White marks on roof. Anyone know what this is?

[HomerJay99]

These marks appeared several years ago over the period of a few weeks on our Welsh Slate roof. We eventually got fed up of looking at them and asked a roofer who specialised in cleaning to see he could get the marks off. Try as he might, using scrubbing by hand and eventually 2 types of acid, he couldn’t get them off - all that happened was the marks became pure white on the test tile that he used the acids on. He eventually gave up and conceded defeat.
The marks are not raised in any way. If you close your eyes and move your finger over them, you won’t feel a difference between the mark and the tile.
The weird thing is that they ONLY appear on one side of the roof and our neighbour (around 100 ft away) doesn’t have them.
Does anyone have any idea what these could be?

 

[datarebal]

Looks like lichen

 

[HomerJay99]

Not lichen apparently.

 

[datarebal]

I'm sticking to Lichen
You can see its growing from one slate to another.

Its a really tough chap being one of the longest living organism on this planet

 

[^woody^]

Not lichen apparently

Who told you that? Its definitely organic

 

[HomerJay99]

Doesn’t come off. We’ve tried all the usual stuff to remove lichen and none of it works. It seems to have removed the actual colour from the slate itself.

 

[HomerJay99]

How would you remove lichen?

 

[Alastairreid]

Have you tried white vinegar? It will come back no matter what you do.

 

[HomerJay99]

Yup, white vinegar was the first thing we tried. Also 2 much stronger acids had no effect apart from bleaching the affected area to nearly milk white.

 

[Mr diy a lot.]

Lichen. Especially when you said it’s taking a layer of the top of the slates. That’s what it does. He is hardy as hell. Vinegar will take a couple days to kill it.

needs moisture and some organic debris to get it going so maybe that part of your roof received more shade than other parts of roof/neighbours etc. tree overhanging it which drops crap on to it or even if you bathroom is below and you have moist air being extracted into your attic space and not being vented properly it could all propagate.

In grand scheme of home maintenance priorities it should be low down the list. I would not worry about it. it adds a period charm to that lovely slate roof.

 

[HomerJay99]

No, that’s not what I meant! There’s nothing actually growing that you can feel on the slates. If you close your eyes and run your finger across a tile, you can’t feel the stained areas from the ‘normal’ tile area at all. We have some similar looking lichen growing on bits of the patio and you can feel a slightly crusty growth there easily. Not on the roof tiles though!

 

[catlad]

Its Lichen: see it a lot on slate.

 

[HomerJay99]

Doesn’t lichen sit on top of the slate though? All these areas feel exactly like the slate areas. Is this significant?

 

[catlad]

I have seen many different forms of plant life living on roofs, some Lichen is crusty to the touch and can be scraped off, others like yours are etched into the slate and is practically impossible to remove. I've seen clumps of sedum like plants thriving on lead back gutters! when you think how toxic lead is! how can they survive? The answer must be that Nature is more powerful than we give it credit for.