The idea behind the strip
A copper strip is a narrow band of the metal fixed along the roof, usually under or just below the ridge tiles. When rain falls on it, a trace of copper dissolves into the water and runs down the slope below. Moss and algae are thought to struggle where that water passes, so a strip is sold as a way of keeping a clean roof clean.
That is the whole principle, and it is why the strip sits high on the roof. Gravity moves the water from the top towards the gutter, carrying whatever the metal has given it.
The principle is easy to state and hard to measure on a real house. Roofs differ in shade, pitch, surface texture and how long the tiles stay wet, and any one of those can outweigh a trace of metal in the rain. Nobody can promise a particular result for a particular slope.
A fair way to describe a strip is as a plausible, slow and passive measure. It asks nothing of the householder once fitted, but it also removes nothing, and anyone offering a firm result for your house is going beyond what can be said.
The limits of a strip
Several things cut down what a copper strip can do, and they are worth knowing before paying for one:
- It does nothing to moss that is already established. The roof has to be cleaned first.
- Rain only carries the metal as far as it runs. On a long slope, the lower courses may get little, and the lowest courses and the valley edges may get none.
- Shaded or north-facing slopes stay damp longer, and a damp tile surface grows moss however much copper passes over it.
- Overhanging branches drop leaves and seed onto the roof, giving moss a fresh bed that the strip does not touch.
- A heavy fall of rain washes the top courses far more than the bottom ones, so the effect is uneven across the slope.
None of that makes the strip useless. It means the strip is one small contributor, and the things that decide whether moss returns are shade, moisture and debris. Where a tree is the cause, cutting back a branch usually does more than any strip. Our page on moss on a north-facing roof explains the shade side of the story.
Fitting one, and its side effects
A strip has to be fixed in place by someone working on the roof, so it brings the same access needs as any roof job: ladder, tower or full scaffold. That is a cost on its own, and the strip is best added while the roof is already accessible, such as when it has just been cleaned or repaired.
The usual spot is below the ridge, with the strip held beneath the top row of tiles so that rain runs onto it. A long roof may take more than one band. Slates and tiles differ in how a strip is tucked in, so the method should be settled when the roof is looked at.
Think also about where the water ends up. Run-off from the roof reaches the gutter, the downpipe and then a drain, a water butt or the ground, and a strip adds a little metal to that flow. If the downpipe empties onto a surface or a planted bed you care about, mention it to the roofer before anything is fixed, so that the position can be thought through.
Ask how it is held on, too. A band that is badly fixed can lift and let rain under the tile above it.
A better order of work
If moss is your concern, the order matters more than the strip. A sensible plan looks like this:
- Have the roof looked at, so you know whether the moss is lifting tiles or only spoiling the look.
- Clean it gently and clear the gutters, so the surface is bare.
- Deal with the causes within reach, such as cutting back overhanging branches and keeping the gutters running.
- Only then consider a strip, as a small extra on a roof that has been put right.
Our approach to cleaning is gentle brushing and treatment rather than force, and our page on whether moss grows back explains why it often does, whatever is fitted on the ridge. A copper strip may slow that, but it is not a fix to rely on.
One practical point on timing: the best moment for a strip is straight after a clean and any repairs, when the surface is bare and the top courses are open and reachable. Fitted onto a roof still covered in growth, it has nothing to protect and the moss just carries on beneath it, which is the very problem the strip is hoped to fix.
For the full picture of cleaning methods, see roof cleaning and moss removal.
