The short answer
Zinc and copper strips can slow moss, but they do not stop it. Rain washes tiny amounts of metal down the slope from a strip fixed under the ridge tiles, which makes the surface less welcoming below it. The effect fades with distance, lasts only while the metal lasts, and does little on shaded or debris-covered tiles. They suit some roofs as a helper, not a cure.
What a metal strip at the ridge is supposed to do, where it falls short, and how it compares with cleaning and treatment.
How a metal strip is meant to work
The idea is simple. A strip of zinc or copper is fixed along the top of the roof, usually tucked under the ridge tiles so that a short length sits just on the slope. When it rains, the water running over the metal picks up a trace of it and carries that down the tiles. Metal ions of this kind are unfriendly to moss and algae, so growth below the strip is discouraged.
It is an old and widely used trick, and plenty of owners have strips fitted. It has one big attraction: once it is in place, it needs no spraying and no regular visits. That makes it appealing on a roof that is awkward to get at.
The result is limited by how much metal actually reaches the tiles, and that depends on rainfall, pitch and the length of the slope. We have no tested figure to give you for how far down a roof a strip protects, and installers differ in what they claim. Treat any precise promise of coverage with caution and ask the supplier to explain the basis for it. Ask the installer too how the strip will be fixed, and whether the ridge tiles are bedded or clipped securely afterwards.
Where strips fall short
The first limit is distance. The metal is diluted as it runs, so the effect is strongest near the strip and weaker further down. A long slope may still go green near the gutter, and that is where moss usually gets going. Several strips at different heights are sometimes used for that reason, but the more you add, the more it costs and the more the roof is interrupted.
The second limit is that strips only act when water runs over them. On a shaded slope, where tiles stay damp but rain does not wash the whole surface, a patch can stay untouched. Debris matters too. Leaves and silt in the overlaps give moss a bed whatever the metal does.
Third, the metal wears. It is slowly used up and may need replacing, and a new strip fixed to an older roof can mean lifting ridge tiles. A strip also does nothing to the moss already growing: it deals with the future, not the present. The roof still needs cleaning first, and any cracked tiles beneath the moss need dealing with before a strip goes anywhere near it. Our answer on why moss grows on the north side of a roof explains why the damp slopes remain a problem.
Strips compared with a biocide treatment
A biocide treatment is applied directly to the whole surface after the roof has been cleaned. Because it covers every tile, it does not depend on rain running over a ridge. It also needs repeating, and how long it lasts depends on weather, shade and the product, which our answer on how long roof biocide lasts sets out.
Strips and treatment are therefore not rivals so much as partners. A strip is a modest, hands-off extra that may stretch the gap between treatments on a roof that is otherwise well kept. It is a weaker choice as the only defence, especially on a shaded slope.
There is also a question about run-off. Metal washed off the roof ends up in the gutters and the drains, and treatment products need care for the same reason. Whatever you choose, follow the manufacturer's instructions, and keep products meant for professionals out of your own hands. Our guide on roof cleaning chemicals and the garden pond covers this side of things.
Should you fit them?
Consider strips if the roof is clean, sound and in good light, and you would like a low-effort way to slow regrowth. They make more sense on a new or freshly cleaned roof than on one already green, and best of all when fitted during other ridge work, when the tiles are already up.
Skip them, or at least do not rely on them, where a tree overshadows the roof, where the gutters often overflow or where the main problem is a damp north slope. In those places, cutting back branches and keeping the gutters clear will do more than any metal.
Fitting a strip means working at the ridge, which is dangerous for anyone without proper access, so leave that to a roofer. If you are weighing it up, we inspect the roof, send pictures and write up one total for whatever is agreed. The wider service is described on our roof cleaning and moss removal page, and strips sit best alongside a cleaned, treated roof with clear gutters and trimmed branches.
More on this in Roof Cleaning & Moss Removal in Crawley
Two more strip questions
Is zinc or copper better?
Both work in the same way, by releasing a little metal when rain runs over them. Copper is often described as the stronger of the two, but it costs more and can stain some surfaces. We have no tested comparison to quote, so ask the supplier what their product is based on and how they expect it to perform on your tiles.
Will a strip clear moss that is already there?
No. It may discourage new growth below it, but existing moss stays until it is removed. The roof should be cleaned first, ideally by a roofer with proper access, and then treated. A strip can be added afterwards if you want a little extra help in keeping the surface clear between treatments.