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Domestic roof construction is the framing and roof covering which is found on most detached houses in cold and temperate climates. [1] Such roofs are built with mostly timber, take a number of different shapes, and are covered with a variety of materials.
Modern timber roofs are mostly framed with pairs of common rafters or prefabricated wooden trusses fastened together with truss connector plates. Timber framed and historic buildings may be framed with principal rafters or timber roof trusses. Roofs are also designated as warm or cold roof depending on how they are designed and built with regard to thermal building insulation and ventilation. The steepness or roof pitch of a sloped roof is determined primarily by the roof covering material and aesthetic design. Flat roofs actually slope up to approximately ten degrees to shed water. Flat roofs on houses are primarily found in arid regions. [2]
In high-wind areas, such as where a cyclone or hurricane may make landfall, the main engineering consideration is to hold the roof down during severe storms. Every component of the roof, as of course the rest of the structure, has to withstand the uplift forces of high wind speeds. This is accomplished by using metal ties fastened to each rafter or truss. This is not normally a problem in areas not prone to high wind or extreme weather conditions.
In the UK, a concrete tiled roof would normally have rafters at 600 mm (24 in) centers, roof battens at 300 mm (12 in) centers and ceiling joists at 400 mm (16 in) centers. The United States still uses imperial units of measurement and framing members are typically spaced sixteen or twenty-four inches apart.
The roof framing may be interrupted for openings such as a chimney or skylight. Chimneys are typically built with a water diverter known as a cricket or saddle above the chimney. Flashing is used to seal the gap between the chimney and roofing material.
A simple rafter roof consists of rafters that the rafter foot rest on horizontal wall plates on top of each wall. [3] The top ends of the rafters often meet at a ridge beam, but may butt directly to another rafter to form a pair of rafters called a couple. Depending on the roof covering material, either horizontal laths, battens, or purlins are fixed to the rafters; or boards, plywood, or oriented strand board form the roof deck (also called the sheeting or sheathing) to support the roof covering. Heavier under purlins or purlin plates are used to support longer rafter spans. Tie beams, which may also serve as ceiling joists, are typically connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart. Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength. The rafters, tie beams and plates serve to transmit the weight of the roof to the walls of the building
Pre-manufactured roof trusses come in a wide variety of styles. They are designed by the manufacturer for each specific building.
Timber trusses also are built in a variety of styles using wood or metal joints. Heavy timber rafters typically spaced 240 cm (8 ft) to 370 cm (12 ft) apart are called principal rafters. Principal rafters may be mixed with common rafters or carry common purlins.[ citation needed ]
Roof framing must be designed to hold up a structural load including what is called dead load, its own weight and the weight of the roof covering, and additional loading called the environmental load such as snow and wind. Flat roofs may also need to be designed for live loads if people can walk on them. In the United States, building codes specify the loads in pounds per square foot which vary by region. The load and span (distance between supports) defines the size and spacing of the rafters and trusses. [4]
The roofing material, including underlayment and roof covering, is primarily designed to shed water. The covering is also a major element of the architecture, so roofing materials come in a wide variety of colors and textures. The primary roof covering [5] on houses in North America are asphalt shingles, but some have other types of roof shingles or metal roofs. Tile and thatch roofs are more common in Europe than North America. Some roofing materials help reduce air conditioning costs in hot climates by being designed to reflect light.
Asphalt shingles is the most used roofing material in North America, making up as much as 75% of all steep slope roofs. This type of material is also gaining popularity in Europe due to lower installation costs. Asphalt shingles dominate the North American residential roofing market, because they are typically less expensive [6] compared to other materials.
In the southern US and Mexico, clay tile roofs are also very popular, due to their longevity, and ability to withstand hurricane winds with minimal or no damage.
In Europe, slate and tile roofs are very popular. Many slate roofs in Europe are over 100 years old, and typically require minimal maintenance / repairs.
Roof space ventilation is needed to regulate the temperature within the roof space. Without proper ventilation, humidity can cause interstitial condensation within the roof fabric; this can lead to serious structural damage, wet or dry rot, and can ruin insulation.
Condensation within the roof space is much more of a problem today due to: much less fortuitous ventilation due to tighter building envelopes with high performance windows and door and no chimneys leading. This tighter envelope means the air temperature in buildings has risen, the warmer the air in the building is, the more water vapour the air can carry.
As the occupied part of the building has become warmer, the roof space has become colder, with high performance insulation and roofing membranes leading to a cold roof space.
When the warm, moist air from below rises into the cold roof space, condensation begins as the air temperature drops to the ‘dew point’ or as the warm air comes into contact with any of the cold surfaces in the roof.
Most building materials are permeable to water vapour; brick, concrete, plaster, wood and insulation all can fall victim to interstitial condensation. This is why UK Building Regulations require roofs to be ventilated, either by the use of soffit vents, ridge vents, or replacement ventilation slates or tiles. [7]
Ventilation of the roof deck speeds the evaporation of water from leakage or condensation and removes heat which helps prevent ice dams and helps asphalt shingles last longer. Building codes in the U.S. specify ventilation rates as a minimum of 1 sq ft (0.093 m2) of opening per 150 sq ft (14 m2) (1:150) with a ratio of 1:300 in some conditions. [8] Warm air rises, so ceiling insulation is designed to have a higher r-value and the insulation is often installed between the ceiling joists or rafters. A properly insulated and ventilated roof is called a cold roof. A warm roof is a roof that is not ventilated, [9] where the insulation is placed in line with the roof pitch. [10] A hot roof is a roof designed not to have any ventilation and has enough air-impermeable insulation in contact with the sheathing to prevent condensation [11] such as when spray foam insulation is applied directly to the under-side or top-side of the roof deck or in some cathedral ceilings. [12]
A more recent design is the installation of a roof deck with foil-backed foam along with a second deck that is air-gapped away from the foil-backed foam to allow air to flow vertically to a ventilation outlet at the peak of the roof—it is a double-deck design with an air gap. This design improves efficiency. [13]
In the Near East, from Roman times, throughout the Middle Ages and up to the start of the 20th-century, roofs of houses were constructed in the following manner: They first arranged the wooden rafters over the walls, at a distance of about 60 cm. between each rafter; over which rafters they laid down horizontally thin strips of wood, or boards (purlins), in close proximity to each other, each board having a thickness of 3 finger-breadths (6.75 cm), or a little less. [14] Over these boards they then firmly secured in place a matting material made of natural plant fibers woven together, such as cane reeds, leaves of rush or papyrus, palm fronds and tree bark. [15] Over this matting material they first placed a thick layer of clay, followed by a layer of soil having a thickness of 20 centimetres (7.9 in), after which they daubed the top with a clay aggregate consisting of a cement mixture of clay, rubble, crushed potsherds and straw. [15] [14] [16] This type of roofing material is well-equipped to withstand wetness and rain downpours. [14]
In Sanaa, Yemen, in the early 20th-century, twisted mats of palm-leaves were laid over wooden beams used to support the ceilings. [17] These mats were held in place by wooden sticks laid at right angles to the beams. [17] Above these crossing sticks, bushes were spread, and these were again covered with mats. The wooden beams and the mats were overlaid with a very hard lime or mortar coating. [17] The thickness of the ceiling-wall seldom exceeded 30 centimetres (12 in), excluding the beams. [17]
In ancient Gaul, Spain, Portugal and Aquitaine, houses were constructed with peaked roofs daubed with lumps of dried mud and covered with reeds and leaves, while other roofs made with oak shingles or thatched. [18] The Colchians in Pontus made their roofs by "cutting away the ends of crossbeams, and [by] making them converge gradually as they lay them across, they bring them up to the top from the four sides in the shape of a pyramid. They [then] cover it with leaves and mud." [18] In Marseilles, during classical antiquity, tiles were not used on the roofs of houses, but rather only earth mixed with straw. [18] The ancient Phrygians built a pyramidal roof of logs fastened together, and covering them over with reeds and brushwood, upon which was heaped up very high mounds of earth. [18]
A roof is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of temperature, and wind. A roof is part of the building envelope.
A floor is the bottom surface of a room or vehicle. Floors vary from simple dirt in a cave to many layered surfaces made with modern technology. Floors may be stone, wood, bamboo, metal or any other material that can support the expected load.
A radiant barrier is a type of building material that reflects thermal radiation and reduces heat transfer. Because thermal energy is also transferred by conduction and convection, in addition to radiation, radiant barriers are often supplemented with thermal insulation that slows down heat transfer by conduction or convection.
A joist is a horizontal structural member used in framing to span an open space, often between beams that subsequently transfer loads to vertical members. When incorporated into a floor framing system, joists serve to provide stiffness to the subfloor sheathing, allowing it to function as a horizontal diaphragm. Joists are often doubled or tripled, placed side by side, where conditions warrant, such as where wall partitions require support.
This page is a list of construction topics.
A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck, roof covering and its associated loads. A pair of rafters is called a couple. In home construction, rafters are normally made of wood. Exposed rafters are a feature of some traditional roof styles.
Framing, in construction, is the fitting together of pieces to give a structure support and shape. Framing materials are usually wood, engineered wood, or structural steel. The alternative to framed construction is generally called mass wall construction, where horizontal layers of stacked materials such as log building, masonry, rammed earth, adobe, etc. are used without framing.
A tie, strap, tie rod, eyebar, guy-wire, suspension cables, or wire ropes, are examples of linear structural components designed to resist tension. It is the opposite of a strut or column, which is designed to resist compression. Ties may be made of any tension resisting material.
Roof shingles are a roof covering consisting of individual overlapping elements. These elements are typically flat, rectangular shapes laid in courses from the bottom edge of the roof up, with each successive course overlapping the joints below. Shingles are held by the roof rafters and are made of various materials such as wood, slate, flagstone, metal, plastic, and composite materials such as fibre cement and asphalt shingles. Ceramic roof tiles, which still dominate in Europe and some parts of Asia, are still usually called tiles. Roof shingles may deteriorate faster and need to repel more water than wall shingles. They are a very common roofing material in the United States.
A batten is most commonly a strip of solid material, historically wood but can also be of plastic, metal, or fiberglass. Battens are variously used in construction, sailing, and other fields.
An attic is a space found directly below the pitched roof of a house or other building. It may also be called a sky parlor or a garret. Because they fill the space between the ceiling of a building's top floor and its slanted roof, attics are known for being awkwardly-shaped spaces with difficult-to-reach corners and often exposed rafters.
A purlin is a longitudinal, horizontal, structural member in a roof. In traditional timber framing there are three basic types of purlin: purlin plate, principal purlin, and common purlin.
A vapor barrier is any material used for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through the wall, floor, ceiling, or roof assemblies of buildings and of packaging to prevent interstitial condensation. Technically, many of these materials are only vapor retarders as they have varying degrees of permeability.
Portal frame is a construction technique where vertical supports are connected to horizontal beams or trusses via fixed joints with designed-in moment-resisting capacity. The result is wide spans and open floors.
A king post is a central vertical post used in architectural or bridge designs, working in tension to support a beam below from a truss apex above.
Sarking is an English word with multiple meanings in roof construction:
A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof. Trusses usually occur at regular intervals, linked by longitudinal timbers such as purlins. The space between each truss is known as a bay.
A post is a main vertical or leaning support in a structure similar to a column or pillar, the term post generally refers to a timber but may be metal or stone. A stud in wooden or metal building construction is similar but lighter duty than a post and a strut may be similar to a stud or act as a brace. In the U.K. a strut may be very similar to a post but not carry a beam. In wood construction posts normally land on a sill, but in rare types of buildings the post may continue through to the foundation called an interrupted sill or into the ground called earthfast, post in ground, or posthole construction. A post is also a fundamental element in a fence. The terms "jack" and "cripple" are used with shortened studs and rafters but not posts, except in the specialized vocabulary of shoring.
An ice dam is an ice build-up on the eaves of sloped roofs of heated buildings that results from melting snow under a snow pack reaching the eave and freezing there. Freezing at the eave impedes the drainage of meltwater, which adds to the ice dam and causes backup of the meltwater, which may cause water leakage into the roof and consequent damage to the building and its contents if the water leaks through the roof.
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