Fig. 165 * shows a method of constructing vaults much used by the ancient Romans. A light temporary centre of wood was first put in place, and on this light brick arches were built to form a framework for supporting the weight of the vault until set. These brick arches were called armatures, and as they became the real support of the vault only very light wooden centres were required. After the armatures were built the spaces between them were filled with rough masonry or concrete, as shown in Fig. 166.
In planning brick chimneys the principal points to be considered are the number, arrangement and size of the flues and the height of the chimney. Every fireplace should have a separate flue extending to the top of the chimney. Two or three stoves, however, may be connected with one flue if it is of sufficient size, and the kitchen range may be connected with the furnace flue without bad results, and often the draught of the furnace will be benefited thereby. For ordinary stoves and for a small furnace an 8x8
* Figs. 165 and 166 are taken from the Brickbuilder', by permission.
flue is sufficiently large if plastered smooth on the inside, but it is generally better to make the furnace flues 8x12 inches and also the fireplace flues, except for very small grates.
The best smoke flue is one built of brick and lined with fire clay tile, or else a galvanized iron pipe supported in the middle of a large brick flue. When the latter arrangement is used the space surrounding the smoke pipe may be used for ventilating the adjoining rooms by simply putting registers in the wall of the flue.
When galvanized iron smoke pipes are used the metal should be at least of No. 20 gauge, and No. 16 gauge for boiler flues. Even then the pipe is liable to be eaten away by rust or acids within ten or twelve years. Fire clay flue lining, on the other hand, is imperishable.
Smoke flues are sometimes made only 4 inches wide. Such flues may work satisfactorily at first, but they soon get clogged with soot and fail to draw well, and should never be used unless it is impracticable to make the width greater.
More flues smoke or draw poorly on account of the chimney not being of sufficient height than from any other cause. A chimney should always extend a little above the highest point of the building or those adjacent to it, as otherwise eddies may be formed by the wind which may cause a downward draught in the flue and make it smoke. If it is impracticable to carry the chimney above the highest point of the roof it should be topped out with a hood, open on two sides, the sides parallel to the roof being closed. The walls and withes (or partitions) of a chimney should be built with great care, and the joints carefully filled with mortar and the flues plastered smooth on the inside with Portland cement, both to prevent sparks or air from passing through the walls and to increase the draught. Chimneys were formerly plastered with a mixture of cowdung and lime mortar, which was called pargetting, but this mixture is now seldom, if ever, used. Portland cement is not affected by heat and is the best material for this purpose.
In building the chimney more or less mortar and pieces of brick are sure to drop into the flue, and a hole should be left at the bottom, with a board stuck in on a slant, to catch the falling mortar. After the chimney is topped out the board and mortar should be removed and the hole bricked up. If there are bends in the flue, openings should be left in the wall at those points for cleaning out any bricks and mortar that may lodge there. The outer wall of a chimney should be 8 inches thick, unless a flue lining is used, to prevent the smoke being chilled too rapidly.
During the construction of the building the architect or superintendent should be careful to see that no woodwork is placed within 1 inch of the walls of a flue, and that all the flues are smoothly plastered their entire height.
The arrangement of the flues is ordinarily very simple. Fig. 167 shows the ordinary arrangement of the flues in a chimney containing a furnace flue and fireplaces in first and second stories, and ash flue for second story fireplace.
Fig. 168, from Part II. of "Notes on Building Construction," shows the arrangement of the flues in a double chimney, with fireplaces in five stories.
Fig. 167. - Plan.