The width of the hearth is ordinarily made about sixteen or eighteen inches beyond the face of the opening with the average size fireplace, twenty inches or even more with larger ones. This width should be increased, of course, if the opening is made considerably larger. The question of materials for the hearth and facing will be discussed in the next chapter.
The chimney itself should extend at least a foot or two above any nearby roof ridge and it should work without any cowl, whirligig or other device of that type on the top. There is no great objection to having the opening a horizontal one at the top of the chimney, although in that case if the flue is nearly straight throughout its course, some rain will find its way down to the hearth in a hard storm. In most cases there is enough bend in the flue to prevent this, and if not it may be avoided by covering the top of the chimney with a stone and having the openings vertical ones on all four sides just under this.
All of the brickwork throughout chimney and fireplace should be laid in first-class cement mortar which consists of one part Portland cement to three parts clean, sharp sand. Although lime mortar was used in all brickwork up to recent years, it is not durable, particularly in the vicinity of heat.
MISCELLANEOUS ODD FORMS
There are many unusual forms of fireplace with which we are not particularly concerned. For example, one sees occasionally an opening shaped like an inverted heart or like an ace of spades. It is possible to make a fireplace of this kind work satisfactorily, but it is by no means certain that this result can be accomplished at the first trial nor that the fire will continue to work properly under all conditions. It is safer always to adhere to the established type of rectangular opening, or to depart from this only to the extent of having the top an arch of large radius. Whenever the top is permitted to vary more than a slight extent from the horizontal there is the danger of having the smoke escape into the room at the top.
There is one other type that deserves special mention and that is the double fireplace, where two openings in adjacent rooms are served by a single flue between them. The only way in which this affects the two vital principles mentioned above is that the cross-section area of the flue should be one-tenth of the combined areas of the openings. The throat will in this case be in the middle of the chimney with the smoke shelf on either side of it. It is essential in a fireplace of this kind that there be no disturbing draft tending to pass through the opening from one room to the other.
Still another type which is even more rarely seen is the open fire in the middle of a room, such as may be desired occasionally in the lounging room of a large club. Such an apparent anomaly could be secured by suspending a metal flue and hood from the roof, so that the lower edge of the truncated pyramidal form at the bottom would form the upper side of the fireplace “opening” at a convenient height above the hearth of brick, stone, tile or concrete. It is conceivable that an effective and thoroughly practical fireplace could be thus devised, having the flue and hood of wrought iron or copper, suspended and steadied by chains or bars from the ceiling and surrounding walls. In such a form the same principle of a fixed ratio between opening (here the entire perimeter of the hood multiplied by the distance above the hearth) and cross-section of flue would have to be observed, and here also it would be well to provide as fully as possible against the presence of disturbing drafts.