I don't have experience with trenches to know how much slope for drainage is desirable but with a 1 in 80 slope (i.e. 6 inches over 40 feet) the water runs very well along the spouting on my shed . >For an accurate slope for drainage, attach a jig to a >3 ft level. A piece of wood cut diagonally from one >corner to 3 inches will give you a slope of 1 inch for >every foot.