Chap. XXII.

CARPENTRY

1311

quantity of timber in the one case to that of the other is as 4 to 3, so that one-third moretimber gives a strength double that of the other.

Where square oak joists are used, and the bearing 12 feet, their scantlings should be 6inches, and laid at a similar distance apart: such a floor contains the same quantity oftimber as if entirely formed of 3-inch plank : the strength of timber being as thesquare of its vertical thickness, it results that the strength in these two instances is2 to 1 : the floor composed of 3-inch plank is only half the strength of the other; buthad the whole been formed 6 inches thick, instead of with joists 6 inches apart, it wouldhave been four times as strong, the square of 3 being 9, and the square of 6, 36.

Naked Floors are divided into single-joisted, double , and framed , floors; and it must beremarked that unsawn timbers are considerably stronger than planks or scantlings cut outof a round tree : when a tree is cut longitudinally, and formed into two pieces, these willsupport less than they would do when united in the original tree, arising from the circularconcentric rings which compose the tree being cut through, which renders the timber morecompressible on one side than on the other; and as the texture is less close where it hasbeen sawn, it is also more susceptible of change from humidity on alternation of temperature.

Joists, -whose width is less than half their vertical thickness, are subject to twist andbend, if not strutted; and for this reason squared timber was usually employed by thebuilders in the middle ages; and we have numerous examples, four or five hundred yearsold, where the timber selected has the pith in the centre, and the concentric ringsnearly entire, being in a sound and perfect condition: experience also teaches us thattimber, whether sawn or unsawn, used for a floor of 16 feet bearing, composed of 12 joists,8 inches square, placed at a distance of a foot apart, is much stronger than another of 24joists, 8 by 4, placed edgeways, at a distance of 6 inches apart, although there is thesame quantity of timber in both cases.

Single-joisted Floors consist of one series of joists, which ought to be let down or halvedon to wall plates of a sufficient strength and scantling to form a tie, as well as a support tothe floors; each joist should be spiked or pinned to thetimbers on which it lies: wherever fireplaces occur, andthe joists cannot get a bearing on the wall, they are let into atrimmer or piece of timber framed into the two nearestjoists that have a bearing; into this the other joists aremortised. As the trimming joists support a greater weight,they must be made stronger than the others, and shouldhave | inch additional thickness given to them for everyjoist they carry. When the bearing exceeds 8 or 9 feetthe joists should be strutted, or they will have an inclinationto turn sideways : the joists in use, being generally thin anddeep, require strutting on all occasions, and a rod of ironis often passed through them, which, being screwed up afterthe strutting pieces are placed, gives the entire floor greatsolidity and firmness. The weight of a square of single-joisted floor varies from 10 cwt. to 1 ton, and the joists should never extend to a greaterbearing than 20 feet in ordinary cases.

To find the Depth of a Joists when the length of bearing and breadth in inches isgiven: divide the square of the length in feet be-tween the supports by the breadth of the joist ininches, and the cube root of the quotient multipliedby 2*2 for fir and 2-3 for oak, gives the depth ininches: a single-joisted floor, which has the samequantity of timber as a double floor, is considerablystronger, particularly if properly strutted, than thelatter. The plates, bedded on the walls, upon whichthe joists are to be tailed down, should have theirdepth equal to half that of the joists, and their widthhalf as much more ; in many instances the plates arenot bedded entirely in the wall, but have one halfresting beyond the face on corbels let into the wall,at a distance of 6 feet apart. To form the entailleor dovetail great care should be used, to prevent thejoist from drawing out of its place, when once pinneddown.

Double Floors are formed of Joists , Binders , and Ceiling Joists : the binders rest upon theplates bedded on the walls, and serve the purpose of supports to the joists which are bridgedon them, as well as to the ceiling joists, which are pulleys mortised into their sides:when the depth of a binding joist is required, the length and breadth being given, dividethe square of the length in feet by the breadth in inches, and the cube root of the quotient,

Fig. 2094,

Fig. 2093.