Thompson's Water Seal on roof to prevent moss?

9 Aug 2006
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United Kingdom
Howdy, first post on the roofing section of DIYnot.

North-facing extension roof of cement tiles has masses of moss between the tiles and around the flashing. A few years I climbed up and scraped it all out, a messy, tedious and tiring job but I'm at ease on roofs thanks to being a climber - I protect myself with a climbing rope thrown over the ridge and tied to something solid.

Don't want to use weed killer because we drain straight into a brook under the house.

Don't want to jet wash because that will attack the surface of the cement tiles, which already release sand when you abrade them.

Going to do it again soon but I'm wondering if I could delay the re-growth of the moss by spraying some spare Thompson's Water Seal on the roof to make it a little less able to hold moisture.

What do you think?
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Sounds reasonable especially not using weedkiller or pressure washer.
If you search on here for COPPER RIDGE you'll find some interesting stuff about copper ridge being quite good at keeping moss at bay
With the Thompson's, I'd clear a small section manually (the same way you did it last time) then treat half the cleaned section & see if it makes any difference over 6 months or so (and see if it causes any other problems)
And it'd be really handy if you posted back on this thread in 6 months or so with the outcome of the experiment..... not a unique set of circumstances I'm sure :)
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Apart from moisture, the other ingredient for moss is shade. If the roof wasn't north-facing then you probably wouldn't have the moss problem. If there are trees putting the roof in even more shade and you've been thinking about removing them (if they're not protected) go for it - the more UV light reaching the roof the less moss you'll have.
No, the roof gets no sun and stays damp for a long time as the westerlies don't dry it. On top of that a tank overflow dripped on to it for a few months until I realised what was happening and went up and bent the ballcock arm down. I'm worried that the accumulation of moss in the gaps between the tiles may actually be forcing them to move outboard slightly.

I will report back next year.

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