Common Roof Structures

Common Roof Structures

Common roof structures are the recurring geometric forms — gable, hip, gambrel, mansard, shed, flat and their variants — that define how a roof sheds water and snow, resists wind, encloses space and is framed. Shape is chosen alongside pitch, span, climate and the intended Roofing Materials, and it has a measurable effect on long-term Roof Integrity: the same covering on the same house performs differently under hurricane winds depending on whether the roof is hipped or gabled, and the same snowfall produces different loads on a steep gable than on a low-slope deck.

This article describes the geometry, typical pitch range, wind, snow and drainage behaviour, framing implications and regional prevalence of each common shape, with reference to the structural provisions of the International Residential Code (IRC) and the National Building Code of Canada (NBC) as adopted provincially. Style-by-style descriptions of pitched roofs are expanded at Types of Sloped Roofs; the members that carry the loads are covered at Roof Structure; slope measurement is at Roof Pitch.

Factors that distinguish roof structures

• Pitch — expressed as inches of rise per 12 inches of run. Pitch governs which coverings are permitted, how fast water leaves the surface, and how much snow the roof retains.
• Span and support — wide, open plans push the design toward trusses, ridge beams or intermediate bearing walls.
• Wind exposure — building codes divide every roof into field, perimeter and corner zones with different design pressures; on steep roofs the highest suctions occur at eaves, rakes, corners and the ridge, and the pattern changes with geometry (for example gable versus hip).
• Snow and rain — codes tie the design snow load to geographic ground snow data and to the roof's shape and slope.
• Drainage — pitched shapes drain to eaves and gutters; flat and butterfly shapes must be engineered to move water to internal drains or scuppers without ponding.
• Compatibility — some shapes accommodate Green Roofs, Cool Roofs, solar arrays and rooftop HVAC far more readily than others.

Design loads that shape the choice

In the United States, the IRC bases wind design on the ultimate design wind speed mapped for the site and, for loads beyond its prescriptive tables, on ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. The IRC's prescriptive framing chapters apply where the ground snow load is 70 pounds per square foot (3.35 kPa) or less; heavier snow regions require engineered design. Minimum roof live loads for construction and maintenance fall with steepness: 20 psf for flat roofs and slopes under 4:12, 16 psf from 4:12 to under 12:12, and 12 psf at 12:12 and steeper, for tributary areas up to 200 square feet.

In Canada, Part 9 of the NBC (adopted, for example, as the British Columbia Building Code) calculates the specified roof snow load from the 1-in-50-year ground snow load: S = C_b·S_s + S_r, where the basic roof factor C_b is 0.45 for roofs whose entire width is 4.3 m or less and 0.55 for all other roofs, S_s is the ground snow load and S_r the associated rain load, with a floor of 1 kPa; bow-string, arch and semi-circular trusses with long unsupported spans are treated separately. Shape enters through drifting and sliding: valleys, lower roofs beside taller walls, and the leeward side of gables and gambrels accumulate more snow than the uniform case.

Major roof shapes

Gable roof

Two planes rise from opposite eaves to a central ridge, leaving triangular gable ends. Pitches from about 4:12 to 12:12 are typical; steeper "Gothic" gables exist in historic buildings. The shape drains and sheds snow well, is simple to frame with common rafters or standard trusses, and ventilates naturally from soffit to ridge. Its weakness is wind: the flat gable end wall receives direct pressure, the rake edge sees high suction, and the roof relies on gable-end bracing. IBHS field investigations after Hurricane Harvey found gable roofs had higher frequencies of underlayment and decking damage than hip or gable/hip combination roofs, although shape did not change the loss of the roof covering itself. Gables are the dominant residential form across Canada and the northern and central United States.

Hip roof

All four sides slope to the eaves, meeting at hip rafters and a short ridge (or a point, on a square "pyramid" hip). Typical pitches are 4:12 to 8:12. The absence of tall end walls and the continuous eave make the shape aerodynamic; after Hurricane Michael, IBHS reported that hip roofs performed better than gable or combination roofs, consistent with earlier investigations. Hips use more lumber and sheathing, and every hip rafter must be deeper than the common rafters it collects — 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. Hips are the standard form in hurricane-exposed regions of the southern U.S. coast and are common in bungalows, prairie-style and many suburban houses across North America.

Dutch gable (gablet)

A hip roof with a small gable set on top of the hip, or a gable whose lower portion is hipped. It combines the wind-resistant eaves of a hip with a gable vent or window at the top. Framing is a hip roof with a short gable-end frame on the hip plane; pitch follows the parent hip roof.

Gambrel roof

Each side has two pitches: a steep lower slope (often 18:12 to 24:12) and a shallow upper slope (roughly 4:12 to 7:12). The break in slope creates near-vertical walls for a loft, which is why the form is standard for barns and "Dutch colonial" houses in the northeastern United States and eastern Canada. The shallow upper plane retains snow and the change in slope collects drifts, so the upper section must be designed for heavier loads; the lower slope sheds quickly. Gambrels are framed with purlins or a knee-brace at the slope break, or increasingly with engineered gambrel trusses.

Mansard roof

A four-sided gambrel: steep, almost vertical lower slopes on all sides, topped by a shallow or flat upper roof. Developed in France and popularized in Second Empire architecture, it maximizes usable upper-floor space and appears in 19th-century civic and commercial buildings and modern mid-rise projects. The low-slope top is effectively a flat roof and needs membrane roofing and drains, while the steep faces take shingles, slate or metal. Snow drifts against the upper roof's edge and ice at the transition are recurring maintenance issues.

Shed (skillion, lean-to) roof

A single plane sloping in one direction, typically 1:12 to 6:12, with one high wall and one low wall. It is the simplest roof to frame — rafters bear on two walls with no ridge — and it drains to a single eave, which concentrates gutter and downspout capacity on one side. Shed roofs are common on additions, porches, contemporary houses and clerestory designs; a pair of offset sheds produces the clerestory or "split-shed" roof.

Saltbox roof

An asymmetrical gable with one long slope, usually the result of a lean-to addition on a colonial New England house. Ridge is off-centre; the long slope is often shallower and retains more snow, while the short slope is steep. Framing follows gable practice with unequal rafter lengths; the ridge must be checked as a beam where rafter ties are not continuous.

Butterfly roof

Two planes slope downward toward a central valley, the inverse of a gable. It admits high clerestory windows and channels rain to a single collection point, which suits water harvesting, but the central valley concentrates snow and debris and any blockage backs water up against the building. Structural design centres on the valley beam and on drains or scuppers sized for the full roof area; ice dams form readily at the valley in cold climates, so it is rare in Canada outside mild coastal areas.

Flat and low-slope roofs

"Flat" roofs are never truly level; the IRC requires built-up, modified bitumen, thermoset (EPDM) and thermoplastic (TPO, PVC) membranes to have a design slope of at least ¼ inch per foot (1/4:12) for drainage, with coal-tar built-up roofs permitted down to 1/8:12. Slope is created by tapered insulation or sloped framing toward internal drains, scuppers or edge gutters. Flat roofs dominate Commercial Roofing because they simplify equipment placement, allow Green Roofs and solar arrays, and cost less per square foot of enclosed space; they are also common on urban houses in mild climates. They carry the full snow load with no shedding, are vulnerable to ponding when drains clog, and require the deck to be checked for rain-on-snow and drift loads beside higher walls. Compatible systems include TPO Roofing, EPDM Roofing, PVC Roofing, Modified Bitumen Roofing and Built-Up Roofing; see Flat Roofs and Roof Drainage.

Dome, barrel and curved roofs

Curved roofs — domes, barrel vaults, bow-string trusses and modern curved metal panels — carry loads partly in compression along the curve. They appear on religious, civic, sports and agricultural buildings and on contemporary houses using curved glulam or steel. Snow slides from the steep lower flanks but accumulates unevenly on the crown, and the Canadian code singles out bow-string, arch and semi-circular trusses for special snow treatment. Coverings must accommodate curvature: standing-seam metal, membranes, shingles on shallow curves, and specialty tile or copper on domes.

Combination roofs

Most houses combine shapes: a hip main roof with gabled dormers, a gable with a shed porch, intersecting gables forming valleys, or a cross-hip plan. Combinations improve appearance and daylighting but add valleys, dead valleys behind chimneys and dormers, and drift zones. Every valley is a concentrated flow path that needs metal or membrane lining, and every roof-to-wall intersection needs step flashing and kick-out flashing. Wind performance tends toward the weaker component: in the Hurricane Harvey study, gable/hip combinations performed between pure hips and pure gables.

Comparison of common roof shapes

Shape	Typical pitch (rise:12)	Wind behaviour	Snow and drainage	Framing complexity	Typical coverings
Gable	4–12	Gable ends exposed; higher deck/underlayment damage than hip	Sheds well; drifts on leeward slope	Low (common rafters or trusses)	Asphalt Shingles, Metal Roofing, Wood Shingles and Shakes
Hip	4–8	Best of the pitched forms	Sheds on all sides; short ridge	Moderate (hip and jack rafters)	Shingles, Tile Roofing, Slate Roofing
Dutch gable	4–8	Similar to hip	Sheds well	Moderate–high	Shingles, metal
Gambrel	18–24 lower / 4–7 upper	Broad upper face and end walls exposed	Upper slope retains snow; drift at break	Moderate (purlin or truss)	Shingles, metal
Mansard	Near-vertical lower / low-slope top	Steep faces shield the top	Top behaves as flat roof	High	Slate, shingles, metal on faces; membrane on top
Shed	1–6	High wall exposed; large single plane uplift	Drains to one eave	Lowest	Metal, membrane, shingles above 2:12
Saltbox	Unequal gable slopes	As gable	Long slope holds snow	Low–moderate	Shingles, wood
Butterfly	2–6 toward centre	Edges lift; central valley sheltered	Concentrated valley; ice-dam risk	High (valley beam, drains)	Membrane, standing-seam metal
Flat / low-slope	¼–2	Perimeter and corner suction dominate	Full snow load; ponding risk	Low (joists) but drainage design critical	Single-Ply Roofing Membranes, Modified Bitumen Roofing, Built-Up Roofing
Dome / curved	Variable	Streamlined	Uneven accumulation on crown	High (arches, curved members)	Metal, membrane, copper

Framing implications

Shape dictates the members required. A gable or saltbox needs a ridge and continuous ceiling joists or rafter ties; where those ties are absent — as in vaulted rooms — the IRC requires the ridge to be a beam supported at each end, and the same rule applies wherever the rafters cannot be tied. Hips and Dutch gables add hip rafters, jack rafters and, on wide plans, hip trusses; gambrels and mansards need support at the slope break; butterfly roofs need a valley beam sized for the full width; flat roofs use joists or open-web steel joists with sloped framing or tapered insulation above. Trusses are available for almost every shape — scissor, attic, gambrel, hip-set and mono (shed) profiles — and shift most of the engineering to the truss manufacturer. The deck, whether plywood, OSB, plank or steel, ties the planes together and is discussed at Roof Deck; see Roof Structure for members, spans and connectors.

Pitch and covering compatibility

Minimum slopes in the IRC set the floor for each covering: asphalt shingles need 2:12 (with two layers of underlayment below 4:12), clay and concrete tile 2½:12, wood shingles and shakes 3:12, slate 4:12, and membranes ¼:12. Steeper is generally better for shingles, tile and slate, while metal panels and membranes serve the low end. Steep-slope roofs also fared better in high wind: IBHS found lower rates of underlayment and decking damage on steep roofs than on moderate- and low-slope roofs after Harvey. Pitch categories and measurement are covered at Roof Pitch and How to Measure Roof Pitch.

Regional prevalence

Gables dominate wherever snow is the governing load — Canada, New England, the Upper Midwest and the mountain west — because they shed efficiently and frame cheaply. Hips prevail along the Gulf and Atlantic hurricane coasts and in much of the Sun Belt, where wind governs and attic ventilation through soffits is prized. Gambrels and saltboxes are regional signatures of the Atlantic colonies; mansards mark Second Empire streetscapes in Quebec, Ontario and the northeastern U.S. Flat roofs are nearly universal for commercial and institutional buildings and, in the arid southwest and dense city cores, for houses as well. In prairie cities such as Calgary, gable and hip forms coexist, and the combination of chinook freeze-thaw cycles, hail and wind makes both shape and covering choice consequential — see Calgary Weather and Your Roof and Best Roof Types for Cold Climates.

Structural and maintenance considerations

• Every roof must be designed for its site's wind, snow, rain and, where applicable, seismic loads; prescriptive tables cover ordinary houses, and engineering is required outside their limits.
• Valleys, drift zones and low roofs beside tall walls need heavier framing or more frequent snow removal.
• Low-slope decks must maintain positive drainage; sagging joists or clogged drains create ponding that accelerates membrane failure.
• Roof shape changes during renovation — adding dormers, raising a gable to a gambrel, or converting a hip to a gable — alter load paths and normally require a permit and engineered drawings; see Roofing Building Codes and Permits.
• Wind-resistant detailing matters most at the zones the codes single out: eaves, rakes, corners, ridges and hips.

Frequently Asked Questions

Which roof shape is best for high winds?

Hip roofs. Their sloped faces on all sides eliminate the exposed gable end wall, and post-hurricane surveys by IBHS after Harvey and Michael found hip roofs suffered less decking and structural damage than gable or combination roofs. Good connections — sealed decks, ring-shank nails and uplift straps — matter at least as much as shape.

What roof shape is best for heavy snow?

Steep gable and hip roofs shed snow and receive lower design live loads than low-slope roofs. Flat, shallow-upper gambrel and butterfly roofs retain snow and collect drifts. In Canada the specified roof snow load is derived from the local ground snow load using the code formula, so any shape can be built safely if framed for the calculated load.

What is the minimum slope for a flat roof?

The IRC requires a design slope of at least ¼ inch per foot for built-up, modified bitumen, EPDM and TPO/PVC membranes, and 1/8 inch per foot for coal-tar built-up roofing. Slope is usually created with tapered insulation toward drains or scuppers so that water does not pond on the membrane.

What is the difference between a gambrel and a mansard roof?

Both have a steep lower slope and a shallow upper slope on each side. A gambrel has two sides and vertical gable ends, as on a barn; a mansard has the double slope on all four sides with a low-slope or flat top, as on Second Empire buildings. Both maximize upper-floor space and both require drainage attention at the slope break.

Can a roof shape be changed on an existing house?

Yes, but it is structural work. Converting a hip to a gable, adding dormers or raising a gambrel changes the ridge support, rafter ties, bracing and wind load path, and the new framing must meet current span and uplift requirements. A permit and, in most cases, engineered drawings are needed; see Roofing Building Codes and Permits.

Internal Links for Further Exploration

• Types of Sloped Roofs
• Roof Pitch
• Roof Structure
• Sloped Roofs
• Flat Roofs
• Roofing Materials
• Roof Deck
• Roof Drainage
• Green Roofs
• Cool Roofs
• Commercial Roofing
• Residential Roofing
• Roof Integrity
• EPDM Roofing
• TPO Roofing
• PVC Roofing
• Modified Bitumen Roofing
• Built-Up Roofing
• Single-Ply Roofing Membranes
• Tile Roofing
• Slate Roofing
• Metal Roofing
• Asphalt Shingles
• Wood Shingles and Shakes
• Best Roof Types for Cold Climates
• Calgary Weather and Your Roof
• Roofing Building Codes and Permits