Shingles are the visible part of a roof system, but they’re not doing the job alone. Underneath them are two layers that matter just as much: the decking and the underlayment.
Decking is the plywood or OSB sheathing that forms the actual structural surface of your roof — everything else is fastened to it. Over time, decking can rot or weaken from prolonged moisture exposure, which is why a tear-off often reveals sections that need replacing before new roofing goes on; this is normal and shouldn’t be treated as a red flag, just a disclosed part of the process.
Underlayment is the water-resistant barrier installed directly on top of the decking, before shingles go on. In colder, storm-prone regions like ours, ice and water shield — a self-adhering, more robust underlayment — is typically used along eaves, valleys, and around penetrations like chimneys, since these are the areas most prone to water backing up under shingles during freeze-thaw cycles.
A roof is only as good as these two layers, even though you’ll never see them again once the shingles go on. It’s worth asking any contractor specifically what underlayment they use and where, rather than assuming all roofs are built the same underneath.