Double floors are used for spans of over 16 feet, to reduce the bearing of the bridging joists; for which purpose large beams called binders are placed - at intervals of every 10 feet, at most - between two of the walls and resting on the other two.

Double Floors PracticalBuildingConstruction01 242

Fig. 282.

Double floors have the advantage of stopping the passage of sound, and rendering the bridging joists stiffer, so that the ceilings are not so liable to crack; and they also act as a tie to the walls of the building.

Double Floors PracticalBuildingConstruction01 243

Fig283.

On the other hand, they are more complicated in construction, which is always a defect; and, moreover, they raise the building and add expense thereby.

It should always be borne in mind that, as the "binders" collect the weight of the floor off the "bridging joists," they must be supported by solid masonry, brick, or ironwork - not placed over openings, as can be done with the ordinary joists. This is sometimes an advantage, especially where the walls beneath are full of openings. In such cases the " binders," carrying, as it were, the whole weight of the floor, can be placed over solid parts, sometimes specially designed for them; and the weaker parts above the openings can be left without any weight at all on them.

Plan.

Scale 1/2" inch- 1 Foot.

Double Floors PracticalBuildingConstruction01 244

Section on lint C D Fig. 284.

In fig. 284 the plan and sections are illustrations of a double floor.

The space between the binders is called a Case Buy, and that between the binder and the wall a Tail Bay.

The " binders " should rest on stone templates on the walls, in a little recess, as fig. 285, so that the air can circulate round the wood, as to build any timbers into the damp walls is always conducive to rot.

Double Floors PracticalBuildingConstruction01 245

Fig. 285. I Scale.

The joints used in this flooring are similar to those last described for single floors, the only addition being that whereby the bridging joists, etc., are fixed to the binder. The joists are generally notched on, as fig. 286; but this is a bad way, as the notches, being cut out of the "binder," weaken it to a great extent.

Fig. 286.

The better plan is, as before explained, to nail fillets on the sides of the binder and notch the joists, instead of the binders which cany the joists and the floor, as fig. 387.

Sometimes the ceiling is put on the underside of the bridging joists where the height of the rooms below will not allow of special ceiling joists; in which case the " binder" is planed and moulded to prevent its being so great an eyesore, and it has to be notched for the bridging joists, or they are cut away where they take the bearing, a process which in no way weakens the framing {vide fig. 288).

The chase-mortise is also a joint that is employed in this kind of floor. It consists of a tenon cut on the ceiling joist and put into the mortise which is cut out of the fixed " binder " with bevelled slide into it, as shown in fig. 389. This joint is very handy in cases where the " binders " (or mortises) are so placed that the tenon cannot be fixed in as usual. It is also used when the space allotted to the depth of the floor will not allow of the ceiling joists projecting below the binder.

Binding joists are only required for spans above 16 feet, unless under exceptional circumstances. The best way to impress on the memory the sectional dimensions required for the different spans is to take 5 inches wide and 8 inches deep as the standard for 10-feet spans, and to add 1 inch in depth and 1/2 an inch in width for every additional 2 feet. Thus we should have the following dimensions; -

8 inches deep by

5 inches wide, suitable

for 10-feet spans.

9 " "

5 1/2 " "

12 "

10 „ "

6 " "

14 "

11 "

6 1/2 „ "

16 "

12 " "

7 " "

18 "

13

7 1/2 " "

20 "

14 " "

8 " "

22 "