Anatomy of a roof

From the peak down to the ground

• Ridge cap and ridge vent — the peak is the most exposed line on the roof. The vent lets superheated attic air out; the cap seals and finishes it. Cut-up three-tab shingles used as cap crack years early.
• Field shingles — the visible weather surface. Nail placement is everything; nails driven high or into the wrong zone let wind peel courses off in the first big storm.
• Starter shingles — a purpose-made first course at the eaves and rakes whose sealant bead bonds the row above against wind uplift.
• Ice and water shield — self-sealing membrane at valleys, eaves, and penetrations, where water concentrates and fasteners are densest.
• Synthetic underlayment — a second water barrier across the whole deck that stays flat and intact where old felt paper tears and wrinkles.
• Roof decking — the plywood or OSB the whole system nails into. Soft or delaminated decking won't hold a nail, and roofing over it guarantees early failure.
• Drip edge — metal at every roof edge that throws water clear of the fascia instead of letting it wick back underneath.
• Flashing — the metal at walls, chimneys, dormers, and penetrations. More leaks start here than anywhere else.
• Valleys — where two slopes meet and the water volume concentrates. Fasteners in the flow path are a classic cause of leaks.
• Pipe boots — the collars sealing each plumbing vent. Cheap rubber dry-rots in a few Mississippi summers; it's one of the most common leaks we find.
• Gutters — carry runoff away from the fascia, siding, and foundation.
• Soffit and fascia — the trim at the eave, and the vented intake that feeds the airflow the ridge vent exhausts.