I take it by 'barnacles', you mean lichen? If so, I give a good dose of tfr, and bit of a scrub, then leave for a while to soak. I then use an old squeegee on pole and basically use it as a soft scraper, and scrape up and down, usually removes worst of it, then wfp. I've only done this on polycarb roof so far, so I'd be careful if its a glass one.