How we ended up doing this
Around Dunedin there are a lot of roofs in an awkward middle stage. They look tired, they have lost some grit, and someone has mentioned replacement. Yet the deck underneath is sound and the inside of the house is dry.
Those are the roofs we built this business around. Some need a wash, some need a handful of small repairs, some are ideal for treatment, and tile roofs need cleaning and sealing instead. The common thread is that each one gets looked at properly before anyone decides what to do.

