9 parts of a roof and what each one does

A pitched roof relies on structural framing, wood panels and waterproof membranes to keep the house dry. The framework gives the roof its shape and strength, while outer layers shed water away from the living space below. Knowing these nine parts of a roof helps you spot failures early. It also helps you understand exactly what a contractor plans to fix.

1. Rafters and trusses

Rafters and trusses form the structural skeleton that gives a pitched roof its shape. Rafters are individual boards, usually dimensional lumber cut on site and nailed into place. Trusses are engineered triangles of smaller wood bound together with metal plates. Factory workers build them and a crane lifts them onto the house.

Both systems carry the weight of the roofing materials and resist wind and snow loads. A visible sag in the roofline from the street can point to a framing problem. Inside the attic, failure looks like cracked lumber or broken metal connector plates. Any cracked framing means a licensed structural engineer needs to inspect the house.

2. Ridge board and roof ridge

The roof ridge is the highest horizontal line where two sloping roof sections meet. In a traditional stick-framed roof, the upper ends of the rafters attach to a ridge board. This is a thick piece of lumber running the entire length of the peak. It gives the rafters a solid surface to fasten against, preventing them from simply leaning on each other.

The ridge defines the roof peak and keeps the rafters properly spaced during construction. A bowing roofline or walls that lean outward at the top are reasons to call a licensed structural engineer, since the cause is structural.

3. Rafter ties and ceiling joists

A heavy roof constantly pushes down and outward on the exterior walls. Rafter ties and ceiling joists are horizontal boards that connect the lower ends of opposing rafters to stop this push. In many homes, the ceiling joists that hold up the top floor drywall double as rafter ties. You can learn more about how a house is framed to see this geometry in action.

If these horizontal connections are missing or damaged, the roof can push the walls outward. Wide ceiling cracks or leaning walls call for a licensed structural engineer, and any repair needs a permit from your building department.

4. Decking

Roof decking consists of flat wood panels nailed across the rafters or trusses, which builders also call roof sheathing. Most modern homes use large sheets of oriented strand board or plywood. Older houses might have solid wood planks spaced slightly apart. This flat layer ties the individual framing members together into a single, rigid structure.

Decking provides a continuous surface to anchor the nails that hold a roof together. A severe leak will soak this wood, causing the panels to swell and rot. You can read about signs of failing roof decking to catch these problems early. Rotten decking creates a wavy appearance on the roof exterior.

5. Underlayment

Underlayment is a protective sheet rolled out and fastened directly over the wood decking. The Asphalt Roofing Manufacturers Association describes underlayment as sheets of water resistant material that give secondary protection from water intrusion. Common types include asphalt saturated felt paper and synthetic rolls.

This layer shields the bare wood decking from rain while the house is being built. Once the house is finished, it catches any water that slips past the outer roofing material. If it is damaged or degraded, small hidden leaks can reach the wood deck below.

6. Ice and water barrier

An ice and water barrier is a self adhered sheet laid over the decking. The Asphalt Roofing Manufacturers Association explains this self adhered underlayment helps prevent leaks from wind driven rain and ice dams at eaves, valleys and flashings.

Roofers stick this barrier to the lowest edges of the roof. They also apply it inside any roof valleys where two slopes intersect to form a trough. ARMA says it helps prevent leaks from wind driven rain and ice dams, so ask a licensed roofer whether your roof needs one.

7. Flashing

Flashing consists of thin sheets of metal bent to cover joints, edges and penetrations on the roof. You will find it wrapped around brick chimneys, lining roof valleys, and tucked against vertical walls. A type of flashing called drip edge runs along the roof edges to guide water toward the gutters.

The metal bridges the gap between the roofing surface and an obstacle. It directs water over the vulnerable seam instead of into it. Flashing fails when the metal rusts through, high winds bend it out of shape, or the sealant around it dries out and cracks.

8. Eaves, fascia and soffit

The roof eave is the lower edge of the roof that hangs over the exterior wall. This overhang directs heavy rain away from the siding and foundation. The fascia is the long, straight board nailed vertically to the exposed ends of the rafters. The soffit is the flat material covering the underside of the eave, bridging the gap from the fascia back to the house wall.

Fascia boards hold the gutters and trim out the roofline. Soffit panels seal the overhang to keep birds and squirrels out of the attic. When gutters clog and overflow, water runs behind the fascia and rots the wood. You can spot this failure by looking for peeling paint, dark water stains or sagging gutters along the edge of the eave.

9. Roof vents

Roof vents allow outside air to flow passively through an unfinished attic. Cool outside air enters through intake vents cut into the soffit. That air warms up, rises, and exits through exhaust vents placed higher up near the ridge. The Department of Energy states that upper vents should sit at least 3 feet above the soffit or eave vents to create this draft.

Balanced intake and exhaust helps remove attic condensation, according to ARMA. The same DOE page, citing IRC section R806.1, gives a minimum net free attic ventilation area of 1/300 of the attic area where high and low vents are balanced, so check the requirement with your local building department. When ventilation fails or gets blocked by poor attic insulation jobs, trapped moisture can build up. ARMA says proper ventilation helps extend the service life of an asphalt shingle roof.

How the roof layers shed water

Every part of a roof relies on gravity and overlap to keep the living space dry. The structural frame pitches the surface downward. Each protective membrane and metal sheet is applied from the bottom up. This is meant to keep the upper piece overlapping the lower piece.

Water hitting the ridge runs down the outer surface, crosses the metal flashing, drops over the drip edge at the fascia and lands safely in the gutter. Any disruption to this overlapping system creates a direct path for water. The failure of one small piece of flashing can compromise the decking and framing beneath it.

Because walking a pitched roof is risky, check these nine parts from the ground with binoculars. You can also inspect the bare framing and the underside of the decking from inside the attic with a bright flashlight.