In this guide
The structure: rafters, trusses and decking
Everything starts with the framing. Rafters or engineered trusses form the skeleton of the roof and set its pitch. Fastened across them is the roof decking, also called sheathing, usually plywood or oriented strand board (OSB). Older homes may have spaced plank boards instead.
Decking is the base everything else is nailed to. During a replacement, the old roof is torn off so the decking can be inspected. Soft, rotted or delaminated panels should be replaced; installing new shingles over bad decking leads to sagging and nail pops.
Drip edge
Drip edge is an L-shaped metal strip installed along the eaves and rakes. It guides water off the roof edge and into the gutter instead of letting it wick back onto the fascia and decking. It also gives the roof edge a clean, finished line and helps keep out insects and wind-driven rain. Many building codes now require it.
Ice and water shield and underlayment
Ice and water shield is a self-adhering rubberized membrane applied to the most vulnerable areas: along the eaves, in valleys, and around chimneys, skylights and walls. It seals around nails and stops water that gets backed up by ice dams or driving rain. In cold climates, codes often require it to extend from the eave edge to a point inside the home's exterior wall line.
Over the rest of the deck goes the underlayment, a secondary water barrier beneath the shingles. Traditional felt has largely been replaced by synthetic underlayments, which are lighter, stronger, more tear-resistant and safer to walk on during installation.
Shingles, starter strips and ridge caps
Starter strips are installed first along the eaves and rakes. They have a factory adhesive strip that seals down the first course of shingles, which is the area most exposed to wind uplift. Then the field shingles go on in overlapping courses, each nailed according to the manufacturer's pattern.
At the top, ridge cap shingles cover the peak and hips. They're thicker, pre-bent pieces designed to wrap over the ridge and hide the seam where two slopes meet.
Valleys and flashing
Valleys are the internal angles where two roof slopes meet, and they carry a lot of water. They can be 'open' with exposed metal, or 'closed' and 'woven' with shingles running across. Either way, they need ice and water shield beneath them.
Flashing is thin metal that seals transitions where the roof meets something else. Step flashing is woven in piece by piece where a roof slope meets a sidewall. Counter flashing is set into chimney mortar or wall siding and laps over step flashing. Pipe boots seal plumbing vents, and headwall or apron flashing protects the top of a slope that meets a vertical wall. Most leaks start at flashing, not in the open field of shingles.
- Step flashing: sidewalls and dormers
- Counter flashing: chimneys and masonry
- Pipe boots: plumbing vent stacks
- Kick-out flashing: diverts water at the bottom of a roof-wall intersection into the gutter
Ventilation: soffit intake and ridge exhaust
A healthy attic breathes. Cool air enters through soffit vents under the eaves and warm, moist air exits at the top through a ridge vent, a continuous vent along the peak hidden under ridge cap shingles. Box vents, gable vents and powered fans are other exhaust options.
Balanced ventilation keeps the attic closer to outdoor temperature, which reduces ice dams in winter, lowers cooling loads in summer and prevents moisture that can rot decking and shorten shingle life. Mixing exhaust types on one roof can short-circuit airflow, which is one reason a ventilation check belongs in every roof replacement.
The edges: fascia, soffit and gutters
Fascia is the vertical board along the roof edge where gutters attach. Soffit is the underside of the overhang, often vented. Gutters collect runoff and downspouts carry it away from the foundation. These parts sit at the boundary between roofing and siding, and they work best when the drip edge, fascia and gutter are installed as a coordinated system.