The first thing to say about lichen is that it is an amazing organism and, if it’s not eating your roof or latching onto your paths, can be quite beautiful to look at.
Here’s the scientific bit (gleaned from Wikipedia!). Lichen is a hybrid fungus/algae organism. The growth form most common on our Nelson, Stoke & Richmond roofs is crustose placodioid, the flat lichen that grows from the centre and radiates outwards.
So why is a harmless looking lichen bad for our roofs ? There are a number of reasons.
First, the crustose lichen secretes organic acid that eats into the roof surface. Whether it’s paint on metal sheeting or tiles, decramastic coating or even concrete tiles, the lichen is burning away the surface of your roof.
Secondly, the lichen retains moisture which will infiltrate those fissures created by the acid. That moisture will freeze and expand, cracking surfaces and corroding metallic roofs.
Thirdly, any dead lichen (as well as moss and other debris) will end up in your gutters, potentially blocking them.
Finally (though there are probably other downsides), it doesn’t look great and can provide the moist conditions for other moulds to thrive.
So what can you do about it ? The first thing is to see what parts of your roof are affected. Often the northern aspects of homes can be completely lichen free while the southern aspects have heavy lichen. The second thing is not to just leave it. We have encountered roofs where the lichen has formed a virtual carpet and it’s touch and go whether it has destroyed the integrity of the roof. The sooner, and more regularly, you address lichen the better it is for your roof.
There are a number of household chemical solutions that will kill lichen, from vinegar to bleach (but whatever you do, DO NOT mix those two as they will produce poisonous chlorine gas !!) but the problem is applying them. Almost by definition if your roof needs treatment it will be slippery and dangerous. In the long run it is probably worth getting specialists, like Nelson Waterblasting, to fix the the problem for you.
Many of the commercial products available to professional contractors are chlorine based but we prefer to use an alkaline salt based product. It kills the lichen just as effectively but also reactivates for a period afterwards in rain to discourage further growth. Whatever the product used there is one big factor that we try to highlight when discussing roof treatment. The solutions will kill the lichen but they DO NOT REMOVE IT FROM THE ROOF. That is only done over a period of time, and that will depend on a number of factors such as exposure to wind and rain, roof pitch and what sort of surface your roof is. On some abrasive decramastic roofs, where the lichen is effectively attached to a sandpaper-like surface, it can take over a year for it to come away naturally without being scrubbed.
It is possible to pressure wash some roof surfaces and depending on the home owner’s priorities that is also something we do regularly, but it is generally a more time-consuming and expensive solution.
