichen took hold. You can book by phone on 01225 285181, by WhatsApp or through the form.
