There are pressure washers that claim to have 1500psi. The volume of water that comes out, however, is insuficient to tackle much more than washing a car. 1500 psi may well do the job, but you will need a good machine. One that you can adjust the pressure would be good, then you can determine if the job is being acomplished without damage to the shingles. You shouldn't need chemicals. The water will do all the work. Are the shingles as old as the house?