For example, 15½ brick are needed for each course of the chimney in figure 9. Assuming the height is 30 feet and one-half-inch mortar joints are used, also that there are 4½ courses per foot, there would be 135 courses. Therefore, 135 multiplied by 15½ equals 2,092 brick; about 100 more will be needed to make the lower portion solid, or 2,200 brick in all. A more general method of estimating that is applicable to more complex structures is given on page 43. Methods of determining the quantity of mortar materials, labor, and cost are also given and can be applied to this example. SMOKE TEST Every flue should be subjected to the following smoke test before the heater is connected with it and preferably before the chimney has been furred and plastered or otherwise enclosed. Build a paper, straw, wood, or tar-paper fire at the base of the flue. When the smoke is rising in a dense column, tightly block the outlet at the top of the chimney with a wet blanket. Smoke that escapes through the masonry indicates the location of leaks. Frequently this test reveals bad leaks into adjoining flues or directly through the walls or between the linings and the wall. Remedy defects before the chimney is accepted for use. Such defects are usually difficult to correct; hence it is wise to watch the construction closely as it progresses. CLEANING AND REPAIRING FLUES Chimneys develop defects which if not promptly repaired cause trouble. Most masonry requires replacement of worn or weathered material or repointing of mortar joints, while flues become clogged and flashings fail. It is advisable to test a chimney every few years for tightness by the smoke test just described; to examine the inside of the flues by lowering a lantern or flashlight on a strong cord down from the top of the chimney or by holding a hand mirror at the proper angle at a stovepipe hole; to inspect the masonry for loose units, which are most likely to occur at the top (fig. 3) where the action of the flue gases, especially when soft coal is burned, disintegrates the mortar; to test mortar joints from the outside by prodding with a knife or similar tool to determine if the mortar is loose clear through the joint so as to leave a hole; and to notice if the chimney is damp because of leaky flashings, absorption of moisture from the ground, condensation, or excessive rain entering the flues. Cleaning Bricks that fall from the top and lodge at offsets or contracted sections can sometimes be reached and dislodged by a long pole or sections of pipe screwed together. They can be caught on a shingle or piece of sheet metal shoved into a stovepipe hole or removed through a clean-out door. A weighted cement sack filled with straw and attached to the end of a rope may be pulled up and down the flue to remove soot and loose material if the offset is not too great.