Always keep in mind that the distance you can reach relies more on the flow rate (GPM).  The 4kpsi DM is rated for 4gpm which will allow you to reliably reach approx 25' with a shooter tip or X-jet on a calm day.  For a full three story reach, we recommend one of the DMs that is 3000psi/5gpm or the gear drive which has a 5.5gpm pump.  So it can put out a little more than 5gpm giving the longest reach of any machine with a GX390.

As for roof cleaning, it is not usually performed with a pressure washer.  Asphalt shingles should not be cleaned with any significant amount of pressure (below 500psi).  The primary goal of roof cleaning is to apply a strong mixture of SH to the surface.  In some regions this can be achieved with an X-jet on a 5gpm, as long as the bleach is strong enough.  However, it doesn't always yield the desired results.  Lastly, when performing a roof cleaning only about half of the contractors rinse the roof.  Most leave the solution to dwell for as long as possible to maximize results.  That means the rinsing is done naturally when it rains.