The roof is the most essential of a house (a house without a roof is not considered a house). It is the part that costs the most, by area and orientation it is the part most exposed to the elements, and it is the part primarily responsible both for indoor comfort and for damage suffered during earthquakes and hurricanes. A well-designed durable roof can compensate for a great number of problems that may arise in other parts of the building.
However, technical aspects are not the only determinants of roof design. Many traditional cultures give more importance to various other criteria, such as religious belief, local lifestyles and social status, and these must be respected in designing housing schemes, especially in order to avoid the depressing monotony of present day housing colonies, which look the same in about all parts of the world.
While traditional, non-technical aspects of roof design are important, these cannot be dealt with in a technology orientated book of this kind. The basically different types of roofs and the main design criteria for roofs in the two major climatic regions, that is, those that are predominantly warm-humid and those predominantly hot-arid, are summarized below.
Common Roof Types
• These can be monolithic slabs, sheets or space frame structures, or simple systems using beams, girders and decking elements of low span capability.
• By definition, roofs with inclinations less than 10° to the horizontal are classified as flat roofs. For rainwater run-off at least 2° slope is needed.
• Strong winds tend to pull off the roof by suction, hence flat roofs are less suitable for hurricane prone areas.
• Flat roofs are most common in predominantly hot arid regions, with low annual precipitation. The roofs provide additional living space (for household activities and sleeping at night) and facilitate vertical extensions of the building.
• Sheet decking must be laid in falls with large overlaps. An ingenious alternative to corrugated sheets are canaletas (trough-shaped asbestos cement roofing elements) which can span entire dwellings without supporting structures, thus saving material, costs and lime of installation. A good material, in terms of strength and durability, is asbestos cement, which most likely will not be used in developing countries in the course of lime (because of the health hazards). Nevertheless, galvanized iron canaletas (eg produced in Mexico) are a good alternative, and continued fibre concrete research will hopefully bring forth an equally good alternative to asbestos cement.
• Space frame roofs, consisting of three-dimensionally triangulated supporting members, are especially suited for large span roofs. They have great lateral rigidity and require only light roof decking.
• These can be single pitched, gabled and tripped roofs, either of monolithic slabs or sheets or with a system of rafters, purling, trusses or space frames.
• Sloped roofs are more common in predominantly warm humid regions with significant rainfall.
• Low pitches are cheaper, requiring less wall construction material and less roofing material (smaller roof surface), but suction forces are strongest at 10° pitch. In hurricane areas, minimum roof slopes should be 30° (about 1: 1.7 or 58 %) and wide overhangs (needed for shading and rain protection) should be avoided.
• Gabled roofs leave end walls exposed; tripped roofs protect all walls, save on wall area and costs, are less susceptible to wind damage, but are more difficult to construct.
• Roofs of courtyard houses should slope inwards for better indoor climate and to facilitate rainwater collection.
• Although roof slopes are often given in degrees, angles are difficult to measure out on the site. Therefore, roof slopes should be expressed in simple relations between height and span (eg 1: 1; 1: 2.5; 1: 10), preferably in round numbers.
• As the main function of roof slopes is to drain off rainwater, the lower the permeability of the roofing material, the less slope is required. Each material therefore has its own appropriate pitch, as shown in the following table.
• These include vaults, domes, bow-string or shell structures, lightweight tensile roofs and a variety of more sophisticated types.
• Vaults and dome-shaped roofs are common in hot dry climates: the curved surface area being considerably larger than the base, receives less solar heat per unit area, thus lowering surface temperatures and facilitating reradiation after sunset. However, the acoustics inside domes can be very unsatisfactory.
• Masonry vaults and domes are likely to fail in earthquakes, while bow-string and concrete shell structures can easily withstand such hazards.
• Tensile roofs, using a system of tough membranes on cables or ropes, can cover wide spans, are relatively economical, but aerodynamically unstable with light deck, and are therefore generally used for temporary structures.
Roofs for Warm Humid Climates
• Sloped roofs with wide overhanging eaves are ideal to facilitate rapid rainwater run-off and to protect and shade outer walls and openings. Horizontal valley and internal gutters should be avoided, as these accumulate dirt and water.
• Flat roofs with good drainage are common in composite and upland climates with warm dry seasons, which permit activities and sleeping on roofs.
• Primary requirements for roofing materials (supporting structure and cladding): low thermal capacity (to avoid heat build-up, which cannot be dissipated at night, since there is no temperature drop); resistance to rain penetration, yet permeable enough to absorb moisture (eg water vapour, condensation) and release it when the air is drier; resistance to fungus, insects, rodents and solar radiation; good reflectivity (to reduce heat load and thermal movements); resistance to impact (hailstones, dropping coconuts, vandalism, etc.); resistance to temperature and moisture fluctuations; freedom from toxic materials (especially if rainwater is collected from roofs).
• Ventilated (double-layered) roofs are most effective in providing good indoor living conditions: the outer layer shades the inner building enclosure (reducing heat accumulation); any heat that builds up between the two layers is carried away by cross-ventilation; the difference between temperatures in the building interior and the ventilated air space is not so large as to cause condensation problems; any rain or moisture that penetrates through or develops beneath the outer skin evaporates or drips along the inner surface to the eaves, so that the inner roof layer remains unaffected.
• Waterproofing with an impermeable membrane can be unsuitable, since water vapour cannot escape and causes condensation.
• Insulating materials prevent release of heat during nights.
• Openings at the ridge (sloped roofs), or just below the suspended ceiling or flat roof, help to discharge accumulated heat.
• Measures for sound absorption should be considered, as tropical downpours can cause unbearable noise.
Roofs for Hot Dry Climates
• As rainwater run-off is no major requirement, flat roofs are most common, providing space for outdoor activities and sleeping.
• Vaults and dome shaped roofs are also common, providing good thermal comfort.
• Primary requirements for roofing materials (supporting structure and cladding): high thermal capacity (to absorb solar heat during the day and release it during the night, when the temperature drops considerably); good reflectivity (to reduce heat load and thermal movements); resistance to embrittlement (caused by repeated cycles of heating and cooling) and abrasion (caused by wind-blown sand); smooth surfaces to prevent collection of sand and dust.
• Double layered roofs (with sufficient air space to dissipate hot air and with the upper surfaces of each layer designed to reflect heat) can be of lightweight, low thermal capacity materials, whereby the outer layer can be of insulating material.
• Wind catchers (towers with openings facing the main direction of wind) are advantageous to redirect higher level breezes into the building.
In some regions it is desirable to exclude the sun during the summer and to use the solar radiation for room heating through windows during winter. This effect can be obtained with an appropriate roof overhang. Its dimension depends on the angle of the solar radiation.
Summary of Common Roofing Materials
Grass roofs (soil roofs with growing grass cover), which are becoming popular in some industrialized countries, have several advantages: use of natural, local material; maintenance of moderate outdoor and indoor microclimate (balance of moisture and temperature); generation of oxygen and humidity; high stability through root reinforcement; good sound absorption. In hot climates, problems may arise in the dry season, requiring watering of roofs to maintain growth, and the roofs are likely to attract insects and small animals, which can be harmful to people. Research is needed to find acceptable solutions.
[Ukrainian] [English] [Russian]