Floor Joists Construction Tips

¶ Joists and Partitions. - A point in the setting of joists too frequently neglected, is the hearing or distribution of the weight on the wall. We have seen carpenters leave some joists resting on a bearing of one inch, while others would have from four to six inches. Now, ordinarily, the insertion of a joist to the depth of four inches in a brick wall, or six inches in a stone wall, is sufficient for practical purposes - that is, on the assumption that all the materials are good, the brick solid, and the joists of proper dimensions and sound timber; but three grains of common sense will show us that little advantage is derived from the depth of insertion, if:, after all, the joist is allowed to bear only upon an inch block, or, as sometimes happens, a trifling pine wedge. No wonder that in some of our would-be fine houses we see the wash-boards and floor parting company, a catastrophe usually attributed to the shrinkage of joists, but often really owing to the above cause.

¶ Lattice bridging is a process of great importance in view of the additional firmness thus given to the floor; no span greater than ten feet should be without a course of bridging in the center, and any greater than twenty ought to have two courses.

¶ Ceilings derive additional security from cracking, by cross-lathing the joists with 1½ by 2-inch lath, to receive the plastering lath; this insures a gradual distribution of any shrinking or sagging that may take place in a particular joist, whereas the abrupt departure from the plane of the ceiling by either of the above accidents is almost sure to cause fissures in the plastering.

¶ Where stud-partitions cross a room of large span, some provision for the support of the weight thus added should be made; the simpler mode of doing this is to double and pin together the joists directly beneath the line of partition, but a more effective method is found in the use of struts, see picture, where the position of the openings will admit of it.

¶ The crowning or cambering of joists is a very good practice; this consists in dressing the upper edge of the joist with a curve in the direction of its length, the rise above a straight line varying from half an inch to an inch, in proportion to the length of the joist; in the first place, this has a tendency to prevent a sagging or deflection of the floor, and in the second place, this sinking must be considerable before the floor in the center falls below the plane of the floor-line at the walls. With the bridging above spoken of and the crowning here described, it is scarcely possible for a well-joisted floor of reasonable span to succumb to any pressure likely to occur in home rooms. We mean by well-joisted, a sufficient number of joists of sufficient dimensions. We would seldom place flooring joists less than sixteen inches between centers, and never, except for very small rooms, recommend the use of less than two and a half by 10-inch joists. We have already expressed our views on the relative proportions of weight-sustaining timbers, and with a few incidental remarks shall close this article.

¶ Latterly, some attention has been given in practice, to supporting floors on iron rests instead of inserting the joists in the wall. We think this commendable in walls of moderate thickness, but as the cost is something of an item, particularly in country houses where it is desirable to avoid great expenditure, other methods to subserve the same object may be adopted. This object is to prevent the disintegration and falling of walls, by the leverage of joists burnt off in the middle during a fire, a catastrophe, happily, of rare occurrence in the country, but which, nevertheless, should be in a measure provided against. By sloping back the end of each joist to the inside face of wall, as represented in the diagram, we make preparation for the emergency. Another expedient now extensively practiced is to project courses of brick from the face of the wall, each course having about an inch projection over that beneath it; this is continued for four or five courses, or until the proper width for bearing is arrived at.

¶ The angle-filling thus formed takes the place of the ordinary wood-bracketing for the plaster cornice.

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