This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
108. Floorbeams are sized or notched on the sill, as shown in Fig. 30, in order to bring their tops to an even line, any variation in the depth of the beams b being removed with the notch at e d, so that all the beams are exactly the same depth from a to c, thus securing an alinement of their upper edges. Nearly all timbers will curl or warp more or less in drying out, or seasoning, and when floorbeams are laid they should be placed with this convex, or crowning, side upwards, in order that any deflection of the beam due to the superimposed weight will bring the floor to a level line rather than produce a sag or dip in the middle of its span.
In the second story of a building the beams are notched over the interties, or girts, in the same manner as over the sill, and in balloon frames, where the girt may be but an inch in thickness, the floorbeams are cut on the under side, forming a channel, or groove, across the beam which rests on the girt, and not only brings the tops of the beams to a level, but also forms a tie across the building. In brick or stone walls, the beams are maintained at the same level by placing small pieces of slate under the shallower beams, and thus raising them to the proper level. Thin slips of wood are sometimes used instead of slate, but as they are seldom made of a uniform thickness, and squeeze readily under pressure, they are not to be recommended. The method of tying such beams into a brick wall is shown at k, Fig. 32, where a strip of iron not less than 3/8 in. x 1 1/2 in. in section, called an anchor, is nailed to the side of the beam, and its end, which is bent around a 3/8-inch iron rod, is allowed to project from 4 inches to 8 inches beyond the end of the beam, and the brick or stone work is then carried up around it, enclosing the anchor and securing the beam.
109. Bridging is the term given to a system of bracing applied between floorbeams in order to prevent them from bending side wise, or curling out of position after they are in place. They also tend to strengthen the floor system when it is not uniformly loaded, by distributing the weight among the adjacent joists. Fig. 31 shows a method by which this is accomplished, called herring-bone bridging, in distinction from other forms of bridging to be described subsequently. The herring-bone bridging shown in Fig. 31, is generally formed of 2"x4" pieces a and b cut between, and well nailed to, the joists c, as shown, and spaced in rows about 8 feet apart.
Trimmer Beams. Where a chimney occurs in the side wall or in the center of a building, it is necessary to frame the floorbeams in such a manner that no wooden beam shall come within 4 inches of the brickwork enclosing a chimney flue. This is accomplished as shown in Fig. 32, the method of framing and the joints required having been already described. When there is to be a fireplace opening and hearth in front of the chimney, the header a, Fig. 32, should be placed from 1 ft. 9 in. to 2 ft. 6 in. away from the chimney breast, in order to provide room for a trimmer arch, as shown at a, in Fig. 80.
This method of framing with headers, trimmers, and tail-beams is also used around the openings for stairs, skylights, dormers, and dumb-waiter shafts. In all cases the header a and the trimmers d, Fig. 32, are made at least twice, or three times the thickness of the regular floorbeams b and k, as upon these trimmers is imposed the combined load of all the tail-beams b.
111. Flooring is applied directly to the top of the joists, after they have been brought to a true alinement, as above described, and is plain or matched, single or double, as the case and character of the building requires.
A plain floor consists simply of second quality boards nailed directly to the floorbeams, each plank being from 6 in. to 10 in. in breadth, and forming a butt joint with its neighbor. Such a primitive floor is seldom used except in temporary structures.
A matched floor is laid with selected material which has been matched, or tongued and grooved, as shown in Fig. 33. This matching, though formerly worked entirely by hand, can now be done by machinery at the mill where the material is cut up from the seasoned timber, and matched flooring is a commercial article purchasable at any lumber yard.
112. Matched floor boards, when laid, are about 3/8 of an inch narrower than the width at which they are purchased, which shortage represents the amount cut away in the milling. At a, b, and c, Fig. 33, are shown three pieces of matched 4-inch flooring, which measures, when laid, but 3 5/8 inches as shown, but the tongue d projects \ inch, making the total width of the board 3 7/8 inches, the remaining \ inch being lost partly in waste in the process of matching, and partly by shrinkage after the matching has been done. At one edge of the board a projecting rib d, called the tongue, is cut, and on the opposite side a groove e is sunk a trifle deeper than the projection of the tongue. The upper edge f of the groove projects about 1/32 inch more than the lower edge, so that when the flooring is laid and the groove is driven tight on to the tongue, as at /, the 1/32-inch clearance between the front of the tongue and the bottom of the groove at m, and between the shoulder of the tongue and the lower edge of the groove at n, leaves the upper edge of the groove to be driven tight against the upper shoulder of the tongue, thereby forming a tight joint on the surface, as shown at p.
The same result is obtained by keeping the under shoulder of the tongue slack, in which case the grooved edge would be square. The tongue is always placed slightly below the center of the thickness, so as to give more wearing surface to the floor and render the edge of the groove less likely to curl up.
When this matched flooring is of hardwood, such as oak, maple, or yellow pine, a shallow groove about 1/32 inch deep is cut along the under side, as shown on pieces b and c at h i. This enables the boards to be laid tight to the beams or under flooring, and causes them to be less affected by any slight unevenness of the substructure.
113. Matched flooring should always be blind nailed; that is, the nails should be driven in the upper angle of the tongue, as shown at k, in a diagonal direction, and, in the hardwoods, the nails should be punched in, so that their heads are well below the surface, and as the groove of the next piece covers the tongue, it hides them from sight. The boards are nailed only on the tongue edge, as the groove edge is held fast by the tongue. Next the wall, on both sides of the room, it is necessary to nail the first and last board through the upper face. As the baseboard will cover these nails the entire floor will present an even surface, without a nail head in sight.
This description of flooring would more often apply in the case of a double floor, though a first-class job of a single matched floor would demand equal care.
114. A double floor consists of a rough, plain, or matched floor of boards (preferably matched), laid when all the beams are in place and bridged, and of a second, or finished floor, of the character just described in detail, laid when all the rough work in the building is complete, and when the joiner work is in progress. In first-class work, the finished floor should not be laid until the base, window and door trim, etc., have been fixed in place.
115. When two thicknesses of flooring are laid, the first, or under floor, is usually laid diagonally across the beams, as shown at a, Fig. 34, and the second, or finished floor, is laid on top of this, but at right angles to the beams, as shown at b. When the rough floor is laid at right angles to the beams, and the finished floor runs in the same direction on top of it, the shrinking of the wide boards beneath may pull two or three of the boards of the finished floor up tightly together, thereby leaving open joints in the upper floor at regular intervals, corresponding very nearly with the widths of the rough boards below. The diagonal laying of the lower flooring obviates this difficulty, and to insure an even bearing all around, it is sometimes necessary to nail between the beams, at their ends, some supporting pieces m, m, Fig. 34, to which the diagonal boards may be spiked. The rough floor should be well nailed to the joists, and care should be taken that the finished floor is nailed only over the joists; when driven into the boards the nails possess little value, and are likely to work loose. For floors 7/8 inch thick, 2 1/2-inch cut nails should be used, and for 1 1/8-inch floors, 3-inch nails.
The thickness of the boards in the lower and upper layers of a double floor are usually 1 inch and 7/8 inch, respectively, but single floors in dwellings should have the boards at least 1 1/8 finches thick. In mills, factories, warehouses, etc., the flooring varies from 1 1/8 to 4 inches in thickness, as will be explained further on.