Take a little time to educate yourself about the various cleaning methods and look at the advantages and disadvantages of each. High pressure may be fine for some surfaces and are certainly not fine for others. Make sure you, and the company you are considering hiring know the difference. You’ll find that some companies clean roofs by using a power washer and some (the good ones) use an approach that avoids high pressure and opts for specialized detergents instead.
If you wanted to replace your roof it is likely you would have called a roofer and not a “roof cleaning” company to come out and blow off your shingles. Likewise, if you were looking to have to repaint your home – you probably would have just called a painter in the first place instead of an under-qualified pressure washing company only to have them screw up your paint so that you ended up needing to call a painter later.
The fact of the matter is, most of what is making your house or roof look first is actually living organisms, namely mold and mildew. If you are looking to truly rid yourself of the problems they cause you to need a “one-two punch” and not just an uppercut. This “one-two punch” can be done by using the proper detergents and a low-pressure rinse. Punch one is the ability of the detergent to actually kill the mold and mildew. This can actually be done in a variety of ways but few also include punch two: removing the stains. Imagine if a super-spy assassin went around murdering the enemy but left bodies everywhere….that’s only half the job right?