Some professionals use a pressure washer on siding, but that is really overkill. It can
damage caulk and force water into the siding.
Try washing your siding with a mild detergent and water.
You can wet the surface and then spray with the detergent/water mix in a garden sprayer. Use a soft brush – the kind you use for washing a car – and put the brush on a long pole to make the job easier.
Work from the bottom up, and keep wetting the area below the part you’re scrubbing to prevent dirty wash water from streaking dry siding. Rinse with clear water.
If your siding has gray, black or green spots, try washing with JOMAX – a great product for removing mildew. Just follow label directions. You spray the product on the surface, wait and then rinse it off. For tougher dirt, you may need to scrub a little.
You will be surprised at how a little elbow grease and detergent can make your siding look like new.