54 FARM BUILDINGS IN SOUTH AFRICA 



amount of slope given to the roof-covering. It may be measured in two 

 ways : — 



1. by the angle (a, Fig. 64) which the roof-covering makes with the 



horizontal, 



2. by the proportion which the rise of the roof bears to the span, 



or distance bridged. 

 The following table shows the connection between these two methods 

 of stating the pitch of a roof. 



A pitch, for a double-slope roof with corrugated iron covering, which 

 is generally adopted is \. This slope is steep enough to prevent hail 

 from lying on the roof. 



Galvanised corrugated iron covering may, however, be laid much 

 flatter than this, as will be seen from instances throughout this book of 

 single-slope or " flat " roofs. 



Thatching requires a steep slope, so as to throw off rain-water before 

 it can penetrate the thatch. This slope should not be less than 45 

 degrees, and in many cases it is made 50 degrees, with the horizontal. 



Types of Roofs. — Ordinary roofs may be divided into two classes, 

 viz. : — 



(1) Single-slope or single-pitch roofs. 



(2) Double-slope or double-pitch roofs. 



Single-Slope Roofs. — These roofs are generally referred to by the 

 carpenter as "flat" roofs. Roofs of this class are illustrated by Figs. 

 86, 87, 88, 92 and 48, and are described in Chap. IX. 



Lean-to roofs which consist of a row of rafters arranged to form an 

 inclined plane against a wall, usually of another building, belong to this 

 class. 



For a flat or lean-to roof, with corrugated iron covering, the rafters, 



