This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Soot Doors are doors of iron which are let into the breasts of inclined flues to enable them to be swept properly. They are made in various sizes, but those less than 40 square inches in area should not be used. Woodwork should not be fixed within 15 inches of a soot door. Fig. 165 shows the type of soot door in common use.
Openings are sometimes left in the smoke flues for the insertion of ventilating valves (see Fig. 166), which should not be fixed nearer than 12 inches to any timber or other combustible material, they being placed as a rule at 12 inches or so below the ceiling line. Sometimes special flues are constructed for ventilating purposes, as on the left-hand side of Fig. 164, a single flue serving for all the rooms adjoining the chimney stack; while on the right-hand side of the same Figure the far more satisfactory arrangement of separate flues for each room is employed. These flues are constructed in precisely the same manner as are smoke flues, and terminate with chimney pot, cowls, or cast-iron airbricks built into the stack.
The insides of all flues, unless lined with fireclay, should be pargeted, i.e. rendered with mortar \ inch thick. The object of this is to prevent the accumulation of soot in the joints of the brickwork, and so lessen the risk of fire penetrating to the woodwork near the flues.
At one time the mortar used for parget was composed of one part of lime to three parts of dry cow-dung. This forms a very tough and fire-resisting composition, and is still used in districts where the cow-dung is to be easily obtained, but in towns ordinary lime and sand mortar has taken its place.
The outside of every flue should also be rendered where it passes through a roof or floor.
In building chimneys it is almost impossible to prevent mortar and pieces of brick falling down them and lodging on the lower sides of bends, and if these obstructions are not removed they considerably check the draught and cause great accumulations of soot. To prevent this defect a sweep, or bundle of rags or hay, is drawn up the flue as the work proceeds and a wire brush is passed through after the work is complete. Sometimes holes 12 by 9 inches are left in the breasts where the bends occur, so that they may be cleared of loose matter.
Fig. 165. - Soot Door.
Fig. 166. - Valve for Ventilating Flue.
Above the highest chimney opening the flues are brought together and carried out through the roof as one stack. The outside walls of chimneys above the roof are usually made 4 1/2 inches thick, but the draught is apt to be retarded by the hot fumes becoming chilled, for which reason it is better to make them 9 inches thick. The best position for a chimney stack is at the ridge of a roof, while the worst is at the eaves. Of course, it cannot be said that a chimney in the latter position is sure to smoke, but there is certainly a greater tendency for it to do so than one surmounting a ridge.
Fires belonging to stacks near tall buildings, cliffs, and tall trees almost invariably smoke, even when the flues are terminated in a cowl or windguard, owing to the air currents becoming diverted in a downward direction.
Every chimney stack or smoke flue should be carried up to a height of not less than 3 feet above the roof, flat or gutter adjoining thereto, measured at the highest point in the line of junction with such roof, flat, or gutter. It is better, however, to carry up all chimneys, where they occur in the lower parts of roofs, well above the highest parts of such roofs, bearing in mind at the same time that a chimney should not be built higher above the roof than six times the least width of the chimney at the level of the highest point in the line of junction with the roof, unless such chimney is built with and bonded to another chimney. Tall chimney stacks may be rendered secure by means of iron braces or flying arches between two stacks.
At least the highest six courses of a chimney stack should be built in cement.
There should be laid level with the floor of every storey, in front of the opening of every chimney, a slab of stone, slate, or other incombustible substance at least 6 inches longer on each side than the width of the opening, and at least 18 inches wide in front of the breast. Such slabs should be laid wholly upon brick, stone, or iron bearers, or upon brick trimmers or other incombustible material, and the slab and incombustible material upon which it is bedded should be solid for a thickness of at least 6 inches beneath the upper surface of such slab.
Where the space between the lowest floor and the concrete over the site is small the hearths are bedded on to the latter, or the height is made up where necessary with brickwork or fine concrete. This applies to all concrete floors at any level throughout a building. Where this space is great the hearths are supported by dwarf or fender walls, as shown in Figs. 167 and 168, according to their height and the weight brought upon them, usually from 4 1/2 to 9 inches thick, resting upon proper footings, which in turn are carried either by the 6-inch concrete over site or by the foundation-bed of the chimney projected out for this purpose. The space within the fender wall is partly filled with hard cone, on top of which a concrete bed is formed from 4 to 6 inches thick, according to the size of the hearth which is bedded upon it, as shown in Fig. 167. Sometimes rough Trimmer Arches, as in Fig. 168, are used to span the space between the main and fender walls, the latter being buttressed by means of cross sleeper walls or tied by means of iron rods to the main wall.
The construction of hearths in upper wooden floors is shown in Fig. 267, Chapter XVIII (Wood Floors).
Hearths may be of stone, brick, tile, or any other incombustible material. When of stone they are usually made 2 inches thick, and 1/2 inch is allowed for bedding them in cement. Tiles are also used for hearths, and vary in thickness from 1/2 to 3/4 in. Marble hearths are made from J to 1 inch in thickness, and great care must be taken to prevent lime mortar coming in contact with them, or in fact with any marble work, as it causes dirty brown stains; for which reason all the built-in parts of marble work are coated with a slip of plaster of Paris.
The London Building Act says that chimneys shall be built upon solid foundations, and with footings similar to the footings of the wall against which they are built, unless they are carried upon iron girders with direct bearings upon party, external, or cross walls.
It will be seen, by referring to the plans contained in Part I. of this Volume, that the loads borne by the foundations of chimneys are much greater in proportion than the loads borne by the foundations of walls against which the chimneys are built. It is therefore necessary to proportion their foundations to meet this greater load, in accordance with the principles set forth in Chapter I. When a stack is very high it is best to form an inverted arch between the lowest jambs.
Chimneys do not always start from the lowest floor, in which case the breasts should be supported on corbels of brick, stone, or other incombustible material, and the work so corbelled out should not project from the wall more than the thickness of the wall measured immediately below the corbel. Fig. 169 shows a corbel suitable for carrying the chimney breasts on an upper floor, and also a method of supporting the hearth upon a system of T and L irons, tiles being supported upon the tables of the T irons as a permanent centering for the concrete bed under the hearth.
In bonding chimneys where the external walls are 4 1/2 inches thick it is almost a universal practice to use stretcher or chimney bond, as shown in Figs. 170. It is most essential to the strength of a stack to make the withes bond well into the external walls, but in stretcher bond this cannot be done in a very satisfactory manner without expensive cutting. The usual way of bonding the withes is by splaying the bricks of one course as shown in Fig. 170. To avoid this defect English and Flemish bond are used, although their use necessitates a great amount of cutting when the external walls are only 4 1/2 inches thick (see Fig. 171). The difficulty is, however, overcome by alternate courses of Flemish and stretcher bond, as in Fig. 172. When the external walls of chimneys are 9 inches thick they are usually built as far as possible in English bond or Flemish bond, as in Fig. 172.
When the flues are of different sizes it is often difficult to build stacks in any particular bond, it being better in such cases to devise a method of bonding for each particular case, and to pay very little attention to the arrangement of the bricks or the faces, as shown in Fig. 173.
Chimney stacks may be made very ornamental in character by the judicious use of cut or specially made bricks.