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.
As will be seen from Fig. 218, the slater's truss has to form a platform to enable the slater or tiler to get at his work without kneeling on or damaging that already executed. Trusses are usually slung from the ridge, and are moved to suitable positions as the work requires. Pads or old sacking should be laid under each truss to prevent the possibility of damage to the slating or tiling while the work is in progress.