Pressure Washing

The right way to clean something depends entirely on what it is made of.  Concrete will take a great deal more than painted wood, vinyl siding or an awning, and using one setting for everything is how surfaces get damaged.

High pressure lifts dirt mechanically.  On a solid concrete drive that is exactly what you want.  On soft or aged timber the same pressure raises the grain and leaves the boards furred; on vinyl siding it can drive water up behind the panels; on painted or coated surfaces it can take the coating off along with the grime; on older mortar it can cut into the joints.

Lower-pressure cleaning is the alternative approach: a gentler wash that relies on the cleaning solution and dwell time to do the work rather than on force.  It is slower and it is the appropriate method on a good deal of what is on a house.  Roofs in particular are not a high-pressure surface in any circumstances we would recommend, and any roof cleaning is looked at before it is quoted.

In practice most properties get more than one method in the same visit: the drive and the walks handled one way, the siding and the trim another.

Cleaning removes what is sitting on a surface: dirt, algae, mildew, general weathering.  It cannot remove what has soaked into the material, and no honest quote will tell you otherwise.

Concrete is porous.  Oil, transmission fluid, rust from a metal item left standing, tannin from leaves that sat wet through a winter and paint spills have all had time to go into the surface rather than sit on it.  Washing usually improves those and often improves them a lot.  It does not reliably eliminate them, and anything that has etched or discolored the material itself is not dirt any more.

Age and condition matter too.  A twenty-year-old drive will not come back looking like a new one, because some of what you are seeing is the concrete rather than what is on it.  Sealers and coatings change the picture again, sometimes for the better and sometimes by failing unevenly when they are cleaned.

Where a surface is old, coated, painted or otherwise uncertain, the sensible step is a test patch somewhere inconspicuous before committing to the whole area.  That is a few minutes of work and it is the difference between knowing and hoping.