Fascia boards are boards mounted at the roofline and work together with soffit boards to cover the exposed area of roof trusses over the side of a house. Not only does this stop dirt, animals, etc from entering the roof space but (and for the majority of us, more importantly) it is an aesthetically pleasing finish to this roof area. Fascia boards are typically made of pressure treated lumber, plastic or metal. Regardless of the material, they can all be cleaned the same way. You will find the necessary materials and equipment to clean your fascia boards at your local hardware and DIY / home improvement stores.
Scrub the fascia. Once you have cleaned all that you can reach, climb down the ladder and move it to the next area. Rinse the cleaned area of all soap while you are getting the next area wet. Continue until all of the fascia is scrubbed and rinsed.
Mix some water with the mild detergent in the bucket. Spray a section of the fascia with water from the hose. This will start to loosen up the dirt and grime so it will clean off easier.