Double Floors----In these the bridging joists, instead of spanning the whole distance from wall to wall, are supported by intermediate balks called Binders (or "Binding Joists"), B B, Fig. 276.
Fig. 277. Scale, 6 feet = l inch.
Plan and Sections of Double Floor.
The stiffness of these floors prevents deflection, and secures the ceiling from cracking. They stop the passage of sound from the room below, and the massive binders are of great assistance to the walls of the building in tying them together. Moreover, if binders are placed close to the walls to carry the ends of the joists instead of wall plates, all timber may be kept out of the masonry except the ends of the said binders themselves.
Double floors are in some ways a complicated and bad form of construction. The bridging joists instead of being merely supporters add their own weight to be carried by the binding joists, and being superposed upon them cause the floor to be very deep, which adds to the height of the walls and the cost of the building.
The binding joists bring the whole weight of the floor and its load to bear upon a few points; if the wall is weak and full of openings this is an advantage, for the binders may be carefully arranged so that their ends fall upon the stronger portions of the wall, leaving the weaker parts unloaded.
The space between two binders is called a "Case Bay" and that between the binder and wall a "Tail Bay,"
Tredgold recommends that binders should be fixed from 4 to 6 feet apart, not more than 6 feet. They should be placed so that they may rest on the piers between the windows, not over the openings; they bear either upon wall plates running the whole length of the wall, or upon stone templates of a sufficient length to distribute the pressure.
A plan and sections of a double floor are shown in Figs. 276, 277, 278.
The binders rest on stone templates, tp; and the trimming of the joists to clear a flue in the wall is shown at A.
The method in which the floor is finished, with an oak border round the hearthstone, is also shown. A similar border is shown in section in Fig. 290. It is sometimes made, for economy, thinner than the floor boarding, which is checked out to receive it.
The ceiling joists are omitted in plan to avoid confusion. It will be understood that they are attached to the under side of the binders, as shown in section, and run at right angles to their direction.