Walkway between houses - drainage
Putting a walkway between ours and the neighbour's house. Not sure on finish layer; maybe interlock. Excavating to 9" depth. There'll be 4" drain pipes running under the walkway, and downspouts will tie into these.
Existing grass has a slope of about 0.4" per foot from the back to the front yard. In order to mate backyard level to front driveway level, I'm planning to have the finished walkway slope about 0.25" per foot, from back to front. It will also slope by this much side to side, so there'll be about a 1-1/2" dip in the center (13 feet between the houses). From what I've read in most places, 0.25" per foot of slope away from a house is sufficient.
My main question is regarding basement windows on the houses. Without ths consideration, the finished surface of a consistently sloping walkway would have been right at the level of the bottom of this window. Given that backyard drainage is being piped under the walkway, and downspouts are also feeding into this piping, only rain hitting the surface of the walkway is of concern. What is a safe drop between the bottom of a basement window, and the walkway surface?