I think i know what your doing
Just put lead box gutters running though back of pitch roof draining out on to tiles at front, i would of thought one every 3ft should be enough
Sounds to me like your paint is to thick try watering it down slightly, u may find if you give a wall two coats with a roller u will need to give the edges 3 but you won t notice brush strokes then and your paint will go futher bargin all round. :D