Roof Moss – Everything You Need to Know (Quite Literally)

This page forms part of our blog which contains a series of help guides for consumers and tradespeople.

With decades of experience in the home improvement and maintenance industry we know that moss removal and prevention is a very popular service in the United Kingdom.

Here are some real messages we received from genuine customers over the years:

The moss on my roof is becoming a real problem, we had our gutters unblocked three times last year and have just discovered our soakaway is now clogged and needs to be jetted. Can you please contact us to arrange a visit as we would like a price for your roof moss removal service. – Carl from Basingstoke

We had our roof jetted three years ago but now the moss has come back with a vengeance, we would like an estimate from you to clean the roof. – Mike from Guildford

We have a conservatory with glass panels and the moss has fallen from the roof and is now collecting on the glass panels below, we are having to clean the glass every week and are looking for a more long-term solution to our roof moss problem. Do you have any suggestions? – Claire from Reading

With that in mind, we decided to create a troubleshooting guide for those of you experiencing issues with excessive roof moss growth.

While any suggestions found on this page should be carried out by someone competent at working on roofs and ladders, the information should be of interest to anyone suffering from the green roof menace.

Moss growing on roof

Considering that a handful of moss is enough to block a guttering system, it’s no surprise that the homeowner was having issues with moss growth!

Our Promise to You

Unlike most other roofing websites, we aren’t trying to sell you a specific product or service.

In fact, we don’t even have our contact details published.

As strange as that may sound, it’s true.

Here’s the deal:

We promise that everything you’ll read on this page is 100% unbiased and is just our helpful guide to roof moss, based on our experience.

For first time visitors to our site; you should know that we are just two former construction and home improvement workers who have nearly 30 combined years of experience in this industry.

Our Guide to Roof Moss Removal and Prevention

This page is all about roof moss and we’ll cover the following:

  • where it comes from and why it’s worse now than in the past
  • will the moss damage my roof?
  • steps that your roofer can take to remove it
  • chemicals and methods to prevent it growing back
  • practical steps to reduce excessive and problematic moss growth
  • things you should avoid doing
  • I heard that copper can prevent roof moss, is that true?
  • gutter guards and filters – do they work?

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.

Moss retains water, which freezes in winter and can result in clay tiles delaminating or crumbling. Photo sourced from Michael Roberts Chartered Surveyors

States – 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

Moss on clay roof tiles

Moss growing on roof

Clean roof

After the moss is removed

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.

(Disclaimer; just a reminder that we are not suggesting or encouraging untrained people to do this type or work, instruct a competent and experienced roofer to do it for you.)

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.

What are the Best Chemicals To Prevent Roof Moss?

Garden Sprayer

Once the moss has been removed, the tiles should be thoroughly treated with a moss and algae preventative chemical.

We cannot stress the importance of this step enough.

Moss will grow back suddenly and prolifically if you don’t first treat the tiles.

The best way to apply the chemical is with a sprayer.

Here’s a list of all the equipment your roofing contractor will need:

A strong moss fungicide such as this one (50% concentration).

Or if you prefer, you can waste your money on a branded product with a weaker concentration of the same chemical such as this one (7% concentration)

A garden sprayer such as this one for large areas.

Gloves and mask which are cheap so there’s no reason not to buy them.

The method of applying the chemical is very simple, but as mentioned earlier, this type of work should be done by someone competent and safe when working at height and on potentially slippery roof tiles.

Here are our recommended steps for moss removal and treatment:

  • place covers over grass, patio and flower beds
  • disconnect gutters from downpipes
  • close windows to the house
  • check the roof for broken tiles and replace as required
  • remove moss by the chosen method
  • complete any remedial repairs
  • clear out gutters
  • treat tiles with the chemicals on a dry day when rain is not forecast for at least 24 hours.

A dry overcast day is perfect for applying the chemical as it won’t get washed away in the rain and it won’t evaporate too quickly in the strong sunlight.

Avoid windy days, unless you want the chemical to drift into lawns, plants and anyone within a 25-metre radius.

How Long Do The Moss Chemicals Last and What is Their “Staying Power”?

This is one of the several questions we have been asked over the years.

Here are some more:

What is the staying power of these chemicals?

and

I want a strong chemical applied so it soaks into the tiles and prevents moss growth for years

and

How long will it remain effective?

Just over a decade ago new regulations were enacted that affected all biocides, pesticides and fungicides on sale in the European Union.

All products must now become inactive once they have been allowed to dry.

You have probably seen the following safety notice on pesticides and similar products used on driveways and patios:

“Keep pets and children off the affected area until the product has dried”

So while these chemicals are great at killing off any moss spores, lichen and algae etc, they have no “staying power” or direct long lasting effect.

This surprises many people but if you think about for a moment, it makes perfect sense.

If the chemicals were to reactivate every time it rained, it would wash off the roof and into the drains and waterways, killing organic material and fish.

Also, if applied to patios, paths or driveways, it would pose a threat to children, pets and wildlife after every spell of rain.

You cannot buy chemicals that reactivate once they have dried. They have been banned in the EU.

Any company or person making claims to the contrary is either misinformed about these chemicals or is lying to you.

So Why Use a Chemical Then?

Even with the most rigorous cleaning, moss spores will remain on the roof, most likely:

  • on the tiles themselves
  • in cracks in the cement work
  • in between the overlapping part of the roof tile that is difficult to power wash or scrape

Moss spores will grow very quickly when they come into contact with water.

It only takes a small amount of them to start sprouting and within a year, the entire roof will be covered in them again.

The purpose of the fungicide chemicals we refer to as “moss killers” is to treat the surface of a cleaned/scraped roof so all moss spores, algae and lichen is killed off and can’t start growing again when it rains.

It’s best to think of these chemicals as a biodegradable surface biocide rather than a permanent solution.

Once a roof has been treated with these chemicals, any future moss growth will be “from new” or via migration from a neighbouring roof. Both types of growth take several years.

[Update 2017: I’ve tested all the popular moss killing chemicals on sale in the UK. See which product is the STRONGEST and most POWERFUL at killing moss and algae by reading my moss killer review]

Reapplying Chemicals – How Often?

How long it takes for roof moss to grow back will depend on these factors:

  • how thorough your roofer was at removing the moss and treating the tiles
  • if your house is semi-detached or terraced, how much moss is on your neighbour’s roof and how quickly it migrates
  • how much sunlight hits the roof as moss loves shady areas
  • how much water is on the roof as moss loves damp areas such north facing roofs or areas under tree branches

As a general rule of thumb, if you were to reapply the chemicals thoroughly every three years then your roof should never experience any significant moss growth.

It’s also worth noting that while roof cleaning or moss removal takes a day or two to complete and requires access equipment, the spraying of chemicals onto the tiles only takes a few hours at most. Many professionals also use long reach poles so they don’t even need to go up on the roof, helping to keep costs down.

It’s, therefore, cheaper to reapply the chemicals every few years than to leave it too long and have to pay someone to remove the moss again.

Some homeowners who have previously experienced excessive or problematic moss growth now incorporate this roof treatment into a regular maintenance program. Every two to three years, they have the roof checked for broken tiles, the gutters unblocked and the tiles treated with a fungicide.

What works best for you will depend on your roof, how bad the moss growth is and many other factors.

Practical Steps to Reduce Moss Growth

As previously stated, moss loves to grow in shady damp areas.

It’s not uncommon to see the north facing aspect of some roofs covered in moss while the south side has little or no moss growth.

Here are two steps you can consider, if applicable to your situation:

Overhanging tree branches or trees nearby – nearby trees block the light that would otherwise dry out the roof. Also, any branches that directly overhang the roof not only block light but also allow rain drops to fall onto the roof, even after its stopped raining. If possible, cut back branches and fell any trees that are too close. As a bonus, you’ll allow more light into your home and garden.

TV aerials and dishes – aerials, poles and dishes are a magnet for birds that often mess onto the tiles below, this along with the extra shade and raindrops falling from the aerial results in extra moss growth. Look up at any north facing mossy roof and you’ll see the most prolific growth is directly under a chimney or TV aerial. As many homes have now switched to digital tv via the internet or cable, you’ve found another excuse to remove and ditch the unsightly aerial

Can I Treat the Roof Tiles Without Removing the Moss?

It’s understandable that homeowners want to keep costs down and one question we hear time and time again:

Can we just spray the roof with a chemical to get rid of the moss? Why do you need to clean/scrape it off first?

If your roof has anything more than a handful of moss growing on it, then it should be scraped or cleaned prior to the application of any chemicals.

If you don’t do this first, here’s what will happen:

  • the chemical will kill the moss but it will still be on the roof
  • the moss will still drop onto the path, patio, conservatory, grass etc
  • it will slowly degrade over the coming months and years into a thick muddy sludge that will periodically block roof valleys, gutters, rainwater pipes and soakaways

Killing the moss isn’t enough, it needs to be physically removed from the tiles too.

Below is a photo showing a blocked guttering outlet, the culprit is a handful of degrading moss and other organic material:

Blocked guttering

Blocked Gutter

Do Gutter Guards and Filters Stop Moss From Clogging Gutters?

One of the most irritating issues experienced by homeowners experiencing prolific roof moss growth is that of blocked gutters.

It’s not uncommon for recently cleared gutters to block again in a matter of weeks if the roof is covered in moss.

There are two types of filter that can used to keep the moss and other types of organic material from blocking the system:

Gutter Guard

The photo below shows a gutter with a gutter guard inside of it.

You can’t see the guard because of moss that has migrated from the roof into the gutter guard.

Moss is now growing inside the gutter guard and has completely blocked it.

Gutter Foam

Blocked gutter guard

The Gutter Hedgehog fares slightly better with moss but will still struggle if the roof has excessive growth on it:

Gutter Hedgehog Review

The Gutter Hedgehog. Not perfect but one of the better gutter guards. Can be purchased from Amazon.

The Gutter Hedgehog has over 300 customer reviews on Amazon so there’s plenty of additional information to read through if you wish.

Filters

Filters are small objects that are placed in the outlet to prevent organic material from entering the rainwater pipe or soakaway system where it could potentially cause an expensive blockage.

As you can see from the photo below, this simple filter will easily clog if the roof has more than a handful of moss growing on it, although it’s easier to unclog this filter than an underground soakaway:

Gutter downpipe filter

Gutter filter in outlet. From Amazon

Filters Meshes and Guards

All filters, meshes and guards have their limitations, most work very well at keeping large leaves out of the gutters/pipes, they don’t perform so well against moss and degrading moss/sludge or fine organic material.

As a general rule, these filters work best when fitted to a recently cleaned/scraped roof where only leaves and twigs are likely to block the gutters.

Filters are not a realistic alternative to roof cleaning/moss removal.

Can Copper be Used to Kill or Prevent Roof Moss?

When copper comes into contact with rainwater and oxygen it will leech a sulphate that prevents moss and algae from growing.

This has to led to several products being developed for the roofing industry:

Copper Ridge:

Copper Ridge

Copper secured to the ridge of a roof. Image source: Copper Ridges

Copper Mesh:

Copper Mesh

Copper mesh or tape. Image source: Westphalia

Limitations of These Products

In theory, these products work very well.

Unfortunately, in the real world there are some serious limitations:

  1. It prevents moss from growing but won’t kill existing moss to any significant degree so the roof needs to be scraped/cleaned first.
  2. Is only effective for about 2-3 metres from the roof ridge downwards.
  3. Can leave the roof looking streaky with uneven moss growth.
  4. Copper is expensive and you need lots of it, copper wire is nowhere near enough.
  5. The copper residue will corrode and damage aluminium guttering if you have it. It also corrodes galvanised fixings and screws used on some roofs.
  6. Works best on flat tiles but not so well of profiled tiles (see below).

Direction of rainwater on profiled tiles

As you can see from the photo above, rainwater and the moss killing chemical leached from the copper is diverted into the channel, so only part of the tile is treated.

This can leave the roof with vertical streaks of moss free lines with moss, algae and lichen either side on the highest part of each tile.

This major drawback, its cost and the fact it only works on the highest part of the roof are probably the reasons why this product never really took off and has only ever been sold in small numbers.

To summarise copper as a moss killer:

Yes, it works at preventing moss growth on a recently cleaned/scraped roof. For medium or large sized roofs, you’ll probably need extra copper further down the roof. You may get streaky lines and uneven moss growth on flat tiles and certainly on tiles with a deep profile. It works best on a simple “up and over” roof with flat tiles. It’s expensive and care needs to be taken if you have aluminium on any other part of your roof as copper will corrode it.

What About Roof Coatings, Paints and Sealants?

Roof coatings to tiles

Roof coating and sealant

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.

It won’t stop it completely as moss will still form on the surface of the tile and also between the overlaps, as you can see from the moss in this photo:

Moss growing on roof tiles

Moss growing between laps

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.

Tip: coatings and sealants are best applied to concrete tiles, they don’t tend to perform so well on clay tiles.

Prices

Whether you are considering roof moss removal, gutter cleaning or filters, roof repairs or any other type of roofing work, go check out our roofing price list where you’ll discover the going rate.

Conclusion

Excessive roof moss is a problem that affects millions of homes in the UK and while some roofs suffer direct damage as a result, this is quite rare and 99% or so do not.

The most common concern is the moss blocking gutters and underground drainage pipes, resulting in costly maintenance bills.

While moss can be easily removed by a professional, there’s no magic product that will prevent it from returning.

Best practices involve chemical inhibitors being applied every few years, possibly in conjunction with a gutter cleaning/maintenance schedule and/or pipe and gutter filters.

Roof coatings and sealants can be used by those who are prepared to pay for it, although these products have their own pros and cons.

We do not recommend removing roof moss yourself unless you have received working at height training and are competent on a roof and have the necessary safety equipment. Roofs are often steep and moss/algae can be very slippery, especially when wet.

Get a Quote

As previously stated, we don’t advertise our services on this website as we want our content to be as unbiased as possible.

If you would like to read reviews of local tradespeople in your area, then hit the link below to see how:

Get a Quote Online

© www.job-prices.co.uk

close

Copy and paste this code to display the image on your site