Government specifications for fireplace mortar, which must be used in laying all work except the fire brick, are as follows: hydrated lime, nine pounds to each 94-lb. bag of Portland cement; mix this with three times its volume of clean sand and enough water to make a smooth workable mortar. Wet all stones before applying the mortar. Don’t use ordinary lime mortar for fireplaces and chimneys.
Construction of the flue is the next important matter. The flue is installed as the chimney is built and must be planned with a fairly close relationship to the fireplace itself. Fireplace experts estimate that the correct ratio is 14 to 16 square inches of flue area to each square foot of fireplace opening.
A round flue is the most satisfactory type. It offers less friction and is easier to clean. Square or oblong flues should be slightly larger than if a round one is used. The easiest method—and one that assures a tight flue—is to use fire clay flue lining, filling in between it and the stone shell with cement. Be sure that the flue is as straight as possible and uniform in size right up to the top. Don’t taper it. If turns must be made in it, make them gradual—never greater than an angle of 45 degrees.
Don’t try to use one flue for fireplace and kitchen stove or another fireplace. Make separate flues for each. They may go up the same chimney but the two or more flues should be separated by at least four inches of solid brick or concrete unless tile flue linings are used. When you have reached the top, test the flue for leaks by lighting a fire and covering the top of the chimney. Escaping smoke will reveal any leaks, which must be repaired at once.
Diagram of a patented fireplace form showing how it heats and circulates the air. The masonry is laid around the form.
SMOKE DOME DAMPER DOWN DRAFT SHELF APRON—TO BE COVERED BY MASONRY WARM AIR OUTLET DAMPER CONTROL HEATING CHAMBER BOILER PLATE FIREBOX COLD AIR INTAKE
Location of your house will affect the action of the chimney, since large objects such as trees or a hill nearby will cause a down draught. If this trouble is encountered, it may be corrected by building an arch over the chimney cap. The arch should be placed at right angles to the cause of the down draught.
The chimney cap itself, which is an important feature affecting the air currents that pull the smoke out, should be about 12 inches high. It can be made of cement and stone chips, modeled by hand. Reinforcing rods or wire mesh may be added if desired. The sides of the cap should curve from the outside edge of the masonry proper to the top of the cap.
A good type of flashing, which must be installed to prevent leaks between the chimney and the roof, is made of sheet lead, which is everlasting. Galvanized iron or copper is also satisfactory. The flashing should be cut to fit the chimney like a collar and is imbedded in the masonry. The roofing should overlap the upper edge several inches, and the flashing must overlap the roofing at the lower edge if set on a slanting roof. In case the chimney comes at the ridge, flashings are fastened over the roofing all around.