By design, many manufactured homes have composition roofs that are low-sloped. A general guideline is that composition shingles are suitable for a roof with a slope of 4/12 or greater. But, in fact, most inspectors do not worry much about a roof that is sloped at 3/12 or greater. The roof slope gauge, below, indicates that this roof is under 1/12 slope.
The concern has to do with basic roof functionality. A roof with composition shingles is not designed to be water-tight. The sufficiency of the roof depends on runoff water quickly exiting the roof. When a roof has a slope that is minimal, the water may be retained on the roof surface and it might even work under the shingles. In that event, whether the leak damages sheathing or not is dependent on the quality of any felts that were installed under the shingles. In most cases, other than trying to peak at an eave to see if there are felts at all, the inspector cannot verify the absolute integrity of underlayments.
I have seen obvious leaks, in attics, under this type of improper installation. From the standpoint of home inspection, usually the safest procedure for the inspector, when a very low-sloped composition roof is seen onsite, is to refer the roof and any of its obvious problems, to a professional roofer. It is my experience that, seldom, does an actual roofer put composition shingles on a very low-sloped roof. Usually, a real roofer selects a different and more suitable roofing material, the exception being at manufactured homes.