We usually charge per sqft. However, the price may vary according to the difficulty of the service, for example: if the roof is very high and dangerous, or we cannot climb and walk on the roof, we need to rent a lift, the client wants service outside the business hours, the place to be cleaned is difficult to have access, the type of roof, the distance from us, the time we will spend washing the area, etc. We need to see the house before confirming the price.