On account of the risk of fire to which the building would be exposed if the ends of the joists rested on or near the chimney breast, it will be noticed on the plan, fig. 267, that the joists are trimmed round the fireplace to avoid any such risk.
A joist, stouter in width, called a trimming joist, T J, is placed an inch or two away from the side of the breast; and at 14 or 18 inches away from the face of this breast - according to the width required for the hearthstones - another joist, called a trimmer, T, is placed, running parallel to the face of the breast, and the ends of it are framed into the trimming joists, T J; and along the trimmer, in equal spaces of about 15 inches, from centre to centre, the trimmed joists are framed, with the other end resting on the supporting wall.
The plan and section, fig. 26S, are intended to illustrate the explanation just given of trimming round fireplaces.
Fig. 269 will show the student how to trim round well-holes, trapdoors, and other openings, where the bridging joists run in a different direction.
Before proceeding to define and illustrate joints usually met with in single floors, it will be as well to explain one or two items which, though belonging to other kinds of floors as well, are connected with single floors.
Herring bone strutting, cross nogging, or stemming, is a means of strengthening and stiffening the bridging or common joists, and consists of small pieces of 3 x 1-inch or 3 x 2-inch stuff, fixed diagonally between the joists, as fig. 270.
There are two independent lengths in a row of it, as A A and B B, and they stretch from wall to wall, thus forming one continuous connection or support between the walls, and rendering each joist perfectly stiff - one row abutting on each side of the top of the joist, and the other at the bottom thereof. A row of this strutting, consisting of two courses, should be inserted every 4 feet apart where the span exceeds 1a feet.
Solid strutting or stemming is an alternate method to the latter, and used in a similar way for the same, and it consists of a solid piece of wood the depth of the joist and 1 1/2 inches thick, let in between the joists in one continuous line from wall to wall.
Pugging is supposed to be a remedy for the prevention of sound being conducted from one room to another by means of the floors. On each side of the joists 1 3/4 x 1/2-inch laths are nailed from wall to wall; and resting on these laths, between the joists, the sound boarding, as it is called, is laid, to carry the mixture which is to deaden the sound (See fig. 271.)
This mixture is laid on the boards to a thickness of about 3 inches, and it consists of either ordinary mortar, with an admixture of hay, or of sawdust or ashes, or of asbestos or silicate cotton in slabs.
Another method is to nail cedar-felt from joist to joist, as on sketch (fig. 272), and fill in the space with sawdust.
Some of these kinds of pugging cause the joists to rot, owing to the dampness of the mixture or the want of air-circulation which they entail; and, to obviate this, some floors are constructed specially with a view to prevent the conduction of sound; but they involve extra expense, and raise the building a very little higher. Some people would call the method a "double" floor; but this is not done for the purpose of strength, though it may be double in construction.
Fig. 273 shows ceiling beams, C B, inserted between the bridging joists, œ J, placed about 8 feet apart, to carry the ceiling joists; which, it will be seen, make the ceiling independent of the floor and not liable to be affected by any vibration of the floor itself.