Question 1

Is moss on my roof a problem?

Accepted Answer

Less than you are usually told. On a sound slate roof it is largely cosmetic. What it genuinely does is block the gutters — moss is the main gutter-filler on most local roofs — and hold water against porous tiles, which in a frost pocket takes the face off them. Worth dealing with; not an emergency. Moss grows on roofs because a roof is a damp, shaded, undisturbed surface, and in a wet part of the country with plenty of tree cover that describes most roofs. It grows faster on porous coverings — older concrete tiles whose slurry surface has eroded — and on shaded slopes, which is why one side of a roof is often green while the other is clear. The two things it actually does are worth separating from the things it is claimed to do. First, it breaks off and washes into the gutters, and on most local roofs that is the main cause of blockages — more than leaves. Second, it holds water against the covering, and on a porous tile in a place that gets a lot of frost, water held against the surface freezes and takes a flake off it. That frost point is genuinely geographical here. Air frost rises sharply with altitude — the Met Office North West range is around 20 frost days a year on the coast to over 75 in the higher Pennines, at roughly half a degree of cooling per hundred metres — and Kerridge, Rainow, Langley and the Forest villages all sit high enough for it to matter. Moss on a concrete tile roof at 300 metres is a different proposition to moss on a slate roof in Broken Cross. How it is removed matters far more than whether it is. A pressure washer on a roof is one of the most damaging things that can be done to it: it strips the surface off concrete tiles, accelerating the very porosity that let the moss grow, it drives water under the laps into the roof space, and on slate it lifts and breaks the covering. There is no version of pressure washing a roof that is good for the roof. The correct method is unglamorous — scrape and brush by hand from safe access, clear the gutters and valleys afterwards, and treat with a biocide, which kills the growth at the root and lets the weather take the residue over the following months. The roof does not look pristine on the day and it improves for a year afterwards, which is exactly the opposite of what a pressure wash does. Finally: moss is one of the doorstep trade's standard openers, precisely because it is visible from the street and sounds alarming. Nobody needs to accept a moss quote from somebody who knocked. If you want it done, ring somebody you chose.

Question 2

Should I remove moss from my roof?

Accepted Answer

Less than you are usually told. On a sound slate roof it is largely cosmetic. What it genuinely does is block the gutters — moss is the main gutter-filler on most local roofs — and hold water against porous tiles, which in a frost pocket takes the face off them. Worth dealing with; not an emergency. Moss grows on roofs because a roof is a damp, shaded, undisturbed surface, and in a wet part of the country with plenty of tree cover that describes most roofs. It grows faster on porous coverings — older concrete tiles whose slurry surface has eroded — and on shaded slopes, which is why one side of a roof is often green while the other is clear. The two things it actually does are worth separating from the things it is claimed to do. First, it breaks off and washes into the gutters, and on most local roofs that is the main cause of blockages — more than leaves. Second, it holds water against the covering, and on a porous tile in a place that gets a lot of frost, water held against the surface freezes and takes a flake off it. That frost point is genuinely geographical here. Air frost rises sharply with altitude — the Met Office North West range is around 20 frost days a year on the coast to over 75 in the higher Pennines, at roughly half a degree of cooling per hundred metres — and Kerridge, Rainow, Langley and the Forest villages all sit high enough for it to matter. Moss on a concrete tile roof at 300 metres is a different proposition to moss on a slate roof in Broken Cross. How it is removed matters far more than whether it is. A pressure washer on a roof is one of the most damaging things that can be done to it: it strips the surface off concrete tiles, accelerating the very porosity that let the moss grow, it drives water under the laps into the roof space, and on slate it lifts and breaks the covering. There is no version of pressure washing a roof that is good for the roof. The correct method is unglamorous — scrape and brush by hand from safe access, clear the gutters and valleys afterwards, and treat with a biocide, which kills the growth at the root and lets the weather take the residue over the following months. The roof does not look pristine on the day and it improves for a year afterwards, which is exactly the opposite of what a pressure wash does. Finally: moss is one of the doorstep trade's standard openers, precisely because it is visible from the street and sounds alarming. Nobody needs to accept a moss quote from somebody who knocked. If you want it done, ring somebody you chose.

Question 3

Does moss damage roof tiles?

Accepted Answer

Less than you are usually told. On a sound slate roof it is largely cosmetic. What it genuinely does is block the gutters — moss is the main gutter-filler on most local roofs — and hold water against porous tiles, which in a frost pocket takes the face off them. Worth dealing with; not an emergency. Moss grows on roofs because a roof is a damp, shaded, undisturbed surface, and in a wet part of the country with plenty of tree cover that describes most roofs. It grows faster on porous coverings — older concrete tiles whose slurry surface has eroded — and on shaded slopes, which is why one side of a roof is often green while the other is clear. The two things it actually does are worth separating from the things it is claimed to do. First, it breaks off and washes into the gutters, and on most local roofs that is the main cause of blockages — more than leaves. Second, it holds water against the covering, and on a porous tile in a place that gets a lot of frost, water held against the surface freezes and takes a flake off it. That frost point is genuinely geographical here. Air frost rises sharply with altitude — the Met Office North West range is around 20 frost days a year on the coast to over 75 in the higher Pennines, at roughly half a degree of cooling per hundred metres — and Kerridge, Rainow, Langley and the Forest villages all sit high enough for it to matter. Moss on a concrete tile roof at 300 metres is a different proposition to moss on a slate roof in Broken Cross. How it is removed matters far more than whether it is. A pressure washer on a roof is one of the most damaging things that can be done to it: it strips the surface off concrete tiles, accelerating the very porosity that let the moss grow, it drives water under the laps into the roof space, and on slate it lifts and breaks the covering. There is no version of pressure washing a roof that is good for the roof. The correct method is unglamorous — scrape and brush by hand from safe access, clear the gutters and valleys afterwards, and treat with a biocide, which kills the growth at the root and lets the weather take the residue over the following months. The roof does not look pristine on the day and it improves for a year afterwards, which is exactly the opposite of what a pressure wash does. Finally: moss is one of the doorstep trade's standard openers, precisely because it is visible from the street and sounds alarming. Nobody needs to accept a moss quote from somebody who knocked. If you want it done, ring somebody you chose.

Is moss on my roof a problem?

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