We pay close attention to the roof structure, trims and especially the shingles. Common issues with asphalt shingle roofs include.
Advanced age with most of the color granules gone. The granules give the roof its color but also stop the sun from deteriorating the asphalt layer. With granule loss the asphalt is expose and deteriorates by drying out. Asphalt is close to the material our black top roads are made of and just like old asphalt roads dry and crack so do shingles once the granules are gone. Eventually the cracks lead to roof leaks.
Deteriorated Sealants. Most all roofs will have some sealants exposed to the sun and elements. Sealant are found on metal flashings and at the roof peaks and other places. These eventually dry out and can lead to leaks. Flashings are held down by nails. The heads of the nails are covered with sealant to prevent corrosion and leaks. With lost sealants the nails heads are exposed, they then rust work loose or the heads just fall off. This can lead to leaks.
The ends of shingles curl up and lift and the finger joints ( the gaps you can see from the ground that give the shingles that line look) get wider because the shingles are old and drying out.
This last one is often found on new construction homes but is one the worst but easiest defect to correct if caught before damage occurs. The nails work up and loosen. This happens because the plywood decking shrinks and contracts with the changing temperatures or as is the case with new construction the nails just are not set flush with the shingle and are left up off the shingle by as much as and inch or more. When the shingles get hot the nails pop through and create a leaks.
Here photos of the most common issues.