Isn’t water a chemical? H2O? If they don’t even have any chemical then it is unlikely that they will be able to much other than rinse off loose dirt… which you could probably do with a garden hose.
A pressure washer alone is capable of cleaning concrete or other flat surfaces but only to the extent that there are no surprises like heavy mold and mildew, rust, oil or paint. Even when we clean concrete we apply a mildewcide afterwards to make sure to concrete is completely clean, the mold is completely dead and that it stays away as long as possible.
Steer clear of companies that pretend to offer $99.00 roof cleaning or house washing – or some other ridiculously low price. There is nearly a 100% likelihood that that is not the final price you get and the whole situation is bad – it just feels icky – like you will need a long shower after they are gone. These ultra-low prices generally don’t even cover what it costs for the chemicals, much less overhead, labor or that little thing called profit!
A truly professional power washing or roof cleaning contractor is one that carries at least general liability insurance so that YOU are covered in the event of an accident or neglect on their part. Having this type of insurance generally means they have been around a little while, know what they are doing, are committed to the business and their customers, have financial stability and are generally running their business in a professional manner.
Not carrying liability insurance generally means, at a minimum, they have never done a commercial project and are probably inexperienced.