1st of all you dont need scaffold,it'll cost to much.
I do 6 every year for the past 3 yrs all just before summer,spring timeish,all done with a extenable lance and a wall cleaner HP that is,as for the ridge,i just slip a fan nozzle on.
Couple of things to look out for,watch where you spray water some cheaper cons tend to leak at the seals esp if water is force in a different angle,not a real money earner i charge £40-£60 per unit theres only one thats a sod for access as one side is about a foot from nextdoors fence