This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol6", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The ordinary builder's ladder is formed of sides consisting of a straight fir-pole cut in half lengthwise, and connected by heads or rungs usually of oak or ash, preferably the former. Before the fir-pole is cut it is desirable to bore the holes for the rungs, so that they may be parallel throughout. The rungs are fixed 9 inches apart, and usually are from 1 to 1 1/2 inch thick in centre and decrease to 5/8-inch diameter at the ends. Their ends are painted with red lead before being inserted, and the projections are then cut off flush with the sides. They are usually fastened at each end with wedges (see A, Fig. 202), to fix every fifth or sixth rung, or pins (B) 1/4-inch diameter may be cut through the sides for the same purpose.
The wedging method is to be preferred, as the sides would necessarily be weakened by cross boring.
To prevent the wedges working out an iron rod 5/16-inch diameter is, in a strong ladder, fixed below every eighth or ninth rung, and bolted on the outside for strength. A rung should not be allowed to be reduced by wear to less than half its original thickness.
In long ladders - say of 100 rungs - the rungs are 9 inches apart at top, and from 12 to 13 inches at the bottom.
For the prevention of accidents in fixing of ladders to platforms of scaffolding their upper ends should reach to a height of 7 feet above the top platform which they serve. If, however, this cannot possibly be arranged a T-piece should be fixed across the top (see C, Fig. 202) to warn the climber that he cannot obtain any higher hold on the ladder.
It is always well to stay ladders in the centre where their length exceeds 25 feet, to prevent bending or swaying. The stay should consist of a wooden piece with an iron clip. The clipping of the rung as shown at D prevents any interference with the hands and feet of the climber.
When the top of the ladder is considerably above the resting-point a stay, as shown at A, Fig 203, should be secured. Care should always be taken that the foot of a ladder is level, and be firmly secured at the resting-point.