rr the jack rafters, P P the plates, pp the purlines, K K the king posts, kk king bolts, qq queen bolts, both are called suspension bolts, CC the collar or straining beams, B B braces or studs, bb ridge boards, c c corbels. 1 3 The pitch of the roof is the inclination of the rafters, and is usually designated in reference to the span as, }, }, &c., pitch, that is, the height of the ridge above the plate is,,, &c, of the span of the roof at the level of the plate. The higher the pitch of the roof, the less the thrust against the side walls, the less likely the snow or water to lodge, and consequently, the tighter the roof. For roofs covered with shingles or slate, in this portion of the country, it is not advisable to use less than pitch; above that, the pitch should be adapted to the style of architecture adopted. The pitch in most common use is the span. Fig. 1 represents the simplest framed roof; it consists of rafters, resting upon a plate framed into the ceiling beam; this beam is supported by a suspension-rod, k, from the ridge, but if supported from below, this rod may be omitted. This form of construction is sufficient for any roof of less than 25 feet span, and of the usual pitch, and may be used for a 40 feet span by increasing the depth of the rafters to 12 inches; deep rafters should always be bridged. By the introduction of a purline extending beneath the centre of the rafter, supported by a brace to the foot of the suspension rod, as shown in dotted line, the depth of the rafters may obviously be reduced. It often happens that the king-bolt may interfere with the occupancy of the attic; in that case the beam is otherwise supported. Again, it may be necessary that the tie beam, which is also a ceiling and floor beam, should be below the plate some 2 to 4 feet; in that case, the thrust of the roof is resisted (fig. 4) by bolts, bb, passing through the plate and the beam, and by a collar plank, C, spiked on the sides of the rafters, high enough above the beam to afford good head room. For roofs pitch and under 20 feet span, the bolts are unnecessary, the collar alone being sufficient. Fig. 2 represents a roof, a larger span than fig. 1; the frame may be made very strong and safe for roofs of 60 feet span. King-bolts or suspension-rods are now oftener used than posts, with a small triangular block of hard wood or iron, at the foot of the bolts, for the support of the braces. The objection to this form of roof is that the framing occupies all the space in the attic; on this account the form, fig. 3, is preferred for roofs of the same span, and is also applicable to roofs of at least 75 feet span, by the addition of a brace to the rafter from the foot of the queenbolt. The collar beam (fig. 6), is also trussed by the framing similar to fig. 2. In the older roofs, queen posts are used (fig. 33), with the foot secured by straps or joint bolts to the tie beam. In many church and barn roofs the tie beam is cut off (fig. 5, Plate II.), the queen post being supported on a post, or itself extending to the base, with a short tie rod framed into it from the plate. Fig. 33. Figs. 7 and 8, Plate II., represent the foot of a rafter on an enlarged scale. In fig. 7, the face of the rafter does not project beyond the face of the plate; the coving is formed by a small triangular, or any desirable form of plank, framed into the plate. The form given to the foot of the rafter is called a crow foot. In fig. 8, the rafter itself projects beyond the plate to form the coving. Fig. 9 represents a front and side elevation and plan of the foot of a main rafter, showing the form of tenon, in this case double; a bolt passing through the rafter and beam retains the foot of the former in its place. Fig. 10 represents the side elevation of the foot of a main rafter with only a small portion of the beam, the remainder being supplied by a rod. In fig. 7, of a similar construction to fig. 1, the tie rod passes directly through the plate. In general, when neither ceiling nor flooring is supported by the tie beam, a rod is preferable. Roofs are now very neatly and strongly framed by the introduction of cast-iron shoes and abutting plates for the ends of the braces and rafters. Fig. 11 represents the elevation and plan of a cast-iron king head for a roof similar to fig. 2. Fig. 12, that of the brace shoe; fig. 13, that of the rafter shoe for the same roof. Fig. 14, the front and side elevation of the R Fig. S4 queen head of roof similar to fig. 3, and fig. 15, elevation and plan of queen brace shoe. Fig. 34 represents the section of a rafter shoe for a tie rod; the side flanches are shown in dotted line. On the size and the proportions of the different members of a roof:Tie beams are usually intended for a double purpose, and are therefore affected by two strains; one in the direction of their length from the thrust of the rafters, the other a cross strain, from the weight of the floor and ceiling. In estimating the size necessary for the beam the thrust need not be considered, because it is always abundantly strong to resist this strain, and the dimensions are to be determined as for a floor beam merely, each point of suspension being a support. When tie rods are used, the strain is in the direction of their length only, and their dimensions can be calculated, knowing the pitch, span, and weight of the roof per square foot, and the distance apart of the ties, or the amount of surface retained by each tie. Rule.-Multiply one half the weight by one half the span, and divide the product by the pitch. Example. What is the strain upon the tie rod of a roof 40 feet span and 15 feet pitch? The weight of the wood-work of a roof may be estimated at 35 lbs. per cubic foot, or on an average at about 9 lbs. per foot square, slate at 7 to 9 lbs., shingles at 1 to 2 lbs. The force of the wind may be assumed at 15 lbs. per square foot. The excess of strength in the timbers of the roof as allowed in all calculations, will be sufficient for any accidental and transient force beyond this. If, therefore, the roof be like fig. 1, Plate II., without ceiling beneath, and retained by a tie rod, we may consider as the weight per square foot for a slate roof: 9+ 7+15 31 lbs. The length of the rafter is 1/2015 25 feet; hence, if the tie rods are 10 feet apart, the amount of surface on each incline supported by the tie is 25 × 10 250 square feet, which multiplied by the weight per square = · 7750 = foot, or 250 × 31 = 7750 lbs. ; applying the rule, x 2015 = 5166 lbs., the thrust on the tie rod. If we estimate the strength of wroughtiron at 10,000 lbs. per square inch of section, or 8,000 lbs. when a thread is cut upon the end, then, =0.646 square inches, or a rod a little 5166 exceeding of an inch in diameter. The rafters (fig. 1, Plate II.), may be considered as jack rafters of long bearings, or as a beam supporting transversely the weight of the roof, and the accidental pressures, and may be estimated by resolving the direction of these pressures into a line perpendicular to the direction of the rafters. Main rafters, as in figs. 2 and 3. The pressure on main rafters is in the direction of their length, when they are supported by braces at or very near the points where the purlines rest; but in addition to the weight of the roof, they support a portion of the weight of the tie beam, and whatever may be dependent upon it. If the frame is like fig. 2, that is, with a king-bolt or post, and the weight is uniformly distributed upon the beam, then one half the weight is supported by the bolt or post, and consequently by the rafter, and the other half by the side walls. Under the same circumstances, the suspension rods (fig. 3), support each of the weight of the beam, &c., and the side walls each. But in general, where the attic is made use of, the load is not uniformly distributed, by far the greatest part is suspended upon the rods. To find the pressure on the main rafter. Multiply one half the weight of the roof, and that portion of the weight of the beam and its load which may depend upon it, by the length of the rafter, and divide the product by the pitch. Example.-What is the pressure upon the main rafter of a slate roof of 56 feet span, 21 feet pitch, frames 10 feet between centres, and form like fig. 3, with an uniformly distributed load on the beam of 8,400 lbs., and a load between the suspension rods of 10,000 lbs. The length of rafter is √282 + 21' = 35 feet. Assuming the load per square foot upon the rafter the same as the preceding example, then of the uniform weight 35 × 31 × 10 = 10,850 lbs., the weight of roof. the weight between rods = 5,000 lbs. 2 If we now assume the resistance of wood at 740 lbs. per square inch,* or 600 lbs. for length exceeding 13 times their thickness, 31083 The proportion of the depth to the width is generally about 10 to 8 or 5 to 4. Hence 52 × 5 = 1.62 × 5 = 8.1 inches = depth. 1.62 × 4 = 6.5 inches = width. Gwilt, in his Architecture, recommends the following dimensions for , portions of a roof: Weisbach. |