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which they are connected should always be thicker than the ordinary joists. A common rule is to make the trimmers and trimming joists ⅛ in. thicker for each joist carried. Single floors may span as great a distance as 18 ft. by using 11-in. by 3-in. joists stiffened with two or three rows of herring-bone or solid strutting.

When the distance between the supporting walls exceeds 14 ft. or 15 ft., it is usual to place binders or girders of wood or iron at intervals of from 6 ft. to 10 ft., and on these to support the bridging joists. Floors so constructed are known as double floors, having two sets of joists, the lower set (ceiling joists) being smaller, and used solely to support the ceiling. Thus the ceiling, being supported independently of the floor joists above, is not liable to be jarred by



the traffic overhead, and the connection between the ceiling and floor being broken by the space between the two sets of joists, sound from above is not so audible below as when the floor is single.

Wooden Binders.—The outline plan of a double floor is given at Fig. 325, and Fig. 326 is a section through the joists, flooring, and ceiling, showing the side of the binder and also the method of supporting it. Fig. 327 is a section through the binder showing alternative ways of connecting the ceiling joists with the binder by mortise and tenon joints. Ceiling joists which have to be got into position after the binders are built in have their tenons inserted at one end into an ordinary mortise, whereas the tenon at the other end has to slide into a chase mortise as indicated at Fig. 328. To avoid weakening the binder, sometimes a fillet is nailed on so as to support the ceiling joists, which are notched to it as shown at Fig. 329. Fig. 330 illustrates the case where