Types of Roofs & Roof Materials
Attics with sloping ceilings are for economy's sake sometimes used in the roof; they are not considered good from a hygienic point of view, because they are usually cold in winter and hot in summer. Care must be taken to provide an air space between the ceiling of the room and the outer covering of the roof; or, if the whole of the room is in the roof, to fill in between the rafters with pugging of silicate cotton, slag-wool, or other non conducting material (picture 37). Roofing felt or Wilesden paper should also be used under the slates or tiles.
In all cases it is advisable to have rough boarding as well as battens under the slates, as the continuous wood surface is a non-conducting medium. The eaves of roofs should project boldly (pictures 37), so as to protect the wall from rain, and at the same time give a good finish to the design, as mentioned in roof slates.
Picture 37, Roof Eaves.
It is not necessary to go deeply into the construction of the various forms of roof, because in the class of house we are discussing they are generally of a simple character.
The couple roof, in which the rafters rest on a wall plate and have their upper ends fixed to a ridge piece, is usual in small buildings. A horizontal beam, called a collar, is spiked to each pair of rafters to counteract their tendency to spread and push out the walls. This form should not be employed for a greater span than fifteen feet, unless purlins, i.e. horizontal timbers, can be arranged to rest on the partition walls to support the rafters in the centre of their length.
(a) King-post Roofs. It is desirable, if this cannot be arranged, to adopt a king-post truss (picture 38) for spans up to thirty feet, which consists of a tie-beam, principal rafters, king-post, struts and purlins.
Picture 38. A King Post Roof.
(b) Queen-post Roofs. The queen-post truss (picture 39) should be adopted when the span is above thirty feet.
Picture 39. A Queen Post Roof.
(c) Hammer-beam Roofs. In large halls or billiard-rooms an open roof of the hammer beam type may be adopted, as shown in picture 40.
Picture 40. An Open Timber Roof.
A table of the least inclinations necessary for different roofing materials is given below, but in order to obtain rooms in the roof, or for the sake of appearance, these inclinations are sometimes increased in practice.
Angle of Inclination for Different Roof Coverings.
Materials for roof coverings were dealt with in house roof design, but pantiles have not been referred to, for, as a rule, they are only used for outhouses, which do not require to be made absolutely watertight. They are fixed on laths, as shown in picture 41, and are "torched" or pointed on the under side with lime-and-hair mortar.
Picture 41. Pantiling.
Flat roofs are generally covered with lead, copper or zinc, but this method is gradually being superseded by steel and concrete covered with asphalt, for the difference in cost is slight, and the durable properties much in favour of the latter materials. The table given above shows that lead roofs should be almost flat, otherwise the lead is liable to creep or crawl down the slope of the roof owing to the heat of the sun.
The edges between two sheets of lead are dressed over semicircular wooden rolls about two feet three inches apart, laid in the direction of the slope.
Drips are formed at the junction of the ends of the lead sheets about every eight feet along the slope of the roof. Nailing should be executed with copper nails (not iron), to prevent any galvanic action being set up between the lead and iron, and to obviate the destruction of ordinary iron nails by oxidation.
Copper is generally used for small ornamental roofs, such as over turrets and bay windows. It oxidizes by the action of the air, and verdigris, forming on the surface, gives a splash of bright green colour, which forms a protection to the copper itself.
Zinc is laid much the same way, but as it expands more than any other metal, great care should be taken to allow for this. The sheets should be held in position by zinc clips, and should be not less than 14 gauge for good work. It is, of course, much inferior to copper or lead.
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