Angle of Inclinaiion of Roofs.—In considering the angle of inclination in roofs, it is generally assumed to be the same throughout the same struc ture, depending chiefly upon the character of the covering material, but also with due consideration to the position, the style, and the purposes the building is intended to serve. It should also be stated that it is not usual to regard the inclination of the roof as a question of degrees, but rather with reference to the fitting proportions necessitated by the relative heights of the roof and the depth of the building, the saddle roof being always pre supposed. In cases where tolerably good tiles are used as covering- mate rial, the inclination may be estimated at from one-lialf to one-third of the depth; with tiles of good quality, the proportion may be from one-fourth to one-fifth; with slate, from one-fourth to one-sixth; while with tarred felt or similar roofing material, as well as with metal, the slope may be less steep. In all forms of timber roof construction, it is iniportant to overcome the thrust proceeding from the oblique position of the roof and to prevent displacements in the longitudinal as well as in the transverse direction; these results are generally attained bv triangular connection.
Roof Consirudion.—Tiniber roofs are constructed in the most varied manner; for the purpose of this sketch the methods employed will be con fined to the more usual systems and to a few characteristic examples. It is understood that the saddle roof only is referred to. As a rnle, the sup port of the material of the roof—or, rather, the immediate support—is effected by inclined timbers or rafters, but occasionally this duty is per formed by horizontal tiinbers, or purlins. These rafters either stand directly on the attic joists or the joists may lie lower than the ends of the rafters. By lowering the joists, as in Figures 26 and 28 (II 3), the con struction, it is true, becomes somewhat more complicated, but in another way some advantages are gained. Among these may be cited the ease with which the space in the attic may be utilized in avoiding the useless angles of the roof; there is also a decided gain in the decreased slope of the roof, and also an increase in the height of the facade, etc. No essential difference in the system of construction results, however, from this lower ing process, while a difference does result from the manner in which the attic joists are supported or sometimes omitted altogether.
brings us to the subject of trusses, which may be classified as follows: (T) Roof-trusses with sufficiently-supported attic joists; (2) Roof-tnisses with suspended joists; (3) Roof-trusses without joists..
(1) with Attie the entire depth of the building is not more than 25 feet, the beams as well as the rafters will require support only at the ends. Each pair or brace of rafters is secured at the apex by mortises and pins and is tenoned into the joists below, with which it forms a triangle and a so-called " roof couple." These couples are set at a distance of from 2X to 4 feet. To
prevent displacements lengthwise, " storm braces" are nailed obliquely upon the lower side of the rafters. In steeper roofs each two rafters are joined together for half their length by a horizontal timber mortised in and called the " collar beam." If the depth of the building is more than 25 feet, " standing-posts " are introduced, which may be either vertical or inclined.
characteristic point in standing-post roofs is that the separate main couples have posts which are mortised into the attic joists and carry purlins proceeding along the length of the roof; these, in turn, take up directly either the rafters or the collar beam of the rafters. The rafters are then notched down to the purlin. For the prevention of longitudinal displacements, struts are used, being mortised in obliquely between the purlins and the posts. In the simple standing-post roof there is in each main couple but one post, which, as a rule, extends up under the ridge beam. The shnple post roof will answer for buildings haying a depth of from 25 to 33 feet; the attic joists must in this case be supported by a wall, or by something similar, at least once in the course of their length.
In the double standing-post roof (fig. 25) there are posts in each main couple, either with a collar beam, as is shown in the left side of the figure, or without one, as on its right side. In the latter case a " straining piece" is used, but of course only in the main couple, and it is placed between the purlin and the posts, which are connected with the latter, as in the Figure, by a corner-brace, and halved with the rafters.
The Roof (fig. 26) has inclined side-posts, which, in the case of the collar beam here represented, carry the purlin above, and, be ing mutually connected by a horizontal timber, the straining piece, are further stiffened by oblique braces. The left side of the Figure exhibits this arrangement. The oblique post without collar beams is shown on the right. Here a connecting strain-piece rests first on the post, and upon the former, in the angle with the rafter, the purlin. Both portions of the Fig ure show also the " sunken-joist course," which may be applied in like manner in the double standing,-post roof. The double standing- and oblique-post roofs are suitable for buildings with a depth of from 33 to 46 feet. The former style furnishes a more solid kind of construction, while the latter affords the advantage of a more open and unobstructed floor space. For buildings with a depth greater than 46 feet, there should be used either the combination of the single standing- with the double stand ing-post roof (triple standing-post roof), or the combination of the single standing- and the oblique-post. In both cases the middle post must be placed under the ridge beam.