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.
Cover all roofs, except to w.c. and woodshed in yard, with best approved Welsh countess slates with 2-inch cover, proper gauge and side lap, and properly secured to boarding with 1 1/2-inch composition nails, all to approval.
An alternative price to be given for covering roof in English tiles p.c. £4, 15s. per 1000, laid to 4 1/4-inch gauge, with all necessary plain ridge, hip, and valley tiles. In this case rafters to be 1-foot 6-inch centres and with 1 3/4 by 1 1/4-inch battens to proper gauge. Each tile to be nailed to battens with zinc nails, two to each tile.