Construction · Lesson 5 of 13Intermediate10 min read

Roofs

A roof carries snow and wind, caps the attic, and sheds every drop of rain that lands on it. It’s built in layers, each lapped over the one below, so water always runs down and out.

Rafters or trusses

A roof’s frame is made of rafters or trusses. Rafters are sloped boards that run from the top of the wall up to a ridge at the peak. A truss is a frame made entirely of triangles: top chords along the roof slope, a bottom chord that also serves as the ceiling joist, and webs between them. As Lesson 2 showed, triangles can’t rack, so a truss turns the roof’s loads into tension and compression in its members.

Truss shapes vary. A raised-heel truss stands taller where it sits on the wall, leaving room for full-depth attic insulation all the way out to the eaves, which helps prevent ice dams. Trusses can also span long distances, which can remove the need for some interior load-bearing walls.

Every member of a truss is part of its design, so cutting or notching even a small web can weaken the whole truss. Repairs or changes need the truss designer or an engineer.

Pitch: rise over run

Roof slope, or pitch, is written as inches of rise for every 12 inches of run: a “6 in 12” roof climbs 6 inches for every foot it goes across. The angle and the length of a rafter come straight from right-triangle geometry.

OSHA sorts roofs by slope for its fall-protection rules: a low-slope roof is 4 in 12 or less, and anything steeper is a steep roof. Steep-slope roofs are typical on single-family homes; low-slope roofs are most common on commercial, industrial and apartment buildings.

angle = tan⁻¹(rise ÷ 12) rafter length = run × √(12² + rise²) ÷ 12

Choose a pitch and the building’s width. The rafter length is measured along the slope from the ridge to the tail of the overhang; real layout also allows for the ridge board and the seat cut where the rafter sits on the wall.

The layers, from the deck up

Roof deck: sheathing panels nailed to the rafters or trusses.
Ice and water membrane: in climate zone 5 and colder, a self-sealing membrane goes along the eaves, from the roof edge to at least 2 feet inside the plane of the interior wall, lapped over the metal drip edge. If an ice dam backs water up under the shingles, the membrane keeps it out of the attic.
Peel-and-stick in valleys and at walls: the valleys and the places where a roof meets a wall are among the most vulnerable spots, so they get a membrane too.
Underlayment: roofing felt or another underlayment over the rest of the deck, a second line of defense under the shingles.
Drip edge: metal at the eaves and rakes that guides water off the edge.
Roofing: asphalt shingles are common on homes; metal, tile and slate are others. Each course of shingles laps over the one below.

Outside air comes in at the soffit, runs up the channel the baffle keeps open, and leaves at the ridge. Sources: DOE Building Science Education Solution Center (baffles, eave membrane, water-managed roof); OSHA (roof slope terms).

Flashing: where the roof meets something else

Flashing is sheet metal or membrane that bridges a joint water could otherwise get into. Step flashing goes wherever a roof meets a wall, such as a dormer: corrosion-resistant pieces at least 4 inches up the wall and 4 inches onto the deck, woven in shingle fashion so each upper layer laps over the one below. At the bottom of a roof-wall joint, a kick-out flashing (diverter) turns the water out into the gutter instead of letting it run down inside the wall. Chimneys, vent pipes and every other penetration get flashing too.

The edges of a roof take the hardest beating from wind: they see the highest pushes and pulls of air pressure, so roof edges in high-wind areas get extra attention.

Roof parts and what they do
Part	What it does	Signs of trouble
Rafters or trusses	Carry the roof’s weight, snow and wind down to the walls.	Sagging ridge or roof plane; cut or cracked members.
Roof deck	Sheathing that the roofing is nailed to.	Soft spots, stains, rot from long-term leaks.
Ice and water membrane	Seals the eaves, valleys and roof-wall joints against backed-up water.	Leaks at the eaves during ice-dam season.
Underlayment	A water-resistant layer under the roofing.	Shows through where shingles are missing.
Shingles or metal	Shed water; each course laps over the one below.	Curling, missing or broken shingles; granules in gutters.
Step and kick-out flashing	Seal roof-to-wall joints and steer water into the gutter.	Stains on walls below a roof-wall joint; rot.
Drip edge	Guides water off the roof edge.	Rot at the fascia and roof edge.
Soffit and ridge vents	Let outside air in at the eaves and out at the peak.	Frost or mold in the attic; ice dams.
Baffles	Keep an air channel open above the insulation at the eaves.	Insulation blocking the soffit vents.
Gutters and downspouts	Collect roof water and carry it away from the foundation.	Overflowing, sagging, water at the foundation.

Attic ventilation

A vented attic breathes. Outside air comes in through vents in the soffits under the eaves and leaves through a vent along the ridge. In each rafter bay with a soffit vent, a rigid chute called a baffle keeps an air gap between the insulation and the roof deck and steers air up toward the ridge.

Without good ventilation, moisture can build up in an attic, causing mold, ruining insulation and damaging wood. Bathroom fans, kitchen range hoods and clothes dryers must vent all the way outdoors, never into the attic: that is a bad practice that can lead to mold, and it may violate local codes.

Ice dams

An ice dam is a ridge of ice at the edge of a roof that keeps melting snow from draining off. It takes three things together: snow on the roof, an upper roof that’s above 32 °F, and lower edges that stay below freezing. Heat leaking from the house warms the upper roof, snow melts there, and the water refreezes when it reaches the cold eaves. Water that backs up behind the dam can work under the shingles and leak into walls, ceilings and insulation, and the dampness can lead to mold.

An ice dam needs snow on the roof, an upper roof above 32 °F, and eaves below freezing. Air sealing the ceiling, then insulating, is the long-term fix. Source: University of Minnesota Extension.

Preventing ice dams

University of Minnesota Extension says the long-term fix starts inside: make the ceiling airtight so warm, moist air can’t leak into the attic, then consider adding insulation. Natural ventilation helps keep the roof evenly cold, but once heat loss is cut, only a little is needed, and Extension does not recommend mechanical attic ventilation as a solution to ice dams. A weatherization contractor can find the leaks with a blower door test and an infrared camera.

In the short term, removing snow takes away one ingredient, but a roof rake or push broom can damage roofing, and anyone working on a roof in winter risks injury. Extension suggests hiring professionals for that work.

StopRoof work is fall work

OSHA calls falls the leading cause of death in construction: in 2024, 389 of 1,034 construction deaths were falls to a lower level. From 2003 to 2013, falls from roofs alone accounted for about a third of construction fall deaths. Workers on roofs and other surfaces 6 feet or more above a lower level must be protected by guardrails, safety nets or personal fall arrest systems. Leave roof work to roofers with the training and equipment, and look at your roof from the ground.

NoteSigns of water getting into the roof edge

The Department of Energy’s Guide to Durable Attics lists warning signs: warped or rotted soffits or fascia boards, bubbled paint on the fascia or soffits, worn or warped shingles at the roof edge, wet attic insulation near the eaves, mold or mildew on attic surfaces, and warped or rotting attic wood.

Check yourself

Tap an answer to check it, or open “Show the answer.”

How much does a 6-in-12 roof rise for each foot of run?
Show the answer
6 inches. Pitch is rise per 12 inches of run, so 6 inches.
What does OSHA call a roof steeper than 4 in 12?
Show the answer
A steep roof. OSHA: low-slope is 4 in 12 or less; steeper than that is a steep roof.
What causes an ice dam?
Show the answer
Heat from the house melting snow on the upper roof, and the water refreezing at the cold eaves. Snow, a warm upper roof and cold eaves together make an ice dam.
Where should a bathroom exhaust fan vent?
Show the answer
All the way outdoors. DOE: bathroom fans, range hoods and dryers should vent outdoors, never into the attic.
What is the job of a kick-out flashing?
Show the answer
Steer water from a roof-wall joint into the gutter. Without it, water running down the roof-wall joint can get inside the wall.

Go deeper

University of Minnesota Extension: Dealing with and Preventing Ice Dams University of Minnesota Extension’s full ice dam guide.
OSHA: Protecting Roofing Workers (OSHA 3755) OSHA’s guide to protecting roofing workers.
U.S. Department of Energy: Guide to Durable Attics (DOE/EE-0339, 2010) DOE’s Guide to Durable Attics: ventilation, air sealing and warning signs.
U.S. DOE Building Science Education Solution Center: Step Flashing at Roof-Wall Intersections DOE building science: step flashing in detail.
Lesson 6: The Building Envelope Next: the walls, windows and doors of the envelope.

Sources

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A free course from Teddy Rook Book. It explains how houses are built in general and follows U.S. sources; the local building code, the building department and the manufacturer’s instructions are the final word.