The short answer
Zinc strips can discourage moss and algae on the slope beneath them, because rain washes a trace of zinc down over the tiles. They do not remove existing growth, they work only on the area below the strip, and they do little on shaded or low-pitched roofs. They work best as an addition to a clean roof, not a replacement for cleaning.
How a zinc strip is meant to work
The idea is simple. A narrow strip of zinc is fixed just under the ridge, or sometimes across the slope further down, with a short length showing above the tile or slate it sits on. When rain falls on the metal, it carries tiny amounts of dissolved zinc down over the covering below. Zinc is unfriendly to moss, algae and lichen, so the surface it runs over is harder for them to colonise.
Nothing is sprayed, nothing is applied by hand once it is fitted, and the strip slowly wears away over time, which is why it does not stay effective forever. The method is passive: the strip does its work only when it rains, and only along the path the water takes. It is sold as a long-term helper, not a cure, and it is best judged by what it does on a roof that has already been cleaned.
What zinc strips do not do
They do not clear moss that has already taken hold. A thick mat on the slope stays where it is, and a strip fitted above it will not lift it off. The roof has to be cleaned first, using a gentle method chosen for its covering, before a strip has anything to protect.
They only affect the area the run-off crosses. The slope below a strip gets some benefit, but the lower courses may get little, and anything to the side or sheltered from the flow gets none. A roof with valleys, dormers or chimneys breaks the water into several paths, so a single strip rarely covers it all. Our answer on why moss is worse on the north side of a roof explains why shaded slopes need more than a strip can give. They also do nothing for gutters, where moss falls and settles.
The strip does not repair, seal or strengthen the covering. If tiles are slipped or cracked, those faults remain.
Which roofs suit them, and which do not
Strips work best on a straightforward pitched roof with a clear run from ridge to eaves, such as a plain gable on a detached or semi-detached house. A reasonable pitch helps, since water runs off in a steady sheet and carries the zinc with it. Concrete and clay tiles are the usual candidates, and slate can take them too, though slate roofs call for care over how the strip is fixed.
They suit less well where the slope is short, heavily shaded or interrupted by solar panels, which block the run-off beneath them. Flat and low-pitch roofs, where water lies and does not run, get little benefit, and our page on flat roof cleaning covers what suits those surfaces. Listed buildings and conservation areas may have rules about visible metal, so check with the council's planning team before fixing anything to a protected roof.
Fitting them without causing leaks
A strip has to be fixed through or under the covering, and every fixing is a possible path for water. On tiled roofs, the strip normally slips under the course below the ridge and is held by a clip or a fixing in the batten, so the tile above does not sit proud of its neighbours. A carelessly nailed strip, a tile cracked in the process or a gap left at the edge turns a moss aid into a leak.
The work happens along the ridge, the top edge of the roof and the windiest, so it needs proper edge protection and access. It is also the place to check the ridge tiles themselves. Ridge tiles have been mechanically fixed as well as bedded since BS 5534:2014, and old bedding that has gone loose should be dealt with before anything else is added to the ridge line. Ask whoever fits the strip to look at the ridge too.
Guarantees and what is fair to expect
Be wary of any product sold with a promise of a moss-free roof. Growth depends on shade, trees, pitch, the texture of the covering and how well the gutters drain, and a strip influences only one part of that. Our answer on what a moss-free guarantee really means explains how to read the wording and what it usually leaves out.
A fair expectation is that a strip, fitted on a clean roof, slows regrowth on the slope below it. Whether that saves you a clean in a given year depends on the roof, and no one can promise a figure. Treat it as one part of a routine: clear the gutters, trim shading branches and have the roof looked at every so often. If you are planning to sell, a cleaned roof is the first thing buyers see, and our answer on cleaning your roof before selling sets out the pros and cons.
What we would do on your roof
We start by looking, not selling. Photographs from the survey show how much moss there is, which slopes are shaded, whether the covering is sound and whether a strip would reach the areas that need help. If cleaning or repair has to come first, you will be told that plainly, and a strip may turn out not to be worth adding. The target is a modest job that cures the fault.
Where a strip does suit the roof, the written quote explains where it goes and how it is fixed, and a written 10-year guarantee covers the workmanship. Tidying up is part of the job. Prefer to see the whole range first? Our roof cleaning and moss removal page lists it. To arrange a visit, ring 01793 626096 or write to us through the website.