Hello Dave and welcome to this board. Evidently there are pros and cons to every method that you may choose. If you are considering using a twelve volt pump for both applications house washing and roof cleaning there are some things to consider. Twelve volt pumps are mainly used for roof washing as they are very durable hence the air pump are also used exclusively by roof washers due to the durability to the Sodium Hyprocloride. If you are trying to save money and use one machine for both applications roofs and house washing I would consider a gas driven pump, the drew back is that these machines are not very durable so you would have to do some research to find what fits you best. If you want to do roofs I would suggest you look for some one in your area that is willing to take you under thier wing and provide you with some training. The key to a successful operation is to have to the proper training that will definitely save a lot of head aches in the future. One name that comes to mind when it comes to roof cleaning is Chris Tucker, a very nice man that will help get going. Good luck to you.