According to the InterNACHI® Home Inspection Standards of Practice (www.nachi.org/sop), the inspector shall inspect the roof-covering materials from the ground level or eaves. Home inspectors should report on the "roof-covering" or the "roof-covering materials" that were observed during the home inspection.
Inspectors should not report upon the "roof system", "roofing", or "roof assembly," the components of which are not readily observable. Components of a roof system or roof assembly are not readily visible during a visual-only inspection. The components of a roof system or assembly may include the roof deck, load-bearing components, trusses, underlayment, substrate, fasteners, thermal barrier, vapor retarder, insulation, ventilation, and roof covering.
Can home inspectors see underlayment? No. Roof fasteners? No. Decking? No.
"Roof covering" is a term specifically used and defined in the International Residential Code (IRC) and building standards. Roof covering is the covering applied to the roof deck for weather resistance, fire classification, or appearance. And since the covering is layered, not all of the covering material is visible during a home inspection. The other terms, such as roofing or roof system, reference a combination of components used together to form a complete assembly.
A system is made of its parts. An assembly is made of smaller components. And most of the smaller parts and components of systems are not readily visible during a home inspection. They are beyond the scope of a home inspection. The focus for the home inspector should be on the roof-covering material. A variety of roof-covering materials may be installed on a house or building to be inspected, such as asphalt shingles, clay, and concrete tile, metal roof shingles, wood shingles and shakes, and metal roof panels. For example, built-up roof covering is a type of roof covering and is defined in the International Residential Code as a common roof covering for houses and buildings with relatively flat roofs and made up of two or more layers of felt cemented together and surfaced with a cap sheet, mineral aggregate, smooth coating, or similar surfacing material.
Homeowners should not expect their home inspector too see everything during a home inspection. Why? Just about everything (every component and system of a house) is limited and restricted in some way for the home inspector during a visual-only home inspection.