A Common Question – “Why is it Only Recently That Roof Moss Has Become a Problem?”

During the last 50 years, the UK and indeed most of Europe, has transitioned from an industrial economy to a services and commerce based society.

With fewer factories and power stations pumping out harmful and toxic chemicals into the environment, our rainfall is now much less acidic than in the past.

Those of you who are old enough may remember the dire government warnings about acidic rain from back in the 1980’s.

Here is a press release about how acidic rain has almost been completely eradicated in the UK over the last 20 years:

Read: Acidic Rain 20 Years On (UK Government Press Release)

What some of you may not be aware of is how sensitive moss, algae and lichen are to acidic rain.

Even the slightest increase in the acidity level of rainwater is enough to kill off and prevent mosses and algae from forming.

Those of us who have been in the roofing industry long enough have experienced this first hand.

Back in the early 1990’s very few customers needed their roofs cleared of moss or cleaned using power washing equipment.

Nowadays some roofing firms get more phone calls about roof moss then anything else.

In fact, an entirely new industry has been created – the roof cleaning/coating industry – which never existed previously as there was very little moss/algae growing on roofs.

Go check out old photos of houses from pre-1980’s, very few will have any significant moss growth on the roof tiles. The same houses today will probably be experiencing at least some growth.

The current roof moss epidemic is a direct result of the cleaner air we now enjoy in western Europe.


Will Moss Damage My Roof?

There are many different materials that are used in the construction of a roof and some will be affected by moss growth while others won’t.

Here is a checklist detailing what is and isn’t damaged by moss growth:

Concrete tiles – No, except for a few corner cases, which are often due to the poor manufacturing techniques of older tiles. Air bubbles, small stones, sand bubbles and other objects in the cement mixture can create small holes that expand over time. Moss may grow in these holes, but it isn’t the cause of the hole. Tile imperfections are less common with newer tiles made to western standards.

Clay tiles – Yes, especially on shallow roofs. These tiles can delaminate if they get wet and then freeze, hence why clay tiles are best laid to steep roofs and kept clear of moss – so they can fully dry out.

Slates – No, not affected.

Cement – Yes, dusty, flaky and “weak” cement can be ruined by excessive moss growth.

Gutters and rainwater pipes – Yes, they can become blocked. Also, thin plastic guttering may bend, warp or snap at the brackets due to the extra weight of excessive moss, hence why gutters should be cleared frequently.

Chimney brickwork – Yes, it gets into the cement cap and between the bricks, leading to moisture ingress, when that freezes, it expands and causes cracks or damage.

Lead/Leadwork – No, this isn’t damaged by moss growth on the roof.

Is the extra weight of moss a problem on the roof? – No, this is a common claim made by some roofing and roof cleaning firms. They suggest that because wet moss is heavy, it could buckle, bow or collapse your roof. In fact, even excessive moss growth is spread so thinly over the roof that no single point or rafter ever exceeds its load capacity. Roofs are designed to hold several feet of heavy wet snow for prolonged periods of time and to hold firm against sustained gusts of wind without collapsing inwards. Thin plastic gutters may be damaged by too much moss in them though (they warp, bend or snap at the brackets).

The Best Way to Remove Roof Moss

There are two practical options you have available.

Both involve going onto the roof so these should be carried out by competent and experienced persons only:

  1. have the moss removed by scrapers and brushes
  2. have the roof power washed

With both methods, it’s very likely that you’ll come across some remedial work that needs to be completed.

This is usually replacing broken or chipped tiles, cement repairs and unblocking rainwater pipes etc.

Both methods are very effective at removing the moss.

If you want to have your roof re-coloured, coated or sealed, then we suggest you clean it first with a power washer.

Power washers remove not only moss, algae and lichen but also pollutants, grease and dust that could prevent the coating from adhering to the tiles correctly.

If you have no interest in re-colouring your tiles, then the manual scraping/brushing method will suffice.


How Long Until the Moss Grows Back?

Without the application of chemicals, you can expect the moss to start growing back within 6-12 months, although it will take a few years before the moss growth becomes prolific again.

Biocide Roof treatment

After your roof has been cleaned we are able to apply a biocide softwash. The wash kills all moss, algae & lichen within 24 hours of being applied and continues to clean your roof tiles for up to 12 months after application.


What About Roof Coatings, Paints and Sealants?

Roof paint, otherwise known as a coating or sealant is a weatherproof liquid applied to the exposed side of roof tiles.

The primary purpose of these products is to add colour to the roof in an attempt to rejuvenate it and improve the roof aesthetically.

Because moss prefers to grow on rough porous surfaces like concrete, clay or tarmac, you may find that a waterproof sealer or coating will reduce the amount of moss growing.

Coatings can certainly reduce the amount of moss that forms but at a cost, the appearance of the roof will be altered too – roof paints are not to everyone’s tastes, this approach would also be at upper end of the cost scale.

If you are already considering having your roof coated to improve its visual appeal then its moss preventing properties could be a deciding factor.

If you don’t like coloured roof coatings, there are translucent products that do the same thing but won’t change the original colour of the tiles.

Even after being coated, these roofs still need an application of moss fungicide every now and again to ensure no moss grows.