This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
The rafters should be spaced close enough to make the roof firm, when planked or boarded. For sheathing, tongued-and-grooved lumber is preferable; and in any case the plank or roof boards should be laid closely - making close joints, both at edges and ends - and should be free from holes or loose knots, and securely nailed to the rafters. Over the sheathing should be laid four layers of roofing felt, the first course, next the eaves, being five layers thick. Each successive layer should be lapped at least 2/3 its width over the preceding one, and firmly secured with cleats. The quantity of felt per 100 sq. ft. of roofing should be not less than 70 lb. The surface under the outer layer of the first course, and under each succeeding layer, as far back as the edge of the next lap, should be well covered with a thin coating of cement, in no case applied hot enough to injure the woolly fiber of the felt. Over the entire surface should be spread a good coating of cement, amounting in all (including that used between the layers) to about 10 gal. per 100 sq. ft., heated as before specified. While the cement is hot. it should be completely covered with a coating of dry slag, granulated and bolted for the purpose, no slag being used that will not pass through a 5/8-inch mesh, and none smaller than will be caught by a 1/4-inch mesh. All chimneys and walls that project above the roof should be flashed and counterflashed with zinc or copper.
Gravel roofs form a very durable and inexpensive covering, but are not suitable for any but practically flat surfaces.