Construction of Chimney Flues.
In a letter to the Insurance World Mr. Thomas Boyd, architect, of Pittsburg, Pa., gives the following practical information:
I have had considerable experience in examining buildings burned by fire, having been associated with my father for eleven years, and during that time have examined hundreds of buildings destroyed by fire. I have traced more fires to the cause of defective flues than to any other source, and I could refer you to buildings, not only in this city but in others, where fires have occurred from this cause, and the insurance men and the public in general stated that the fire occurred from “unknown causes,” as it was first seen many feet away from the flues.
In seventy‐five cases in one hundred where fires occur from “unknown causes,” it can be traced to defective brickwork. Ordinarily, an architect specifies that the brickwork shall be well slushed, and that the flues shall be well pargeted or plastered on the inside. This is a great error, as no flues should be plastered on the inside, and no walls having flues in them should be slushed, as the term is generally understood.
The flues should in all cases be built smooth on the inside, and all the joints should be filled full of mortar, the vertical joints as well as the bed joints. The lining of the flue or the four inches surrounding the flue should always be kept in advance of the brickwork, and the brick adjoining the lining and the second and third brick, and so on, should be shoved in soft mortar up against each other. This will fill all the vertical joints from bottom to top as laid. The slushing that is ordinarily put in from the top only goes down into the joint about ½ inch, thus leaving an opening the entire length of the wall, and in some cases an opening which a mouse could crawl through. As it is only a question of time when all the plastering that can be put on the inside of a flue will fall off, it will leave these vertical joints between the bricks open into the flue, and as the joists cross through these joints in the brickwork, fire is liable to take place ten or twenty feet away from the flue. I have taken down many old buildings in which these joints were filled with carbon or soot.
If the flues are built as above described, any competent builder or architect can find out whether the mechanics doing the work are slighting it or not; but if the inside of the flue is plastered or lined with terra cotta or any other material, you cannot tell whether the wall is properly built or not until this plastering falls off, which it will in the course of a few years. Thus all buildings erected with plastered flues are liable to burn at any time.
I have made a practice for a number of years of building flues without lining them, and then when the house is built, or as each story is erected, I put a dense smoke in the flue and close the opening at the top. If there is a hole the size of an ordinary pin head, the smoke will find it and penetrate into the interior of the wall adjoining the flue.