The composition shingles that we see on houses are designed for sloped roofs. They will shed water but they are not waterproof membranes. Ideally, a roof will have a slope of 4/12 or better, but generally, if a roof is 3/12 or steeper, the inspector probably won't say much about it.
This is a roof slope gauge. It is on a composition roof. The slope is about 1.25/12. Now, as an inspector, I have to cite that as a potential problem. In some cases, like at manufactured homes, the factories use minimal slope but we hope that they use a good system of underlayments and felts below the surface. But, even at a manufactured home, the roof better have more slope than this.
Some sellers might think that the inspector is a bad guy for referring to this as a worry. After all, the average home owner and do-it-yourselfer has no clue about slope. In this case, it was a pretty easy call. The low slope was resulting in leaks and rot at the underside of the sheathing.
This photo was taken at an attached shed where the problem was obvious, but parts of the house were equally low sloped. A roofer needs to be called in to remedy the roof problems here.