Generally, minimum slope for water to run off is 1% (1/8" per 1'). However, minimum slope for a flat roof by building code here is 2%. (1/4" per 1'). You have more than enough slope for your roof, however you will likely need some sort of membrane roofing since it is very low slope.
Accordingly, how much slope is needed for drainage?
All of your drain pipe (with one exception) must slope slightly downhill. Usually 1/8" per foot is plenty for drainage, this is the same as the commonly recommended 1% slope. Note that I said the drain pipe must be sloped rather than the French drain.
How much slope does a septic line need?
My experience is that many waste line installers simply install a straight sewer line from house to septic tank or house to sewer main, regardless of the building slope, provided that we have at least 1/8" per foot, preferably 1/4" per foot, or more. Lower slope or nearly-flat drain lines are more likely to clog.
What is the minimum fall for sewage pipes?
Standard Horizontal Drainage Pipe Slope
|PIPE DIAMETER||MINIMUM SLOPE|
|2 1/2" or smaller||1/4" per foot|
|3" to 6"||1/8" per foot|
|8" or larger||1/16" per foot|