ROOFS. A roof is the covering of a structure. Its chief pur pose is to enclose the upper parts of a building as a protection against wind, rain and snow; in communities of dwellings, domes tic buildings, industrial and commercial structures, fire-resistance is a necessary property of roofing.
Roofs may be flat, or may be inclined at an angle to suit the roof covering. Covering materials are selected for pitched roofs according to their resistance to rain and snow, durability in changes of temperature and in acid atmospheres, and for their artistic qualities in relation to architectural design. Modern buildings erected in concrete usually have flat roofs, consisting of concrete slabs surfaced with asphalt or with waterproofed cement coatings. Sufficient fall must be given to drain water off quickly and adequate gutters formed in the concrete to discharge the water in heavy storms.
For buildings erected in steel framework flat concrete roofs may be adopted, but in order to keep the eaves of a building at a lower level, while providing additional storeys in the accomoda tion, it is more usual in cities to adopt a Mansard type of roof, having the general form shown in fig. 5, but constructed in steel. Cross members are avoided as much as possible—except where required as floor girders—and the frame is made rigid by strong steel gussets at the changes of direction of the slopes. Two storeys in the roof—or more—can be obtained in this way.
While timber framework continues to be used for supporting roof coverings of lead, copper, zinc, special felts, asbestos sheets, slates and tiles—both for flat and pitched roofs—steel has largely replaced timber for the main supports and for many transverse members in the larger buildings, and even for semi-permanent industrial buildings and stores, because of the rapidity with which materials can be prepared, assembled and erected. Steel is also economically justified for use in roof framing except for dwellings and small buildings, or for dye-works and bleach-works where rapid oxidation renders steel unsuitable. Roofs are often made with considerable projection at the eaves. This serves to protect the upper portion of the external walls and also possesses aesthetic value in design.
"Lean-to," "shed," or "pent" roofs are developments of the same form of construction as the flat roof, one end of the joists (now called "rafters") being lifted to form a decided slope, which enables slates, tiles, asbestos sheets and other materials to be employed in small units.
Simple roofs in general use with a double slope are called "coupled rafter roofs," the rafters meeting at the highest point upon a horizontal ridge-piece which stiffens the framework and gives a level ridge-line. In some old roofs the rafters are con nected without any intervening ridge-plate, with the result that after a time the ridge instead of remaining level takes on a wavy outline, due to the fact that some of the timbers have settled slightly owing to decay or other causes, whilst others have re mained firm. The lower ends of the rafters should rest upon a wood plate bedded on the top of the wall ; this assists in spreading the weight over the wall, and provides fixing for the timbers. The simple "couple roof" consists merely of two sets of rafters pitched from plates on the walls on either side of the building and sloping upwards to rest against a common ridge-piece. There are no ties between the feet of the rafters, which therefore exert a considerable thrust against the supporting walls ; this form of roof is only suitable for spans of io to 12 feet. Where the ends of the rafters are connected by ceiling joists which form a level ceiling the joists act as ties and prevent any outward thrust on the supports. Such roofs are termed "couple close roofs." When used for spans between 12 ft. and 18 ft. a binder supported by an iron or wood "king" tie every 5 or 6 ft. should be run along across the centres of the ceiling joists and the latter spiked to it. When ties are fixed about half-way up the rafters the roof is called a "collar roof," and may be used for spans up to i6 ft. This type of roof is commonly used in dwelling-houses where the fir framing is generally hidden from view by the ceilings. Where required extra support is obtained from partitions and cross walls. If the span is above 20 ft. without intermediate support, it is necessary to employ "principals" and "purlins." American usage sanctions much greater spans than English for simple raftered roofs. Frequently spans as great as 3o ft. or more are roofed without trusses or interior supports, provided that the feet of the rafters are adequately tied by means of ceiling or floor joists at or near the plate level. Principals are strong trusses of timber rigidly framed together and placed at intervals of about i o ft. to support the weight of the roof covering. Purlins—stout tim bers running longitudinally—are supported on the principal rafters at intervals of 6 to 8 ft., and on these the common rafters are fastened. Principals, or '"roof trusses" as they are more often called, are framed together in various ways; they may be of timber members with iron fastenings, or entirely of steel, or again a combination of steel and timber. In the latter case they are called "composite trusses" and the use of steel is confined to tension members.