The short answer
They can slow moss on the part of the roof below them, but they do not stop it altogether and they do nothing for moss already growing. The strip is a narrow band of zinc or copper fixed under the ridge. Rain running over the metal carries traces of it down the slope, which discourages growth. Shade, damp and shape of roof still decide how much moss you get.
A metal strip at the ridge is a quiet, long-term helper. It is no substitute for a clean, and it works best on a roof that has just had one.
What the strips do
A moss-control strip is a thin band of zinc or copper, usually fitted just beneath the ridge tiles, at the very top of the slope. Each time rain falls on the metal, a small amount of it dissolves and runs down the tiles or slates below. Those traces make the surface less welcoming to moss, algae and lichen.
Is it a quick fix for a mossy roof?
No. The effect is gradual and preventive. It takes rain, and time, for the metal to be carried down the slope, and moss that is already established keeps growing until it is removed. Fitting a strip to a heavily covered roof without cleaning it first leaves the moss in place, holding water at the laps and against the tiles.
So the order of work counts. The roof is cleaned first, any treatment is left to do its job, and the strip is fixed afterwards so that it protects a fresh surface rather than a mossy one. Think of it as a maintenance aid that helps keep the roof clear for longer, not as a cure.
Our answer on moss growing back after cleaning looks at what else affects regrowth.
Where strips help and where they do not
The metal only treats the water that runs over it, so the shape and slope of the roof decide how far the benefit reaches.
Why do they not work everywhere?
A long, simple slope sends water straight down from the ridge, which suits a strip well. Complicated roofs with hips, valleys and dormers divert water sideways, and parts of the roof never receive any run-off from the metal. A valley collects water from both of its sides, but a strip on one slope does not feed a different one. Anywhere beneath a dormer or a chimney can be missed entirely.
Shade is the other limit. Moss loves a north-facing slope under trees, and a damp, shaded patch may still grow it despite the strip. In those places the answer is often to cut back the tree, clear leaf litter and brush the roof regularly. Our page on moss on the north side of a roof explains why the cold slope suffers.
So strips suit a clean roof with a long run of plain slope, and they do far less on a broken-up roof.
Zinc or copper, and how they are fitted
Both metals work in the same way, so the choice is mostly about looks and cost. Zinc weathers to a dull grey. Copper starts bright and darkens over time. VM Zinc states that its zinc has a BRE Environmental Product Declaration of 100 years, which says something about the life of the metal itself, though not about how well a strip controls moss.
Can strips be fitted to any roof?
They go under the ridge tiles or the top course of slates, so a roofer has to lift the ridge or the top row to slide them in and fix them to the battens, the timber that carries the covering. It is a small job, but it means working up at the ridge, and it is best done alongside other ridge work. The ridge itself needs to be sound, and Marley notes that mortar by itself is no longer trusted to hold bedded ridge tiles.
Where a house has a listing or a conservation area around it, visible metal alters its appearance, so ask Somerset Council's planning team before fitting.
Where the ridge needs attention anyway, repairs during a roof clean is the sensible time to add strips.
Strips, cleaning and coatings compared
Strips are one tool among several, and each deals with a different part of the problem.
What is the best way to keep moss away?
Cleaning deals with the growth already on the roof. A biocide treatment, used to the terms of its authorisation, kills what remains and slows return. Strips work quietly in the background to make the surface less welcoming. A coating, covered on our roof coating page, changes the surface itself. Cutting back trees and keeping gutters clear remove the conditions moss likes.
Which mix suits you depends on the roof and how much upkeep you want to do. A tidy, sunny slope may need only a clean now and then. A shaded one with a dense canopy may benefit from several measures together. The roof gets read first, with photographs, and the advice follows what is likely to help in your case rather than a strip on every roof.
The roof cleaning and moss removal hub covers the range of options. Send a photo of the slope. You then receive one total in writing for whatever suits.
Metal strips, answered
Do zinc strips stain tiles?
Zinc can leave a pale streak below the strip on some surfaces, and copper may leave a faint green or brown trace as it weathers. The effect is usually subtle, though a trial patch on your own roof is a sensible first step. We note the colour of the tile on survey.
How long before a strip makes a difference?
It takes repeated rainfall, so expect the effect over a season or more, not a week. It will not clear moss already growing, and the benefit stays limited to slopes the run-off reaches. Cleaning first and fitting the strip afterwards gives the metal a clean surface to protect.
The full picture: Roof cleaning and moss removal in Taunton ยท More short answers