I tell clients that ignoring moisture control is like wearing wet underwear to work: you’re appearance is acceptable but you are destined to have difficulties in the future. Most moisture related problems are cumulative, not catastrophic and that is why so little attention is paid them. Moisture and its’ effects remain, like underwear, unseen (most of the time).
Roof Water Formula: for every 1000 square feet of roof (a typical roof has about 2000 square feet) 620 gallons are generated for every inch of rainfall. With an average of 50 inches of rain per year that 1000 square foot roof generates 31,000 gallons of water per year.
Where does it go? Much of it accumulates outside the foundation walls. During rainy periods it may enter the underfloor space. Best Practice? Pipe it WELL AWAY from the house. The photo shows downpouts piped along a narrow space in order to avoid draining onto the neighbors yard. This is cheap and easy: it’s best to pipe using buried smooth wall PVC drainpipe with cleanouts to allow for maintenance and fewer leaf clogs.