Fig. 11. Bad Joist Anchor
For the same reason, the ends of all the joists, with or without anchors, should be beveled so that, in like conditions, the joists will readily fall out without injury to the wall. [Fig. 10] illustrates the correct method of attaching the anchor to the joist. The dotted lines show how the joist would drop without damaging the wall. [Fig. 11] shows the destructive effect caused by the anchor being placed at the top of the joist. The importance of these points cannot be emphasized too much as walls have had to be rebuilt which by proper framing construction would have stood intact. After the joists are placed, the brickwork is continued up between, and leaving a small "breathing" space around, them. The same method of joisting is followed at the upper floors.
Fig. 12. and Fig. 13. True Corbeling Between Joists
If the lower part of a wall is thicker by a brick than the upper part, it should be carried up its full thickness nearly to the top of the joists Fire Stops where ft is stepped back to the inside face of the upper part, thus forming with the plastering a fire stop at the top of the joists, while a projection of a quarter brick length should always be provided as a fire stop at the bottom of the joists, as shown in [Fig. 12]. If the wall is the same thickness throughout, the brickwork should be corbeled out between the joists two inches, the full height of the joists, to form a fire stop as in [Fig. 13]. The object of the fire stop is to block all possible passage of fire from the space between the joists to that between the furring strips on the wall, or the reverse. Without these fire stops, a fire originating in the floor could communicate with the furring space on the wall above, or originating in the furring space could communicate with the floor. With the stops, the fire is confined to certain spaces and is retarded instead of spreading. These corbels also serve the wholesome purpose of checking vermin of all kinds from passage through the floor and wall spaces.
Ceiling Lath
[Figs. 12 and 13] also show the proper way of placing the lath at the corner of the ceiling so as to take full advantage of the fire stops. The ceiling lath, usually placed first, should be started far enough away from the side walls so that when the side wall lath is placed tight, as it ought to be, against the underside of the floor joist, there will be space enough for the plaster to push through and form a key touching the bottom brick of the corbel. As the corbel by construction is necessarily the distance of a mortar joint above the bottom of the joists, the openings are thus completely sealed by the plaster key. In cheap speculative buildings, these fire stops are too often omitted or a pretext for them is resorted to by projecting only one brick at the top or bottom of the joists. This, however, is as good as no fire stop at all. [Figs. 14 and 15] show the lath as they ought not to be placed and also how false corbeling leaves the passages really unstopped, thus defeating altogether the purpose of fire stops.