The short answer
Partly. Zinc strips fixed under a row of tiles release tiny amounts of zinc into rain water as it runs down the roof, and that makes the tile surface less welcoming to moss, algae and lichen. They reduce growth rather than stop it, they only work on the slope below the strip, and they do nothing to remove moss already there. On a roof that is clean and where moss keeps returning, they can help. On a roof that is already mossy, clean it first.
How zinc works
Zinc is a metal that, when wetted by rain, releases ions in solution. Those ions run down the face of the tiles and are toxic to moss spores and algae. A strip is fixed near the top of a slope, usually just below the ridge, with a few centimetres showing beneath a course of tiles. Each shower carries a thin film of zinc over the tiles below, and the growth finds it harder to take hold. The effect depends on rain, so it works best where rainfall is regular and the roof is steeply pitched enough for water to flow.
Where they do and do not work
A strip protects the area below it, with the effect weakening further down the slope. On a long slope, the lower courses may get little benefit. A strip does nothing on the slope above it or on a neighbouring slope, so a house with moss on several slopes needs strips on each. Shaded, damp roofs under trees get a heavy supply of spores and a constant moisture source, which can overwhelm the effect. On low-pitched roofs the water moves slowly and spreads the zinc poorly. Our answer on why moss grows on one side of the roof explains why north slopes are the hardest to protect.
What strips do not do
They do not kill moss that is already growing, nor lift it. If you fit strips to a mossy roof, the old growth stays until it is removed. They also fade as the zinc surface weathers and as the supply runs down, so protection lasts for a time, not forever. Our guide to roof biocide treatments explains the alternative, a liquid applied across the whole surface.
Fitting and cost
Strips are fitted by lifting a course of tiles near the ridge, sliding the strip underneath and re-fixing, so they need access and care to avoid breaking tiles. They are cheap as a material, and the cost is mostly the labour of working at height. The tile that holds the strip is lifted a little, which is why the job is not for an unsupported ladder. We do not publish a figure. Your fixed quote follows the survey. If the roof needs cleaning too, the strips are best fitted in the same visit.
Staining and effects on other materials
Zinc run-off can streak lighter or discolour some tiles and lead, and the strip itself can leave a pale line on dark tiles. Run-off collected from the roof reaches gutters, water butts and the garden, where zinc is harmful to some plants and aquatic life in larger amounts. Check this before you fit strips over a pond or a water butt feeding a vegetable bed. Our guide to roof cleaning, run-off and wildlife covers those concerns.
Strips against a biocide
A biocide is sprayed over the whole roof surface, so it protects every course at once, but it needs reapplying as it wears off. A zinc strip is fitted once and works only below itself, with a weaker effect further down. Some owners use both: a treatment after cleaning for the first protection, and strips to slow the return. If your roof is large, steep or hard to reach, strips can reduce the number of visits. If it is small and easily cleaned, a periodic treatment may cost less over the years.
Tree cover tips the balance. Under an oak or a beech, the debris and shade deliver more moss than any strip can stop, and cutting back branches, where you are able to, helps more than zinc. A roof that dries quickly in sun will not grow much moss anyway, which is worth remembering before you spend money on prevention.
Checking the strips later
Look at the strips from the ground every year or two. A strip that has slipped, bent or turned grey and chalky is near the end of its useful life, and fresh growth below it tells you the same thing. Strips can be replaced the way they were fitted, by lifting the tile above and sliding in a new one, and the job is quick if access is already in place.
Are they worth it?
Strips suit a roof that has been cleaned, faces regular rain, is not heavily shaded and has a history of regrowth. They are a modest, low-effort way of slowing things down, not a cure. Many roofs do just as well with periodic cleaning and a biocide. We tell you honestly which is likely to suit yours. The rest of what we do for moss is on the Basingstoke roof cleaning hub.
Every roof is different. A free survey with photos settles it for yours.
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