hi there i've been in the roofing trade for over 35 years specializing in removing moss from roofs, we have heavily research into this, and found moss damaging to roofs. roof coating are beneficial but it has to be done right, for example our company taken a range of coating for sale on the market put then to a lab for testing, some lasted on 3 months whereas others lasted 18 years so watch how you buy from. you should be looking to pay no less than £80 for 15 litres many thanks cal