How To Build A Fireplace For A Log Cabin

The chimney-and-fireplace combination consists es­sentially of four parts—the fireplace itself, the throat, smoke chamber, and flue. The shape, construction, and relative dimensions of each of these parts is important.

Determining the size of the fireplace is regulated somewhat by the size and shape of the room. Some builders make the overall width equal to about one-third the longest dimension of the room. Others follow the plan of using a size that merely looks right. The opening should be wider than it is high, or at least square. Never make it higher than wide. A fireplace whose opening is less than 30 in. high is difficult to tend, so that this figure can be taken as a minimum, with the maximum about 40 in. for fireplace openings less than 6 ft. across. The lower opening is less likely to let smoke escape into the room. Other factors de­termining the fireplace size include the type of fuel to be used. Half-length cord-wood requires an opening at least 2 1//2 ft. wide, while if large logs are to be burned the width should be 6 ft. and the depth nearly 30 in. Depth of any fireplace should be 18 in. or more.

While a fireplace hearth may be of solid construc­tion, it is convenient to build an iron grate or ash-dumping device into it at the back, and make a cham­ber below it into which the ashes fall. When there is no cellar beneath the cabin, the ashes can be removed through an opening built into the outside wall of the chimney and equipped with a tight-fitting iron door. It is best to keep the hearth level with the floor throughout, so that refuse can be swept into it. Some builders follow the plan of elevating the hearth a few inches in the fireplace opening, pointing out that less stooping is required when adding fuel or removing ashes, and that a somewhat better distribution of heat is obtained. Sometimes the hearth curves upward 2 in. or so at the front edge of the fireplace opening, to form an ash pocket. The hearth can be of flat stone or con­crete, made as smooth as possible to facilitate tending.

A stone fireplace should be rugged and massive in construction. The distance from the side of the open­ing to the outer corner of the jamb should be in the proportion of about 16 in. for every 30 in. of opening width. The mantel above the opening can be built as an integral part of the fireplace, with flat stones; or it can be a hewn or split log supported on pegs set in the masonry. Stone chimneys and fireplaces should be at least 1 ft. thick.

Supporting the mass of masonry above the opening is a matter demanding care. When the fireplace is of massive construction, having jambs substantial enough to resist a considerable lateral thrust, an upward-curving arch can be used. Generally, however, a heavy iron beam, preferably of T-shape in cross section, is set above the opening to support the weight. This beam should be sufficiently large to prevent sagging when it becomes hot. The T is placed in an inverted position. Some forms of dampers are built as an integral part of a supporting bar.

The U. S. Department of Agriculture has made a study of proper fireplace construction. Many of the recommendations in this chapter are based on the results of that study. One important point that the government experts stress is proper shape and con­struction of the fireplace opening.

A fireplace opening can be compared somewhat to a reflector on an automobile headlamp or other source of more or less concentrated light. In the fireplace, heat waves must be reflected by the back and sides of the fireplace into the room. This is the only possible way that a fireplace can heat a room, other than by direct radiation from the fire itself. A fireplace does not circulate heated air as a furnace or stove does. On the contrary, it sucks in cold air, sometimes to the dis­comfort of occupants of the building, and sends it, together with hot gases, up the flue.

So slope the back of your fireplace forward, begin­ning the slope about one third the distance from the hearth; and build the sides so that they are straight vertically but slope inward from front to back. Thus the opening is widest at the front, and deepest at the hearth and for a distance upward of about one third the height. Line the sides and back with firebrick laid flat, with their longer edges showing.

By sloping the back surface of the fireplace forward, the opening leading into the smoke chamber and flue is restricted in width. The recommended width for this opening or throat is 4 to 5 in., preferably the smaller figure. It is important that the narrowest part of this opening or throat be 8 in. above the arch or lintel of the fireplace opening, no matter what the diameter or height of the fireplace. Some builders place the throat even with the top edge of the opening, and have a smoky fireplace as a result. It is equally important that the throat extend the full width of the fireplace. Do not start narrowing in the top of the fireplace until after the throat has been reached.

Although a fireplace without a damper will work well if otherwise properly designed, a damper is a highly desirable piece of equipment. Besides providing control over the behavior of the fire by regulating the draft, a damper can be closed when the fireplace is not in use, to keep mosquitoes, flies, birds, squirrels, and other pests from entering the room.

Such fireplace dampers consist of an iron lid hinged at the back of an iron frame that forms the throat. Construction of the throat is simplified because the iron provides a support for the masonry. A damper for a fireplace 36 in. wide or smaller can be purchased for less than five dollars. A handle projects through the front of the fireplace to provide a way of regulating the damper. While the maximum diameter of the throat may be greater than 4 or 5 in., the effective width is regulated by the damper.

Every fireplace should have a smoke shelf and cham­ber. The shelf is a flat area immediately behind the damper, and extending back until its rear edge is in line with the back of the flue. To understand the necessity for a smoke shelf, it is necessary to ob­serve the action of gas and air currents in a fire­place. Hot gases, sparks, and smoke from the fire rise into the throat of the fireplace because of the difference in air pressure inside and outside the fire­place and chimney. The narrow throat directs these gases along the front surface of the smoke chamber and flue. These gases rise rapidly and cause downward air currents to flow along the back of the flue. If the chimney is improperly constructed, these downward currents will be directed into the throat in such a way that the upward rush of gases and smoke will be checked, and a smoky fireplace will result.

This condition is prevented by making a shelf back of the damper. The downward air currents then strike the shelf and bound forward, intersecting the upward currents at such an angle that they do not cause inter­ference, but are carried back up the chimney. A dam­per, by creating an upward-sloping path, helps in properly deflecting these downward currents.

A smoke shelf, therefore, is necessary. Its depth is not governed by the size of the fireplace opening, but only by the relative positions of the flue and throat. It should be over 4 in. deep and should extend the full width of the throat.

Between the top of the throat and smoke shelf and the bottom of the flue is a space known as the smoke chamber. It is relatively large in volume, extending as it does over the combined width of the smoke shelf and throat, and the length of these parts. The smoke cham­ber provides a space in which the downward traveling air currents can expand and thus lessen their speed. It also exerts a cushioning action by providing a pocket to hold the smoke temporarily when gusts of wind blow down the chimney. Without a smoke chamber, such gusts would force smoke into the room. The sidewalls of the fireplace can begin tapering in towards the flue when they reach the base of the smoke chamber; but they should be built straight before that. Although the amount of taper can vary, the Department of Agriculture suggests that the walls be drawn inward 1 ft. for every 18 in. This lessens the friction, as com­pared with steeper slopes. Finish the walls of the smoke chamber with cement mortar applied at least ½ in. thick, and made smooth to reduce friction.

This cement mortar is of the type that should be used for laying up the fireplace and chimney, with the exception of the firebrick. The government specifica­tions for the mortar are: Mix 9 lbs, of dry hydrated lime with one bag of Portland cement (94 lbs.); then add three times this volume of clean sand and sufficient water to make a smooth-flowing, easily worked paste. Instead of the hydrated lime, ¼ cu. ft. of slaked lime putty can be used. The stones should be wet when mortar is applied. Do not use ordinary lime mortar for fireplaces and chimneys.

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How To Build A Log Cabin