Roof Structure

Roof Structure

The roof structure is the framework of rafters or trusses, ridge members, ties, purlins and sheathing that carries the weight of the roof covering and everything that lands on it — snow, wind, workers, equipment — down to the walls and foundation. It fixes the roof's shape and slope, provides the surface to which Roofing Materials are fastened, and is the part of the roof that must last the life of the building. A sound structure is the foundation of Roof Integrity: coverings can be replaced every few decades, but sagging rafters, split trusses or delaminated sheathing compromise every layer above them.

This article covers the components of wood-framed roof structures in houses and small buildings, the loads they resist, how spans and connections are determined under the International Residential Code (IRC) and Part 9 of the National Building Code of Canada (NBC), and the signs that call for a structural engineer. Roof shapes are described at Common Roof Structures; the sheathing layer in detail at Roof Deck; steel and concrete systems for Commercial Roofing are noted briefly below.

Load path and design loads

Every roof structure is designed for a set of loads defined by the building code:

• Dead load — the permanent weight of framing, sheathing, underlayment, covering and fixed equipment. Asphalt shingles on plywood typically produce a light roof; clay tile, slate or a ballasted or green roof can multiply the dead load several times, which is why re-covering with a heavier material requires a structural check.
• Live load — temporary loads from maintenance, construction and occupancy. The IRC minimum roof live load is 20 pounds per square foot for flat roofs and slopes below 4:12, 16 psf from 4:12 to below 12:12, and 12 psf at 12:12 or steeper, for tributary areas of 200 square feet or less.
• Snow load — based on mapped ground snow. The IRC's prescriptive framing tables apply where the ground snow load is 70 psf (3.35 kPa) or less; beyond that, engineered design is mandatory. In Canada the specified roof snow load is S = C_b·S_s + S_r, using the 1-in-50-year ground snow and rain loads for the site and a basic roof factor of 0.45 for roofs no wider than 4.3 m or 0.55 otherwise, never less than 1 kPa.
• Wind load — both downward pressure on the windward slope and, more critically, suction (uplift) on leeward slopes, eaves, rakes and corners. The IRC uses the site's ultimate design wind speed and, beyond its prescriptive limits, ASCE 7.
• Seismic and other loads — earthquake forces, concentrated loads from HVAC units and solar arrays, and ponding on low-slope roofs.

Wood design also allows for duration of load: because wood carries short-term loads better than permanent ones, span tables multiply the benchmark design values by 1.15 for snow and 1.25 for seven-day construction loads, adjustments that are already built into the published tables.

The load path runs from covering to sheathing, sheathing to rafter or truss, rafter to ridge and wall plate, plate to studs and foundation. Every link — including the nails and metal connectors — must be continuous; wind failures almost always start at a broken link rather than in a member that was too small.

Framing members

Rafters

Rafters are sloped members, usually 2×6 to 2×12 lumber at 12, 16 or 24 inches on centre, cut and installed on site ("stick framing"). Common rafters run from the wall plate to the ridge; hip and valley rafters run diagonally at the intersections of planes and collect jack rafters. The Canadian code requires hip and valley rafters to be at least 50 mm deeper than the common rafters and at least 38 mm thick, and requires opposing rafters to be located directly opposite each other and tied at the peak, or offset by their own thickness and nailed to a ridge board at least 17.5 mm thick.

Ridge board versus ridge beam

A ridge board is a non-structural nailer at the peak; the IRC requires it to be at least 1-inch nominal thickness and no shallower than the cut end of the rafter. It works only when the rafters are restrained from spreading by ceiling joists or rafter ties. Where those continuous ties are absent — vaulted ceilings, split-level rooms — the ridge must be a ridge beam designed by accepted engineering practice and supported on a wall or column at each end.

Ceiling joists and rafter ties

Rafters on a ridge board push outward at the walls. Ceiling joists that run parallel to the rafters and are nailed to them at the heel joint resist that thrust; where joists run the other way or sit higher in the attic, rafter ties perform the same job. The nailing required at the heel joint rises steeply with load: the IRC's table calls for 3 to 34 16d common nails per connection depending on rafter slope, spacing, ground snow load and roof span. Raising the ties above the plate reduces the allowable rafter span, and the code provides adjustment factors for that condition.

Collar ties

Collar ties are short horizontal members in the upper third of the attic that hold opposing rafters together at the ridge under wind uplift. The IRC requires at least 1×4 lumber at no more than 4 feet on centre, and permits 1¼-inch by 20-gauge ridge straps to replace them. Under the Canadian code, 38 × 89 mm collar ties on slopes of 1 in 3 or steeper may also be counted as intermediate support that shortens the rafter span, provided ties longer than 2.4 m are braced at mid-length. Collar ties are not a substitute for rafter ties; they resist different forces.

Purlins, struts and dwarf walls

On long rafter runs, a purlin — a horizontal member under the rafters — supported by struts to a bearing wall below reduces the effective span. Purlins are also the primary framing in many metal-building and agricultural roofs, where they span between trusses or rigid frames and carry the panels directly. Dwarf (knee) walls under rafters serve the same purpose in one-and-a-half-storey houses.

Trusses

Metal-plate-connected wood trusses are factory-built triangulated assemblies of top chords, bottom chords and webs. The IRC requires them to be designed to ANSI/TPI 1, with design drawings prepared by a registered design professional where the jurisdiction requires, and braced during and after erection according to those drawings. Trusses span farther than sawn rafters for the same lumber and install in hours, but their webs fill the attic and they must not be cut, notched or loaded (for example with a water heater or HVAC unit) beyond the design without verification from the designer. In Canada, trusses not designed under Part 4 must meet the prescribed test-load criteria of Part 9.

Attribute	Site-built rafters	Engineered trusses
Design responsibility	Builder, using code span tables or an engineer	Truss manufacturer's engineer (ANSI/TPI 1)
Typical span	Limited by lumber size; 2×10 Douglas fir-larch No. 2 at 12 in. o.c. spans about 26 ft at 20 psf live and 10 psf dead load	Commonly 30–60 ft; longer with special profiles
Attic space	Open; usable for storage or conversion	Filled with webs unless attic or room-in-attic trusses are specified
Site labour	High; skilled cutting of hips, valleys and bird's-mouths	Low; crane-set, then bracing and sheathing
Flexibility	Easy to modify for dormers and vaults with engineering	Modification prohibited without designer approval
Ridge	Ridge board with rafter ties, or ridge beam	No ridge beam needed; loads go to bearing walls
Typical use	Custom homes, additions, complex roofs, older housing stock	Most production housing in the United States and Canada

Spans and deflection

Prescriptive span tables let a builder select rafter size and spacing without calculation. The IRC's Table R802.4.1(1), for a 20 psf live load with no ceiling attached, gives for Douglas fir-larch No. 2 at 12 inches on centre and 10 psf dead load maximum spans of 16 ft 10 in. for a 2×6, 21 ft 4 in. for a 2×8 and 26 ft 0 in. for a 2×10; at 20 psf dead load the same members drop to 14 ft 7 in., 18 ft 5 in. and 22 ft 6 in. Snow-load tables use the same format with higher loads. For other species, grades and loads, the code refers to the American Wood Council's Span Tables for Joists and Rafters and its online calculator, which covers all commercially available species and grades listed in the NDS supplement. Canadian builders use Span Tables 9.23.4.2.-A to -G of the NBC/provincial code, selected by the specified snow load for the site.

Two criteria govern each table entry: strength (bending and shear under the full load) and stiffness (deflection). The IRC limits deflection of rafters steeper than 3:12 with no finished ceiling to L/180, ceilings with gypsum board to L/240, and brittle plaster ceilings to L/360, where L is the span. A 20-foot rafter at L/180 may therefore sag up to 1⅓ inches under design load — acceptable structurally, but visible in a shingle line, which is why many designers use L/240 for roofs regardless. Lumber species and grade matter: southern pine and Douglas fir-larch are stronger and stiffer than spruce-pine-fir, the common Canadian framing lumber, so a Canadian rafter of the same size often spans less.

Decking and sheathing

Sheathing turns individual rafters or trusses into a diaphragm that resists wind and seismic racking and provides the nail base for the covering. Options are:

• Wood structural panels — plywood and oriented strand board (OSB). Both are engineered panels that, although manufactured differently, meet the same performance criteria and are stamped with a span rating. The IRC requires panels to conform to DOC PS 1 or PS 2 or the Canadian standards CSA O325 or O437, with allowable spans taken from the code table or APA's Engineered Wood Construction Guide (Form E30). The NBC lists CSA O121 plywood and related CSA panel standards. Common residential roof sheathing is 7/16-inch OSB or ½-inch plywood at 24-inch framing spacing; heavier tile and slate roofs, and snow-country roofs, often use ⅝-inch panels.
• Panel edge support — where the span rating requires it, unsupported panel edges between framing members are supported by metal H-clips or 38 × 38 mm blocking. APA's installation guidance calls for a minimum of 8d common nails, a 1/8-inch gap at all panel edges and ends to allow for moisture expansion, and code-compliant ventilation of the space below. Panels installed tight buckle when they swell, producing the ridged shingle lines visible on many roofs.
• Lumber sheathing — solid boards on older houses, and spaced (skip) sheathing for wood shingles and shakes, which the IRC prohibits in Seismic Design Category D₂.
• Steel and concrete decks — corrugated steel over open-web joists, and cast or precast concrete, are the norm on Commercial Roofing and are usually engineered under the building code's structural chapters rather than prescriptive tables.

The sheathing layer, its thickness, damage signs and replacement cost are detailed at Roof Deck; the underlayment above it at Roof Underlayment Guide.

Uplift connections

Wind suction tries to lift the roof off the walls. Toe-nailing alone is inadequate in exposed sites, so metal connectors — commonly called hurricane ties or straps — tie each rafter or truss to the top plate, and the plate to the studs. The IRC requires roof assemblies to have uplift resistance per its tables; toe-nailing per the fastening schedule is permitted only where the calculated uplift per rafter or truss does not exceed 200 pounds, or where wind speed does not exceed 115 mph in Exposure B, the pitch is 5:12 or steeper, the span is 32 feet or less and framing is spaced at 24 inches or less. The value of a continuous load path is evident in post-hurricane surveys: IBHS observed damage progressing from roof cover, soffits and fascia above roughly 110 mph to roof decking above 130 mph, structural roof damage above 140 mph and collapse above 150 mph, with the worst outcomes where the deck or roof-to-wall connections gave way. Canadian Part 9 framing likewise shifts from simple nailing to prescribed bracing and fastening as the site's hourly wind pressure and seismic acceleration rise.

Integration with roofing systems

• Metal Roofing, Tile Roofing, Clay and Concrete Tiles and Slate Roofing need framing and sheathing checked for their dead load and, for tile and slate, solid sheathing rather than spaced boards.
• Single-Ply Roofing Membranes such as TPO Roofing and EPDM Roofing require a deck with positive slope and adequate stiffness to avoid ponding.
• Green Roofs, ballasted systems and solar arrays add sustained dead load and point loads that must be verified against the original design.
• Added Roof Insulation changes attic temperature and moisture conditions; Roof Ventilation must be maintained so that sheathing stays dry.

Signs of structural problems and when to involve an engineer

Warning signs include a sagging or wavy ridge line, dished roof planes between rafters, cracked or split rafters and truss chords, separated or corroded truss plates, cut or removed webs, water-stained or delaminated sheathing, rafters pulling away from the ridge, walls bowing outward at the eaves, and doors or windows on the top floor that suddenly bind. Any of these warrants inspection — see How to Inspect Your Roof and Post-Storm Roof Damage Assessment.

A structural engineer should be engaged when the building is outside prescriptive limits (ground snow over 70 psf, high wind or seismic zones, spans beyond the tables); when trusses have been altered or damaged; when a heavier covering, solar array, rooftop unit or green roof is proposed; when ceilings are to be vaulted or rafter ties removed; after fire, impact or significant water damage; and whenever a ridge beam, hip beam or purlin needs sizing. Permit requirements for structural work are outlined at Roofing Building Codes and Permits.

Canadian context

Houses and small buildings in Canada are framed under Part 9 of the NBC as adopted by each province, which provides the same kinds of prescriptive tools as the IRC — span tables selected by specified snow load, fastening schedules, hip and valley rafter rules, collar tie and truss provisions, sheathing standards and H-clip edge support — and refers designs outside those limits to the Canadian Wood Council's Engineering Guide for Wood Frame Construction or to Part 4 engineering. Ground snow loads vary widely, from near 1 kPa on the prairies to several kPa in mountain and coastal communities, so identical house plans require different rafter sizes in Calgary, Whistler and St. John's. See Calgary Weather and Your Roof for the local load environment.

Frequently Asked Questions

What are the main components of a roof structure?

Rafters or trusses form the sloped frame; a ridge board or ridge beam joins them at the peak; ceiling joists or rafter ties keep the walls from spreading; collar ties hold the peak together under uplift; purlins and struts shorten long spans; sheathing of plywood or OSB ties everything into a diaphragm; and metal connectors complete the load path to the walls.

What is the difference between rafters and trusses?

Rafters are individual sloped members cut and assembled on site, leaving an open attic and allowing easy modification. Trusses are factory-engineered assemblies designed to ANSI/TPI 1 that span farther and install faster but fill the attic with webs and cannot be cut or altered without the designer's approval. Trusses dominate production housing; rafters remain common in custom and older homes.

How far can a 2×8 rafter span?

It depends on species, grade, spacing and load. Under the IRC table for 20 psf live and 10 psf dead load, a Douglas fir-larch No. 2 2×8 at 12 inches on centre spans 21 ft 4 in.; at 24 inches or under snow loads the span is shorter. Spruce-pine-fir, standard in Canada, spans less, and Canadian builders use the NBC Part 9 tables for their local snow load.

How much weight can a roof structure hold?

Roofs are designed for the code loads at the site: a minimum live load of 12–20 psf depending on slope, plus the local snow load and the dead weight of the roofing. Heavy coverings, ballast, green roofs, solar arrays and rooftop equipment count against that capacity, and a structural engineer should verify the framing before such loads are added.

How do I know if my roof structure is failing?

Look for a sagging or wavy ridge, dished planes between rafters, cracked rafters or truss chords, loose truss plates, sheathing that feels spongy underfoot, walls bowing at the eaves and upper-floor doors that start to stick. Any of these, or a roof that has been overloaded by snow or altered without engineering, calls for a prompt professional structural inspection.

Related Pages

• Common Roof Structures
• Roof Deck
• Roof Anatomy and Parts Explained
• Roof Pitch
• Roofing Materials
• Roof Integrity
• Roof Underlayment Guide
• Roof Insulation
• Roof Ventilation
• Sloped Roofs
• Flat Roofs
• Residential Roofing
• Commercial Roofing
• Green Roofs
• Solar Panels and Your Roof
• TPO Roofing
• EPDM Roofing
• Metal Roofing
• Tile Roofing
• Slate Roofing
• Clay and Concrete Tiles
• How to Inspect Your Roof
• Post-Storm Roof Damage Assessment
• Roofing Building Codes and Permits
• Calgary Weather and Your Roof