Will the roof look clean straight after treatment?

If the job began with hand clearing of thick moss, the roof will look visibly better as soon as the team leaves, since the cushions along the tile edges are gone and the lines of the courses show again. The gutters will have been cleared of what fell. What will not yet have changed is the staining and the thin growth that the treatment is working on.

On a roof that was only treated, with no bulk to remove, there may be little to see on the day. Tiles may look slightly damp or darker for a short while. This is what a gentle method looks like at the start, and it does not mean the job was skipped. We take photographs before the work and after it, giving you something to measure the following weeks against, and a clear start point.

Moss usually dulls first, turning from bright green to a rusty brown or grey. Over time the brown fades as weather takes it off. Algae staining, the dark streaks you see on many roofs, tends to lighten more slowly, and it is often the last thing to go. Lichen, the crusty grey and orange patches, is the most stubborn and can take the longest to loosen.

None of this runs on a fixed clock, and rain is the helper. A damp spell clears a roof faster than a dry one. The sequence is described step by step in how long a biocide takes to work, and the staining side is covered in black streaks on roof tiles.

A treatment kills growth, but it cannot repair the surface underneath. If a tile has been stained deeply, worn by years of moss or lost some of its colour, the mark can stay after the growth has gone. Older concrete tiles in particular may look faded once their cover of moss is removed, because the moss had hidden the wear.

This is useful to know. It is not a failure of the treatment but an accurate view of the roof, and the survey photographs help you see it. A roof can be clean, healthy and still look aged. If appearance is your priority, a coating is one option, and our page on roof tile coatings sets out what it can and cannot do.

Slopes in the sun may clear before those in the shade, and thin patches before thick ones. For a while the roof can look uneven, with a clean-looking south face and a still-mottled north face. People sometimes read that as a poor job.

It is better read as the weather working at different speeds on different slopes. Give it time before you judge, and take a photograph from the same spot each month so you can see the change. If one slope looks no different after a long wait, send us the picture. We will say whether to wait or to treat again, and if it is the second, a written quote follows with a single total.

A fair test is to look from the ground, in dry daylight, after a few showers. Check that the moss has gone from the tile edges and valleys, that the gutters flow freely and that the roof has lightened overall. You are not looking for a brand-new surface, since no clean can give you that.

Also look for the things the moss was hiding. Cracked or slipped tiles sometimes show once the cover is gone, and we would tell you and show you the photograph. For the bigger picture of what the visit includes, see the roof cleaning and moss removal page. For how long the result lasts, our answer on moss returning covers it.

Memory is a poor judge of a roof, because you rarely look at yours. The most useful tool for judging the outcome is a set of photographs taken from the same place at regular intervals. Our before and after pictures give you the start and the finish of the visit. Add your own from the garden or the pavement once a month for a few months, in similar light, and the trend is clear.

Compare like with like. A bright, dry afternoon flatters a roof, and a grey, wet morning does the opposite. Look at the edges of the tiles, the valleys and the lower courses, because those are the places where growth was thickest. If after a good while you still see areas that look unchanged, send the pictures over and we will talk it through. Our roof cleaning and moss removal page explains what a visit covers if you want to check it against what you received.