Roof Framing & Roofing

The roof is a building's first line of defense against weather. It must support its own weight plus snow, wind, and construction loads; shed water completely; withstand decades of ultraviolet radiation, thermal cycling, and wind; and complement the building's aesthetic design. Residential roof construction involves two distinct systems: the structural framing (rafters or trusses that carry the loads) and the roofing system (sheathing, underlayment, and shingles or other covering that sheds water).

Stick-Framed (Rafter) Roofs

Stick framing (also called conventional framing or rafter framing) builds the roof one piece at a time on site. Each rafter is individually cut and installed. Components:

• Common rafters: The main spanning members, running from the exterior wall (top plate) to the ridge. Rafters are typically 2×8, 2×10, or 2×12 at 16" or 24" o.c., sized based on span, slope, species, and load.
• Ridge board: A non-structural board (typically 1× or 2× lumber, one size deeper than the rafters) at the peak of the roof. The ridge board is a nailing surface and alignment guide for opposing rafters — it does not carry vertical loads. Common rafters lean against each other at the ridge, creating outward thrust at the walls.
• Ridge beam: A structural beam at the peak that actually supports the rafters. Used with cathedral (vaulted) ceilings where there are no ceiling joists to resist the outward thrust. The ridge beam must be supported by posts at each end.
• Ceiling joists: Horizontal members spanning between opposite exterior walls at the wall plate level. Ceiling joists serve two purposes: they support the ceiling finish below, and they act as ties connecting opposite rafters to resist the outward thrust created by the rafter loads. Without ceiling joists (or equivalent ties), the roof would push the walls outward.
• Collar ties: Horizontal members connecting opposing rafters in the upper third of the rafter span. Collar ties are not substitutes for ceiling joists — they do not effectively resist thrust at the plate level. They serve primarily to reduce rafter deflection under wind uplift and to brace the rafters.
• Hip rafters: Diagonal rafters running from the ridge to an exterior corner, forming the hip of a hip roof. Hip rafters are typically double the rafter depth because they span farther and carry loads from jack rafters on both sides.
• Valley rafters: Diagonal rafters running from the ridge to an interior corner where two roof planes intersect (a valley). Valleys are critical waterproofing points.
• Jack rafters: Shortened rafters running from the wall plate to a hip rafter (hip jacks) or from a valley rafter to the ridge (valley jacks).

Birdsmouth cut: A notch cut in the bottom of each rafter where it bears on the wall's top plate. The birdsmouth provides a flat bearing surface and prevents the rafter from sliding off the plate. The horizontal cut (seat cut) bears on the plate; the vertical cut (heel cut or plumb cut) fits against the outside of the wall.

Trussed Roofs

Manufactured roof trusses (discussed in M4 Lesson 4.5) have largely replaced stick framing in production homebuilding. Trusses are designed by the manufacturer, fabricated in a factory, delivered by truck, and set in place by crane or by hand.

Advantages of trusses over rafters:

• Faster installation (a crew can set a house full of trusses in a few hours)
• Engineered for specific loads and spans (optimized material use)
• Can span the full building width without interior bearing walls (open floor plans)
• Consistent quality (factory fabricated)
• Lower cost for simple roof configurations

Advantages of rafters over trusses:

• No lead time required (built from stock lumber)
• Easier to frame complex roof geometries (dormers, intersecting roofs, steep pitches)
• Provide usable attic space (truss web members obstruct the attic)
• Easier to accommodate changes in the field
• Better for small additions and renovations where crane access is limited

Roof sheathing (plywood or OSB, typically 7/16" or 15/32") is nailed over the rafters or trusses to create a continuous structural surface. The sheathing:

• Provides a nailing base for roofing materials
• Acts as a diaphragm for lateral load distribution
• Braces the rafters or truss top chords

Installation: Panels are installed with the long dimension perpendicular to the framing, with end joints staggered. The standard nailing schedule is 8d nails at 6" o.c. on edges and 12" o.c. in the field. In high-wind zones, closer nailing (4" o.c. on edges) is required.

H-clips (panel edge clips) are metal clips inserted between adjacent panels at unsupported edges (between rafters) to prevent differential deflection. Required when panel edges are not supported by blocking.

• Fascia: A vertical trim board attached to the rafter tails (the ends of the rafters extending beyond the wall). The fascia covers the exposed rafter ends, provides a finished appearance, and serves as the attachment surface for gutters. Typically 1×6 or 1×8, wood or composite.
• Soffit: The horizontal surface under the roof overhang (eave). Soffits are typically vinyl, aluminum, or fiber cement panels. Vented soffits contain perforations that allow outside air to enter the attic — a critical component of attic ventilation.

Underlayment

A layer of material installed over the sheathing and under the final roofing — the secondary water barrier. If the roofing above fails (blown shingles, driven rain), the underlayment provides backup protection.

• Asphalt-saturated felt (tar paper): The traditional underlayment. #15 felt (approximately 15 lb per square) is standard for most applications. #30 felt is heavier and more durable, used for steeper slopes and as a substrate for some roofing types.
• Synthetic underlayment: Polypropylene-based sheets that are lighter, stronger, and more tear-resistant than felt. Less slippery when wet. Increasingly the standard in new construction.
• Ice and water shield (self-adhering membrane): A rubberized asphalt membrane that adheres directly to the sheathing, creating a waterproof seal even around nail penetrations. Required by code in cold climates along the eaves (at least 24 inches past the interior wall line) to prevent ice dam leakage. Also installed in valleys, around penetrations (chimneys, vent pipes), and at other leak-prone areas.

Asphalt Shingles

Asphalt shingles are the dominant residential roofing material in North America, used on approximately 75% of all homes. They are economical, widely available, and come in hundreds of colors and styles.

Types:

• Three-tab shingles: Flat shingles with three tabs cut into the exposed portion. The most economical option but thinner and less wind-resistant than architectural shingles. Being phased out by many manufacturers.
• Architectural (dimensional/laminated) shingles: Multiple layers of asphalt laminated together to create a thicker, more dimensional appearance. Better wind resistance (many rated to 110–130 mph), longer warranties (30–50 years or "lifetime"), and the current industry standard.

Installation sequence:

1. Drip edge: A metal L-shaped flashing installed at the eaves (under the underlayment) and rakes (over the underlayment) to direct water away from the fascia and into the gutter.
2. Underlayment: Installed from eave to ridge, overlapping each course by at least 2 inches (4 inches for horizontal laps). Ice and water shield at the eaves in cold climates.
3. Starter strip: A row of shingles (inverted or purpose-made starter strips) along the eave, with the adhesive strip at the edge. This seals the first course of shingles.
4. Shingle courses: Installed from eave to ridge, each course offset (staggered) from the one below to prevent water tracking through alignment. Standard exposure is 5" to 5-5/8" per course. Four to six nails per shingle, placed per manufacturer's instructions (typically 1" above the cutout line). In high-wind zones, six nails per shingle and hand-sealing of the adhesive strip may be required.
5. Ridge cap: Special ridge cap shingles installed along the ridges and hips, overlapping the field shingles.

Metal Roofing

Metal roofing is gaining market share in residential construction due to its durability (40–70 year lifespan), energy efficiency (reflects solar heat), and performance in severe weather (high wind and fire resistance).

Types:

• Standing seam: Panels with raised seams that interlock without exposed fasteners. The most weather-tight metal roof. Concealed clips allow thermal expansion.
• Exposed fastener panels: Corrugated or ribbed panels with screws driven through the face. Lower cost but the exposed fasteners are potential leak points as gaskets deteriorate over time.
• Metal shingles: Individual metal pieces shaped to look like traditional shingles, slate, or tile.

Flashing is sheet metal (aluminum, galvanized steel, copper) or membrane material installed at roof transitions and penetrations to direct water away from vulnerable joints. Flashing is the most critical detail in any roofing system — the majority of roof leaks occur at flashing locations.

Key flashing locations:

• Step flashing: Where a roof meets a vertical wall (sidewall). Individual L-shaped pieces are woven with each shingle course, stepping up the wall.
• Counter flashing: Metal embedded in or attached to the wall surface, overlapping the top of the step flashing. Directs water running down the wall over the step flashing.
• Valley flashing: Metal or ice-and-water shield in roof valleys. Open metal valleys (visible metal) or closed-cut valleys (shingles cover the flashing) are both accepted methods.
• Chimney flashing: A combination of base flashing, step flashing, counter flashing, and a cricket (small ridge built behind the chimney to divert water).
• Pipe flashing (boot): A rubber or metal/rubber collar that fits around vent pipes, sealing the penetration.
• Drip edge: Metal flashing at eaves and rakes directing water into the gutter.

Proper attic ventilation removes moisture and heat, extending roof life and reducing cooling costs. The principle is simple: cool outside air enters at the eaves (through soffit vents), flows up under the roof deck, and exits at the ridge (through a ridge vent) or near the peak (through gable vents or roof vents).

Building codes require a minimum ventilation area of 1 square foot of net free area (NFA) per 150 square feet of attic floor area, reduced to 1:300 if a vapor barrier is installed on the warm side of the ceiling or if the ventilation is balanced between high and low vents.

Ventilation components:

• Soffit vents: Perforated panels or individual round/rectangular vents in the soffit
• Ridge vent: A continuous vent along the ridge, covered by ridge cap shingles. Provides the most uniform exhaust ventilation.
• Insulation baffles (rafter baffles): Rigid foam or cardboard channels installed between rafters at the eave to maintain an air gap above the insulation, ensuring that soffit vents are not blocked by insulation.