This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The framing of the roof is one of the most difficult problems with which the carpenter has to deal, not because of the number of complicated details, for these are few, but because of the many different bevels which must be cut in order to allow the rafters to frame into one another properly.
There are many kinds of roofs, and before describing the different varieties it will be well to consider briefly the purpose of the roof and its development from simple forms to those which are more elaborate and perhaps more ornamental. The primary object of a roof in a temperate climate is, of course, to keep out the rain from the interior of the building and at the same time to keep out the cold. The roof must, therefore, be so constructed as to free itself from the falling water as soon as possible, that is, it must be built to shed water and, therefore, it must be sloped or inclined to the horizontal in some way. If it is necessary for the sake of economy or for any other reason to construct the roof practically flat, it must be made more secure against the passage of water than if it is made sloping, and some provision must be made to carry off the water, and to cause it to collect in one or two low places in the roof surface, from which pipes or down spouts may be provided to take it away to a safe place outside the building. The roof covering must be of some material through which water will not readily penetrate, such as tin, galvanized iron, lead, or zinc or copper among the metals, or a composition of tar and gravel, if metal is not suitable to the purpose. This would be for a roof which is nearly flat; for a roof which slopes, and will shed the water, thus getting rid of it more quickly, a covering of slates, tile, or wood shingles may be employed, as well as tin, copper, or other metals. The slates or shingles would have to be laid in several thicknesses, the number depending upon the steepness of the slope of the roof, and in order to accomplish this they would have to be laid overlapping each other. In order that the water escaping from the roofs may not run down the sides of the building, making unsightly streaks and wetting the windows and doors, it is necessary that the roofs should project somewhat beyond the face of the walls all around. The projecting part of the roof is called the eaves.
Copyright, 1912, by American School of Correspondence.