If you have to go with a roof pitch less than 3 12 then it would either have to be a metal roof or a roof covered with a rubber membrane to prevent water penetration.
Good roof pitch for shed.
Basically the steeper the pitch the better it can shed snow and rain.
Roof pitch for the shed roof pictured above the back has a 5 12 pitch.
It is expressed as x y where x rise and y run for example 4 12 or 5 12 and so on.
There are advantages and disadvantages to having a steep pitch versus a shallow or low pitch the most obvious of which is the roof s ability to shed or withstand heavy snow rain or wind.
This slope works well for sheds that are fairly narrow like 4 to 6 feet but for sheds that are 8 or wider the shed roof becomes unsightly high.
Having a steeper pitched roof allows for more space in a shed loft.
A roof pitch calculator can make things much easier.
Worth noting is that the higher the first number in the equation is the steeper the pitch of your shed roof will be.
This image shows a relatively shallow roof pitch of 15 degrees.
Shed roof pitch calculator.
Asphalt shingles typically have a minimum slope rating of 4 in 12.
The loft space would be much larger with a roof pitch of 45 degrees.
Simply plug in the values of your roof and it will show you slope percentage degree and even length.