Hi Tom, great question. Based on your described requirement, I would recommend you opt for a professional grade petrol pressure washer. For a heavier roof clean as you have described, you will need a washer powered by a minimum 8 horsepower sized engine. The cleaning specifications you are looking for would be a minimum of around 3500PSI and a flow rate of 14 litres per minute or above. This sized pressure washer will strip the roof back to a state ready to re-paint and it will do it in satisfactory time.
I would also suggest that you use a Turbo Head nozzle (these are often an extra accessory for many pressure cleaners), which are great for tough cleaning jobs: the nozzle will spin at a fine degree spray with the water hitting the surface area from multiple angles, which quickly breaks down caked-on dirt and grime.
For any roof cleaning job, you will likely also need an extension hose which is another optional accessory for most pressure washer models.
For lighter roof cleaning requirements where the roof is not as old and you don’t need to strip off rust etc. you can opt for a smaller petrol machines powered by a 5-7 horsepower sized engine and would have a pressure rating of around 2500 – 3500 PSI, with flow rate of around 9-12 litres/minute. These are classified as light trade/heavy domestic machines, which are slightly cheaper than professional grade machines, making them a great price proposition. People do use these slightly smaller machines for jobs similar to your description, however they will take a longer time to complete the task.
Brands such as Powershot, Gerni, Crommelins and Jetwave all have high quality models for both professional and light trade/heavy domestic requirements. Thanks.