UKC

My son is considering buying a property (semi-detached, bedrooms upstairs) which has lots of moss on its roof. Level 3 Survey recommends removal in case it damages the roof. I am also aware of the National Federation of Roofing Contractors' advice that moss on roofs is rarely a problem unless moss blocks gutters etc and warns against risk of damage to tiles from removal, esp. power-washing and foot fall  (https://www.nfrc.co.uk/article/2012/09/07/moss-on-your-roof-advice). Also urges people to exercise 'extreme caution' about claims that painting tiles will increase thermal efficiency and extend life of roof tiles.

Thoughts from the UKC hive mind welcome.

Small potential for moss to result in dampness in battens etc. but probably has to be quite extreme for this to be an issue.

We live in a semi detached property, our roof had quite a bit of moss on the roof, falling off in to the gutters etc.

Had ours cleaned and treated with some anti moss spray a few years ago, it's still spotless, unfortunately our neighbours didn't want it doing, so we still get a bit of their moss in our gutters. 

I can sympathise with both points of view.  Moss provides lots of insects etc for birds.

I have seen that you can use biological washing powder to kill roof moss. I'd want to do a bit of research about whether it could do any harm to the roof covering first though. 

We've just bought a bungalow with clay roof tiles. The survey said exactly the same. I was up a ladder this weekend clearing out the gutters (lots of moss!) and dislodged some of the moss I could reach on the tiles. The moss has certainly done damage - in some cases the top surface of the tile has become loose. I'm hoping I can get a few replaced rather than a new roof - but I don't think the warnings are necessarily wrong. It is fairly heavily mossed (on the shady side - maybe 10% coverage)

Thanks all for the replies - very helpful

If you use a slow spin you should be OK

It might just be because I live in Glasgow, but for me removing moss is pointless. It comes back within weeks.

I just leave it. The birds like to forage in it for insects, and do a good job of keeping it somewhat under control by stripping some of it off every spring for nests. I just keep the gutters clear and leave it at that.

As I'm a botanist I'm probably a little biased but why would moss be a problem? Mosses have no roots so how can they damage a roof other than maybe keeping it damp at times. Maybe I'm missing something obvious but I can't see a problem.

Get some crows. The ones that live around us delight in sitting on the roof pulling the moss out and throwing it around.   

I was told on a previous home with concrete roof tiles and a lot of moss, not to remove the moss as it was not recommended. The surface of tiles would most likely be damaged by pressure washing/ heavy brushing causing future problems and premature replacement. Damage to the tiles was done on the removal of the moss and not the moss itself.

Recently I had a sales person of a roof cleaning company at my door and when I mentioned the above, he responded by saying that they sealed the tiles with paint so there would not be a problem. He skirted around whether the cleaning actually damaged the tiles, but just repeated that they sealed the tiles anyway so it wasn’t a problem. He could get their surveyor to visit and discuss if necessary!!

I did notice on a neighbour’s roof they cleaned and painted that they replaced various tiles after cleaning. Whether they broke them walking on them, by pressure washing or whatever I don’t know.

Anyway, take what you will with that, but makes me think the professional opinion not to remove moss I had years before was more likely to be the best. I’ve never removed any moss in my current roof (except guttering) and not aware of any problems.

You can spray the moss with wet and forget, it is reasonably effective. I have never used it on a roof but it is what it was developed for I believe.

It claims to be all eco too.

My north-ish facing roof sheds a fair bit of moss into the gutters during storms. I've cleaned them out once or twice in a decade. In recent years it's developed quite a visible ecosystem of blotchy mosses and lichens, much better than stark red cement-tile.

Can't say I worry about it doing any damage to the roof.

jk

In the interest of balance, I live in Glasgow too and had a lot of moss on my roof a few years back. I've cleaned the south-facing side with a wire brush one hot summer day in 2022 when it was nice and dry and so far it hasn't come back. I'll tackle the north-facing side this summer.

I've also heard that putting a strip of zinc under the ridge tiles keeps a roof moss-free, there's some clever, natural chemical in the run-off apparently. No idea of that's true or not.

Apart from someone who sells paint for roof tiles (or someone who wants to paint your tiles) who has ever claimed it will increase your insulation?? New one on me 🤣

We have had extensive roof leak issues with various roofers of varying degrees of reliability looking at it. Dislodged moss blocking leaded gullies is definitely a problem. But they have also suggested moss growth may be lifting roof tiles. I don’t know if that’s true, but that’s what they have said.

I’ll tell you what not to do: roll stones down your roof thinking they’ll gather the moss. Total waste of time. 

As a plant scientist (which is certainly not the same as a botanist), the little buggers have done damage! Might be holding water then freeze/thaw. Alternatively they might chuck out organic acids like plant roots do.  

I can't say what it was exactly, but I was quite bad at cleaning my work van and it got quite green, especially in the hard to clean areas. As the years went by I found the greenery had eaten away at the door seal glue so the seals eventually started peeling off. 

You need to roll them up the roof for that to work 🫸🪨🗻 always gets tricky towards the top but you'll get there eventually!

You can put "gutter hedgehog" in, which gets round the gutter issue. 

Moss can act like a sponge and cause damp issues if its established and growing on roofs. Rain should fall on a tile and gravity should take it down onto the next tile and so on. If moss is growing and rooting up under the tiles and flashings then it can suck water up and under and lead to problems. 

We thought that but it doesn't. It just takes longer to become an issue then is more of a faff to sort.

Moss interferes with water run off on concrete tiles and is best removed. Brushes are best as pressure washing does damage the tiles and doesn’t clean the overlaps on tiles. Prevention includes heave metal strips (such as zinc mentioned above) but these will discolour tiles. I use Mosskill spray once every three years.

I always think it's worth bearing in mind that part of the surveyor's job, by informal incentive as someone paid by the buyer, is to make the house sound as bad as possible to help you negotiate the price down. If they see something that could possibly be construed as a negative, they'll definitely mention it and talk up the risks, regardless of how likely it is to cause any real problems. There's no incentive for them to go out on a limb and say "there's some moss on the roof but it'll be fine".

Conversely, of course, there are all kinds of very real problems that wouldn't be obvious to a cursory glance and won't get picked up by most surveys at all.

That would be my assessment too - extra little bit of insulation, and ecologically good. Win win.

As someone who works in building conservation I can advise thusly:

How worried you should be depends on the amount of moss and the roof covering. For all roofs a heavy build up of moss presents the following risks (most to least serious): Blocked gutters, a build up of organic matter allowing rooting plants to establish, water retention, acidic run-off.

I cannot emphasise enough how important it is to clear gutters. The impact of overflowing gutters can be severe.

If you have good quality concrete tiles you may not suffer any issues provided you clear out the gutters very regularly. If you have slate, stone slate or clay tiles then the increased water retention can increase the risk of frost shattering. 

Moss can be removed by hand or by water-jets. Avoid very high pressure water jets and only aim them down the roof. 

Thanks NobleStone and others - really appreciate the input

I use 0.000694 rpm...