56 FARM BUILDINGS IN SOUTH AFRICA 



A roof hipped at both ends is termed a paviMm roof. 



Remarks on Roof Trusses. — Reference to the accompanying Figs, 

 illustrating different types of roof trusses, will help the reader to under- 

 stand the following remarks. 



The roof-covering is carried (through the purlins) on a series of rafters. 

 Each pair of rafters in a double-slope roof forms a couple. The rafters 

 composing each couple would tend to thrust out the walls on which 

 they rest. To counteract this tendency each pair of rafters is braced 

 by a cross timber, stretching from the foot of one rafter to the foot of 

 the other, and known as a tie learn since it ties the feet of the rafters 

 together. 



When the span is large and the rafters are consequently long, the 

 latter tend to sag. This is prevented either by the introduction of cross 

 timbers, termed collars, higher up the rafters than, but parallel to, the 

 tie beam; or by the insertion of diagonal timbers stretching from tie 

 beam to rafters and called struts. 



The tie beam, when long, is prevented from sagging by timbers, 

 usually vertical, connecting the tie beam to the rafters. These are 

 termed king-posts or queen-posts, according as they are at mid- span or 

 quarter-span respectively. They also take up the thrust of the struts. 



The rafters (forming a couple) are connected to each other at their 

 top ends by means of a cleat, though this is sometimes unnecessary, e.g. 

 when the principal in question is provided with a king-post. 



Each couple, together with its strengthening timbers such as tie- 

 beam, collar, struts, etc, constitutes a principal or roof-truss already 

 mentioned. 



Wall Plates. — The principals rest on 4£" by 3" wall plates, being 

 attached thereto by strips of hoop iron bent over, and nailed to, the 

 foot of the rafters, and also nailed at each end to the wall plate. 



The wall plate is built into the wall along the inside edge of the 

 latter, and is firmly attached to the wall by strips of hoop iron having 

 one end nailed to the wall plate and the other end built into the masonry 

 or brickwork. 



The roof is thus protected against being lifted by violent winds. 



The wall plates are connected, at the corners of the building, either 

 by a halved joint (Fig. 77), or by dove-tail halving (Fig. 78). The latter 

 type of joint is further illustrated by Fig. 78a. 



Roof Trusses for Different Spans. — The accompanying figures illustrate 



