its obvious the guys that make these heavy brushes have never cleaned windows wfp day in/day out for any length of time. 
even for conny roofs you d be better off with a SL brush or radius sill
Or if you prefer the quick & easy way to clean the likes of really filthy conservatory roofs, frames, sills, cladding etc back to white then just softwash it instead and agitate if need be with an unger cobweb brush which weighs much less than any wfp brush. Not only that gets into all the nooks & crannies no wfp brush can do easaly if at all. Oh I almost forgot it works rather well on cobwebs too
