When the joists run at right angles to the fireplace the trimmed joists are carried on a trimmer, which is framed into two trimming joists on either side of the opening, as shown in Fig. 267.

The proximity of wood to flues is governed in London by the Building Act, which states that no timber or woodwork in any wall shall be placed nearer than 12 inches to the inside of any flue or chimney opening, or within 2 inches from the outer face of any chimney or flue which is less than 8 1/2 inches thick, unless the face is rendered. Woodwork under any chimney opening must not be placed nearer than 10 inches from the upper surface of a hearth.

When joists run into bays, as in Fig. 265, they should either be trimmed into trimmers or rest upon beams or binders placed across the opening, in order to obviate the necessity of increasing the depths to suit the increased span.

The rule for finding the thickness of trimming joists is to make them 1/8 inch thicker than the common joist for every joist supported by the trimmer, which is itself usually made the same size as the trimming joists.

Bearing Of Joists

The ends of joists of single floors are often built into the walls for a distance of about 4 1/2 inches, but this is not a good method of construction, as these ends are liable to decay and the loads carried by the joists are concentrated on small bearings. Where the joists are built into the walls they should rest upon 4 1/2 by 3-inch wood wall-plates or on iron bearing bars, so that the load may be distributed throughout the length of the wall. It is by far the best plan to carry the joists on wall-plates supported on iron brackets, or on offsets as at L and D, or on corbels as shown at A in Fig. 266, so that the air may circulate round the ends and so render them less likely to decay.

The joists are joined to the wall-plates, as shown in Fig. 254.

2. Double Floors are used when the span is greater than is convenient for single joints. They consist of bridging joists supported on a second series of beams called Binders, as shown in Fig. 265.

Binders, when of wood, should be spaced (says Tredgold) at intervals of from 4 to 6 feet centre to centre, but in practice this higher limit is often exceeded. When the binders are of iron, as is now generally the case, they should be spaced in the most economical manner, this condition being found by experimental calculations. The ends of the binders should not be placed over openings; but where the exigencies of the case make it impossible to do otherwise, strong wall-plates or templates should be used over the openings to throw the weight on to the piers.

Tredgold s Rules For Sizes Of Floor Timbers 522

Fig. 267.

The Sizes of Binders are exhibited in the table already given, but it is generally more convenient to bear in mind some simple rule as a guide to proportioning them, such as the following: For binders of 6 feet span use timber 6 by 4 inches; and add 1 inch to the depth and 1/2 inch to the breadth for every additional 2 feet of span. This will be found to agree exactly with the table.

A formula very frequently used for determining the cantlings of binders is given on page 152, but it will be found to give scantlings somewhat larger than those generally used in practice.

Bearing Of Binders

Binders usually bear about 6 inches at each end in stone or wood templates built into the wall. Pockets are formed round the ends, as shown at E in Fig. 266, so that the air may have a free circulation round them.

The following are approximately the Loads on Floors: -

For ordinary dwelling-houses

1 cwt.

per sq. ft.

For floors of public halls used for concerts and dancing .



For warehouses .

2 to 4


In the case of warehouse floors, some reasonable estimate of the loads likely to be placed upon them should be made, and the floors calculated to meet the heaviest probable load.

Bridging joists are fixed to the binders by means of any of the joints shown in Fig. 254.

Strutting is frequently introduced above the binders, as shown at F and G, Fig. 266.

When it is undesirable to decrease the strength of binders by notching or housing the bridging joists into them, it is better to suspend the latter from the former by means of iron stirrups, as shown at H, Fig. 266.

3. A Triple-Joisted or Framed Floor, as its name implies, is constructed with three series of joists, namely, - Bridging1 Joists, Binders, and Girders, as shown in Fig. 268.

Girders are usually spaced at intervals of about 10 feet centre to centre, with their ends bearing from 9 feet to 12 inches on stone templates forming the bottom of a pocket.

The binders are usually tusk tenoned into the girders, as shown at K, Fig. 266.

The Sizes of Girders are exhibited in the table already given, but an easily remembered rule is as follows: For girders of 10 feet span use timbers 9 by 7 inches, and add 1 inch to the depth and 3/4 inch to the width for every additional 2 feet of span.

Ceiling Joists

When these are used they are spaced at intervals of from 12 to 15 inches, and are mortised as shown at H, Fig. 266; or, when it is desirable to weaken the binders as little as possible, supported upon fillets, as at F and G. Ceiling joists are made from 2 1/2 to 2 inches wide and 3 1/2 inches deep for a span of 6 feet, 1/2 inch being added to the depth for every additional foot to the span. They should project beneath the joists to which they are fixed, in order that a proper key may be formed for the plaster ceiling.

Tredgold s Rules For Sizes Of Floor Timbers 523

Fig. 268.

Ventilation Of Floors

Floors should be ventilated as thoroughly as possible, to prevent their timbers from decaying. For this purpose ventilating grids are built into the wall, as shown at A and L, Fig. 266, the brickwork above the openings being carried by a stone lintel. It is particularly desirable that the principal timbers in double and triple-joisted floors be ventilated, as serious accidents might happen in the event of their decaying.