The Roof

Before there were walls there were roofs. The first shelters people made were little more than a covering propped against the weather (branches leaned together, hides stretched over a frame, reeds bent into a vault), and in many languages the word for roof still stands for the whole house: to be “under the same roof” is to live together. The Roman writer Vitruvius, imagining the first buildings, describes people copying the nests of swallows with twigs and mud, and eventually learning to make pitched roofs covered with reeds and leaves so that the rain would run off. Seventeen centuries later the abbé Laugier made that primitive hut, with its gable roof on four tree-trunks, the origin of the whole of classical architecture.

The roof is also the part of a building that says most about where it is. A traveller can tell the country from the train window by the roofs alone: the steep slate of northern France, the low red tiles of Tuscany, the flat earth terraces of the Maghreb, the thatch of the English village, the sweeping grey tiles of a Chinese temple, the heavy thatched gables of a Japanese farmhouse. Each is an answer to the same questions (how much rain or snow falls, what materials lie at hand, how wide the span, what the building should mean), and this chapter follows those answers.

What a roof must do

A roof has to shed water, and it has to do it for decades without attention. Water runs downhill; a sloped roof hurries it off before it can find its way through the joints of the covering. The steeper the roof, the faster the water runs and the more the pieces of the covering can overlap, so the rule is simple: the more porous the material and the wetter the climate, the steeper the roof. Snow adds weight (a metre of wet snow weighs several hundred kilograms per square metre), and a steep roof lets it slide off before it becomes dangerous.

A roof must also resist the wind, which lifts shallow roofs as a wing is lifted and pushes against steep ones like a sail; it must throw rain clear of the walls with its eaves; and in hot countries it must shade. Finally it must be held up. Spanning a room with a sloping covering creates two structural problems that every tradition had to solve: the slopes push outward at their feet, and the covering needs a framework of purlins and rafters to lie on.

Pitch, climate and covering

The oldest written rule of roof pitch comes from China. The builder’s chapter of the Kaogong ji, compiled in the Warring States period, says in eight characters that a thatched roof is raised by a third and a tiled roof by a quarter. The commentators have argued ever since about what exactly the fractions measure, but the principle is plain: straw holds water in its fibres and must be steep; fired clay sheds it and can be flatter.

℟Thatch a third, tile a quarter

Raise a thatched roof by a third and a tiled roof by a quarter: the rougher and more absorbent the covering, the steeper the roof must be.

thatch: rise = ⅓ · tile: rise = ¼

Every covering has its lowest workable pitch, and roofers everywhere learned these limits by experience long before building codes wrote them down. Thatch wants 45° or more; small flat clay tiles, hung on laths and overlapping by two-thirds, about 35°; slate somewhat less; the large interlocking tiles of the Mediterranean (whose ancestors are the Greek pan-and-cover tiles) lie comfortably at 15 to 20°; sheets of lead or copper with raised seams can be almost flat. Where the climate is dry enough, the roof can be flat altogether, a terrace of beaten earth or plaster on which people sleep in summer and dry their crops.

℟Each covering has its lowest pitch

Pitch a roof no lower than its covering allows (roughly 45° for thatch, 35° for plain tiles, 20–25° for slate, 15–20° for interlocking tiles, a few degrees for sheet metal) and steeper where snow lies long or rain is heavy.

thatch ≥ 45° · plain tile ≥ 35° · slate ≥ 22½° · pantile ≥ 17½° · metal ≥ 5° · membrane ≈ 1–2°

The steep roofs of northern Gothic churches are the most dramatic result. Over their stone vaults rise timber roofs of 55° to 60° (the pitch of an equilateral triangle, whose sloping sides equal the span), covered in slate or lead. The timber framework of Notre-Dame in Paris, burnt in 2019 and rebuilt to its old form, was nicknamed “the forest”. In the south, where Roman tiles were never abandoned, the Romanesque and Gothic churches of Italy kept low roofs that are hardly visible from the street, and the classical pediment, with a slope of little more than 12°, preserves the pitch of the Greek tiled temple (The height of the pediment).

℟The steep northern roof

Over the vaults of a northern church raise a timber roof of about 55–60°, its section close to an equilateral triangle, so that snow slides off and the slates or tiles overlap generously.

equilateral roof: pitch = 60°, rafter = span

A family of forms

However varied their pitch and covering, roofs are built from a small number of geometries. A single slope makes a shed or lean-to. Two slopes meeting at a ridge make a gable roof, closed at each end by a triangular gable wall. Four slopes meeting along sloping hips make a hipped roof, which protects every wall and needs no gables; on a square plan the hips meet at a point and the roof becomes a pyramid, on a round plan a cone. Combine a hip below with a small gable above and you have the hip-and-gable roof, the favourite of all East Asia. Break each slope into two pitches and you have the gambrel (on two sides) and the mansard (on four). Curve the surface and you have the barrel, the dome and the shell.

℟Name a roof by its slopes

Classify a roof by how its planes meet: one slope (shed), two meeting at a ridge (gable), four meeting at hips (hip, pyramid), a hip below a gable (hip-and-gable), two pitches each side (gambrel, mansard), or a curved surface (barrel, cone, dome).

Each form has its logic. The gable is the cheapest way to roof a rectangle, and its triangular end is an invitation to ornament: in Greece it became the pediment, and Laugier insisted that a pediment is honest only where a roof really ends (A pediment is the end of a roof). The hip is more complex to frame (every hip rafter is a different length), but it braces the roof against wind from any direction and lets the eaves run unbroken round the building, which is why it dominates in typhoon-prone East Asia and in the Caribbean. The sawtooth, a row of sheds with glazed uprights facing away from the sun, was invented for nineteenth-century factories that needed even daylight; the butterfly, which gathers rain into a central gutter, is a twentieth-century form.

Holding it up: the truss

Two rafters leaning against each other push their feet apart, and a wall of ordinary thickness cannot resist that push for long. The European solution, known to the Romans and universal by the Middle Ages, is to tie the feet together with a horizontal beam. The three members form a triangle, a truss, and a triangle cannot change its shape without changing the length of a side. The rafters are in compression, the tie beam in tension, and the whole frame bears straight down on the walls without pushing them outward.

A long tie beam sags under its own weight, so a vertical member, the king post, hangs it from the apex; on wider spans two queen posts and a straining beam do the same. Struts from the posts to the rafters shorten the rafters’ free length. Trusses are spaced at intervals along the building and carry purlins, which carry common rafters, which carry the battens and the covering. English carpenters developed the hammerbeam, in which short brackets cantilevered from the walls carry arched braces, to roof the great hall of Westminster (1390s) without a tie beam across its twenty-metre span; the iron and steel trusses of nineteenth-century railway stations and factories are the same triangulated idea in new materials.

℟Tie the feet of the rafters: the truss

To roof a wide span, join each pair of rafters to a tie beam at their feet so that the three form a triangle that cannot spread; hang the tie beam from the apex by a king post, or by two queen posts on larger spans.

Holding it up: the raised-beam frame

Chinese carpenters solved the same problem differently, and the difference explains the most famous roof silhouette in the world. In the northern tailiang frame (“lifting beam”), a beam spans between the columns, short posts or blocks stand on it, and a shorter beam rests on them; on that stands a shorter beam still, and so on up to a king post under the ridge. Each beam-end carries a purlin. There are no triangles: the beams work in bending, like shelves, and the roof thrust is taken by the stiffness of the timber and the weight of the tiled roof pressing down.

A truss makes a straight roof, because each rafter is one straight timber from eave to ridge. The raised-beam frame makes whatever roof its builder chooses, because each purlin stands on its own post and can be set at any height. In the south a lighter frame, chuandou, puts a slender column under every purlin and threads tie-beams through them; again, every purlin height is free.

℟The raised-beam frame: purlins at any height

Instead of a triangle, stack beams one above another, each shorter than the one below and raised on short posts, and lay a purlin on each beam-end; the heights of the purlins then set the section of the roof, which may be curved at will.

The curved roof of China

The freedom of the frame was used to one end: the concave roof. An ancient poem in the Classic of Poetry praises a newly built palace whose roofs spread like the wings of a bird and soar like a pheasant in flight, and for three thousand years Chinese builders sought that lightness. The state manual of the Song, the Yingzao fashi of 1103, gives the method. First the ridge purlin is raised above the eave purlins by a third of the depth of a great hall. Then a straight line is drawn from ridge to eave purlin, and the first purlin below the ridge is lowered from it by a tenth of the rise; a new line is drawn from there, and the next purlin is lowered by a twentieth; then by a fortieth, and so on. The procedure is called jǔzhé, “raising and depressing”.

As a bird spreading its wings, as a pheasant taking flight.

The curve that results is steep near the ridge and flat at the eaves. Practical reasons are often offered: the flattening throws rain far out, clear of the timber columns and the earth platform; the lifted eaves let the low winter sun into the hall while shading it from the high summer sun. But the chief reason is surely the one the poet gave. A heavy tiled roof that might look crushing becomes, by its curve, something that seems to float, and the Song carpenters compounded the effect by letting the eaves grow outward and rise at the corners (Let the eaves grow out at the corners) and by raising the corner columns themselves (Shengqi: raise the corner columns). By the Qing dynasty the curve had become steeper and the corners more exaggerated, especially in the south, where eave tips curl up like the prows of boats.

Japan inherited the curved roof and transformed its structure. Because Japanese builders wanted deep, low eaves in a rainy climate, they separated the roof that is seen from the roof that keeps out the rain: above the visible rafters and ceiling they built a second, steeper roof, the noyane or hidden roof, on great cantilevers (The hidden roof: separate the eaves from the weather). The visible eaves could then have any curve the designer wished, usually a gentler one than in China. Korean roofs, lifted at the corners and curved in plan as well as in section, occupy a middle position between the two (see Korean architecture).

Mansart and the roof as a storey

In France the roof became a storey. The broken roof (a steep lower slope, almost a wall, pierced by dormer windows, and a shallow upper slope above it) appears in the sixteenth century at the Louvre, but it bears the name of François Mansart (1598–1666), the most refined French architect of his generation, and it spread through the building works of Louis XIV under his great-nephew Jules Hardouin-Mansart. Its virtue was economy: the attic, which under a steep gable is a cramped triangle, becomes a row of good rooms.

℟The mansard: a storey in the roof

Break the slope of the roof on all four sides: a steep lower slope, nearly a wall, which holds a storey of rooms lit by dormers, and a shallow upper slope, often invisible from the street.

lower slope ≈ 60–75° · upper slope ≈ 15–30°

Two centuries later the mansard had a second life in Haussmann’s Paris, where building rules measured the height of a façade to its cornice and so allowed one or two more storeys within the roof; from there, as the emblem of the Second Empire style, it crossed to London, New York and the new capitals of Latin America. The gambrel, a mansard on two sides only, did the same work for the barns and houses of the Netherlands and New England, giving a wide loft under a roof that needed no long timbers.

The flat roof and after

The twentieth century made the flat roof a manifesto. Reinforced concrete could span a room with a level slab, and Le Corbusier’s Five Points of 1926–27 demanded that the roof become a garden, restoring the ground the building had covered (The flat roof as a garden). For the modernists the pitched roof was historical costume; the flat roof was the sign of a new architecture. It was also a technical gamble, since the membranes of the time cracked and leaked, and houses in wet climates paid for their principles.

Other new roofs followed new structures. Engineers built barrel vaults and hyperbolic shells of thin concrete, steel space frames and cable nets; the butterfly roof, falling to a central valley, became a mid-century emblem of informality. At the end of the century the pitched roof returned, first ironically in postmodern houses whose gables were drawn like a child’s picture, then seriously in sustainable architecture, which rediscovered deep eaves for shade, steep roofs for rain-water and snow, and green roofs for insulation, lessons every vernacular tradition had known.