Anatomy of a Complete Roofing System

The deck is the structural sheathing — typically plywood or OSB panels — fastened across the roof framing. It is the surface everything else is attached to, and it is the one part of the assembly you almost never see.

Because it sits under the old roofing, its true condition is usually confirmed only after tear-off. Deteriorated or delaminated panels are replaced before the new assembly goes on; skipping that leaves a sound roof sitting on an unsound base.

A self-adhering waterproofing membrane that bonds directly to the deck. Unlike the layers above it, it is designed to seal around the fasteners driven through it.

It is used strategically at the areas of a roof most exposed to standing or backed-up water rather than as a blanket across the whole roof. Where exactly it belongs on a given house depends on the roof and on current requirements, and it should be confirmed for your property rather than assumed from a general article.

Underlayment is the secondary water-shedding layer installed across the deck beneath the shingles. Synthetic underlayments are the common choice on residential work today.

It sheds water that gets past the covering — driven rain, a lifted shingle, wind-blown moisture — rather than acting as a waterproof seal. It also protects the deck during installation before the shingles go down.

A metal edge profile installed along eaves and rakes. It carries water off the roof edge and into the gutter instead of letting it run back under the roofing or behind the fascia.

It is a small piece of metal that quietly protects the most exposed inch of the entire roof.

The first course at the eaves and rakes is not simply another field shingle turned around. Starter is a purpose-made product with its sealant positioned so the first visible course bonds down correctly at the edge.

Cutting up field shingles to improvise a starter course puts the sealant in the wrong place — at the edge of the roof where wind pressure is highest.

This is the visible roof covering: the main body of shingles installed in overlapping courses across the roof. On most residential replacements these are architectural shingles.

It is the part everyone shops for, and it is one layer of ten.

Flashing handles the places where the roof stops being a plain slope: walls, chimneys, valleys, skylights, vent pipes and any other penetration or transition.

These details are where roofs most often leak, and they are handled with formed metal and correct layering rather than with sealant. Whether existing flashing is reused or replaced should be stated in the scope of work, not assumed.

A roof assembly includes how air moves through the space beneath it — intake at the lower part of the roof and exhaust near the ridge working together.

Ventilation is considered as part of the roof, not as an add-on. Replacing a covering does not by itself change how the roof and the space below it breathe.

Ridges and hips are finished with material made for that purpose. Cap shingles are shaped and thicker so they bend over the ridge line without cracking and close the roof off cleanly.

This is also where ridge ventilation is commonly integrated, so it is a finishing detail and a functional one at once.

Roofing nails are what actually hold the covering to the deck. Type, length, quantity and — critically — placement within the shingle all matter.

Fasteners placed outside the shingle's designated nailing area, overdriven or underdriven, undermine the whole assembly no matter which shingle was purchased. It is the least visible line on an estimate and one of the most consequential.