The short answer
Zinc or copper strips fixed at the ridge can slow moss on the slope below them, but they do not stop it. Rain carries a trace of metal down the tiles, which discourages growth only a short way. Shade, trees and damp matter far more, and a strip has no effect on moss that is already established. Treat a strip as a small help after cleaning, not a cure.
How a metal strip is meant to work
The idea is simple. A narrow strip of zinc or copper is fixed beneath the ridge tiles so that its lower edge sits on the top course of the slope. When it rains, a minute amount of the metal dissolves and travels across the tile faces. Moss, algae and lichen dislike that metal, so growth below the strip is slowed.
You can see the same effect on a roof that carries a lead or copper flashing. The tiles directly below the metal often stay cleaner than those beside them, with a clean streak following the path the rain takes. A ridge strip tries to copy that on purpose, across the whole width of the slope.
The effect depends on the metal reaching the tiles. It needs rain, a slope that carries water evenly, and a surface the water actually wets. Where any of those is missing, the strip has little to work with.
Why the effect is limited
Metal runs down a roof in very small amounts, and it thins out as it travels. The few courses nearest the strip may stay clearer, while the lower part of a long slope gets almost none. On a steep pitched roof with a short run, that distance may be enough. On a wide, shallow or long slope, the bottom half is left to the weather.
A strip also works only on what it touches. Moss that is already growing is not killed by a trace of metal arriving from above, so a strip fixed to a mossy roof will not clean it. The roof has to be cleared first, and the strip can only help to keep it clear afterwards.
Then there is the cause of the moss itself. Moss establishes itself wherever a roof stays damp: on the north slope, under overhanging branches, beside a taller building, or where the tiles are rough and hold water. A strip does not change the shade or the damp. Our page on trees and moss on roofs explains why a shaded slope grows moss so readily, whatever is fixed along the top.
What fitting a strip involves
A strip is not simply laid on the roof. To sit correctly, it has to be tucked under the ridge tiles or the top course, which means lifting and refixing the ridge. The ridge is the row of tiles capping the apex. Wind loads it harder than any other part of the roof, so anything lifted there has to be put back properly.
BS 5534:2014 says the strength of mortar cannot be counted on by itself for ridge and hip work, so every bedded ridge needs a mechanical fixing as well. Dry-fix systems are tested to BS 8612:2018. A strip fitted at the ridge should therefore go in alongside fixings that hold the ridge down, and not rely on bedding mortar to do the job.
A strip also needs to suit the roof. Zinc and copper behave differently, and the metal should be compatible with the lead, fixings and gutters it will shed water onto. Where the house is listed, or within a conservation area, ask B&NES planning before adding anything visible at the ridge. Metal strips are usually hidden, but appearance and material still count on a protected roof.
What keeps moss off more reliably
The measures that do the most are plain ones. The first is to remove the moss gently and fully. Brushing or scraping it off by hand, followed by a soft wash if the surface needs it, leaves the tile intact. Pressure washing can strip the surface from a tile or slate, and MyBuilder's cost guide treats fragile tiles as a reason to avoid high pressure.
The second is a treatment that stays on the roof after cleaning. A roof biocide is applied to the tiles and left to work over the following weeks, which tackles spores on the surface itself, over the whole slope and not just below a strip.
The third is to take away what feeds the moss. Cut back branches that shade a slope, clear leaves from valleys, and keep gutters running so water does not sit at the eaves. Moss thrives where debris collects, and clearing it costs little.
Where a strip does make sense is as an extra on a slope that is already clean, sunny enough to dry, and unlikely to get shade. There it may add a little protection. It is rarely worth fitting on a roof that grows moss because of damp and shade.
Strips on flat and low-pitched roofs
Metal strips suit tiled or slated pitched roofs, where water runs down a visible face. They do nothing useful on a flat roof, where water sits and drains slowly and the covering is a membrane. Moss and algae on a flat roof need a different approach, described under cleaning a flat roof.
On a low-pitched tiled roof, water moves slowly and the thin trace of metal carries even less distance. A strip is least likely to help where moss is most likely to grow.
If you are weighing up a strip, we will tell you directly whether your roof is a candidate. You pay nothing for the survey on this roof, and the figure we then write down does not move. For most roofs the better spend is on cleaning, a treatment and sorting out the shade and damp, rather than a strip alone. For the whole service, see roof cleaning and moss removal.
