In the previous examples or problems, attention has been given to putting the student in line of thinking in materials. Under this heading, consideration will be given to the combination of materials in a roof covering, partly because a good roof is one of the prime necessities of a successful structure, and also because, in a brief study of one type of roof of which a model can easily be constructed, the elements of how to think in construction within the province of the specification can be clearly illustrated. The particular covering considered is canvas. Undoubtedly each student is familiar with the use of canvas, as a deck covering for steamships, sailing vessels, etc., down to the small duck-boat completely constructed of canvas; but it is possible that few have ever considered the various steps necessary to make it available for water and wear-resisting uses.

Canvas is a material which, in its natural condition, is far from waterproof, and is one of the materials most easily and quickly destroyed by mildew. Its one original desirable quality is its extreme strength and toughness. Therefore there must be incorporated with this tough material, the properties of resisting the passage of water and the ravages of decay. Take a good sample of 10-oz. duck; put it on a board, and strike a dozen times in one place; take up, rub in the hands, and observe how little damage has been done to the texture.

Build up of 7/8-inch matched flooring dressed on the upper side, a board about 3 feet square, putting 2 by 4-inch cleats on the under side. These cleats represent the rafters, the 7/8-inch stuff the roof boards. Obtain from different stores samples of 10-oz. duck, and mark each for identification, making a schedule in which the price of each is noted. Cut all samples to exactly the same size; and, after drying thoroughly to drive off all moisture, get them weighed by a druggist on very accurate scales, and on the schedule note the weight of each piece. Soak and wash each thoroughly, rubbing so as to remove all size or filling soluble in water. After thoroughly drying, have each carefully weighed again, and note the weights on the schedule. From this schedule can easily be determined, which is the best for the money. It may be found, for example, after soaking out all soluble matter, that the material which costs 18 cents a yard is considerably cheaper than that costing 17 cents.

A choice having been made, buy a yard; and, when thoroughly dry, secure it with ordinary tacks about 5/8-inch long, spaced -one inch apart, to the boards prepared, stretching it as much as possible so that it will be tight and flat. Wet it thoroughly and observe that wetting has a tendency to shrink it; but, as it cannot shrink on account of being securely nailed to the board, each thread is drawn very tight and made smaller. By examining closely, it will be seen that the spaces between the threads are increased and that water will run through as through a sieve. When in this condition, put on a good coat of white lead and oil paint (this is about the only material which should be painted wet), and observe that a large amount of paint is taken up; in fact, through the openings between the threads, the paint goes, reaching and covering the underside of the cotton. The water will not unite with the paint, and passes off before the paint dries, allowing the threads to swell into the soft paint, so that when the paint is dry the fabric is practically embedded in paint, with less on the upper surface than anywhere else. Two more coats on top furnish the necessary protection. This treatment protects the canvas from mildew and makes it waterproof; the original fabric furnishes the strength.

After the paint is dry, subject the sample to the harshest of treatment. Grind with the heel, drop bricks, etc., to see just how much it will resist; if not successful in making a hole, use a hatchet, patch the hole so made by tacking over it a piece of duck of sufficient size to cover it; and wet it, paint it, etc., as before. When dry, place the boards level, nail down 1-inch strips around the edge, laid in white lead, and cover with one inch of water. Let this stand for fortyeight hours. Now cut up the duck, and see if any leaks exist; if so, carefully examine the covering to determine exactly the cause. Next, roll the duck loosely, and put away for two weeks in a dark, damp place, and then examine for mildew.

It is not the intention of the above problem merely to explain how a canvas roof is made, but to set the student to thinking about roofs. A shingle roof, or a tar-and-gravel roof, or a slate roof, differs from the above only in the manner in which it accomplishes the same ends; and all other styles should be observed and studied along similar lines, so that the proper material will be used in each of the different varieties of buildings.