diate timbers tenoned into the
trimmer are known as the trimmed joists. This system has to be resorted
to when it is impossible to get a bearing on the wall.

[Illustration: FIG. 25.--Double Floor, with Steel Binders.]

A trimmer requires for the most part to be carried or supported at one
or both ends by the trimming joists, and both the trimmer and the
trimming joists are necessarily made stouter than if they had to bear no
more than their own share of the stress. In the usual practice the
trimmer and trimming joists are 1 in. thicker than the common joists,
but there are special regulations and by-laws set out in the various
districts and boroughs (see _By-laws_, below) to which attention must be
given.

[Illustration: FIG. 26.]

[Illustration: FIG. 27.--Construction of a Medieval Floor.]

The principal objection to single flooring is that the sound passes
through from floor to floor, so that, in some cases, conversation in one
room can almost be understood in another. To stop the sound from passing
through floors the remedy is to pug them (fig. 24). This consists in
using rough boarding resting on fillets nailed to the sides of the
joists about half-way up the depth of the joists, and then filling in on
top of the boarding with slag wool usually 3 in. thick. Also to further
prevent sound from passing through floors the flooring should be tongued
and the ceiling should have a good thick floating coat, in poor work the
stuff on ceilings is very stinted. In days gone by, ceiling joists were
put at right angles to the floor joists, but this took up head room and
was costly, and the arrangement is obsolete.

[Illustration: FIG. 28.--Herring-bone Strutting.]

[Illustration: FIG. 29.--Solid Strutting.]

Double flooring.

Double flooring (fig. 25) consists of single fir joists trimmed into
steel girders; in earlier times a double floor consisted of fir joists
called binding, bridging and ceiling joists, but these are very little
used now and the single fir joists and steel girders have taken their
place. Steel girders span from wall to wall, and on their flanges are
bolted wood plates to receive the ends of the single joists which are
notched over plates and run at right angles to the girders (fig. 26).
The bearings of the joists on the wall also rest on wall plates, so as
to get a level bed, and are sometimes notched over them. Wall plates,
which are usually 4-1/2 in. X 3 in. and are bedded on walls in motar,