steps, whose joists are framed into the beam for the support of the flooring. This beam is cailed the apron piece, and that which sustains the rough strings at the upper end is called the pitching piece. The joists of the half-pace are sometimes turned into the pitching piece, and sometimes bridge over it; but the steps of both flights are always supported by string pieces, as before. The upper ends of the string pieces at the landing rest upon an horizontal piece of timber, called, as above, an apron piece. The scantlings of the strings, of course, vary with the length of the inclined part. The depth given to joists of similar length will be more than sufficient. ROOFS. 2027. The first obvious consideration in constructing a roof is the slope to be given to it, which depends on the climate against which it is to serve as a protection, and on the materials to be employed in covering it. In hot countries, rain more rarely falls than in temperate ones; but when it comes, it descends very abundantly, which, added to the temperature of the air, makes it unnecessary to give a great slope to the roof, from which the water immediately runs, and the air dries it almost at the instant of the rain's cessation. In cold countries the rain is more searching, the air is more impregnated with moisture, and snow often lies for a long time on a roof; circumstances which require a greater proportional slope to be given to it. Again, roofs covered with lead, zinc, or copper, do not require so great a slope as those covered with tiles or slates. (See ROOFING, in Glossary.) 2028. Though among architects there does not appear to have been any fixed principle by which the slope should be determined, we find that in different climates suitable slopes have been adopted for similar materials. Thus in the southern parts of Europe we find the roofs very flat; whilst as we proceed into its northern parts the roof acquires a very considerable elevation. We shall here transfer to our pages the notice of this subject in the Encyclopedie Methodique, which we consider extremely important and interesting, inasmuch as it shows that necessity was the parent of beauty in the inclination of the roofs of the ancients; and in the times of the middle ages it had some influence even in the production and developement of the lancet arch. 2029. The researches and observations made respecting the roofs of a great many ancient and modern buildings, situate in different countries, satisfy us that the slopes of roots which have lasted best are always proportioned to the temperature of the climate. Before entering into the consideration of any law for determining the slope of a roof, it will be proper to comprehend the meaning of the word climate as here introduced, which we shall use in the same way as it is understood by geographers. According to them, the climates of the globe are comprised under belts or bands, of unequal size, parallel to the equator. Of them there are twenty-four between the equator and the polar circle, each of half an hour; that is, the length of the longest day of a place situated at the beginning of the climate is always shorter by half an hour than that of the place situated at the extremity of the same climate, or at the beginning of the succeeding one, proceeding from the equator towards the polar circle. This difference in the length of the day, caused by the greater or less obliquity of the tropic with the horizon, is one reason of the different degrees of temperature of countries corresponding to the different climates. We are not, however, to assume that the temperature will be exactly the same for all places under the same climate, since there are many circumstances which tend to make a place more or less damp, in which cases the slope of the roof should rather have a relation to a more northern spot. In the roofs of the Continent covered with the hollow tile, as in the south of France for instance, less slope is required than with the Roman tiles (see the word TILE in Glossary), which are in sections alternately flat and circular; and these, again, require less slope than the common plain tile or slate. From the observations that have been made, we find that the slope of roofs covered with hollow tile, thus, of the south of France, should be after the rate of three degrees for every climate, beginning from the equator and proceeding northward, and that when the Roman tile is used, an addition of three degrees should be made to such inclination; an addition of six degrees, if covered with slates; and of eight degrees, if covered with plain tiles. According to this law, the table which will be presently subjoined has been constructed, and a comparison of it with ancient buildings gives a remarkable corroboration of its value. Thus, at Athens, situated about the middle of the sixth climate, the slope of a pediment would be about 160; and that of the Parthenon is actually about 16°; that of the temple of Erectheus, 15°; of Theseus, 15°. In Rome, which is about one third of the way up the seventh climate, the Roman tile requires an inclination of 22°. The actual slope of the pediment of Septimius Severus is 23°; those of the temples of Concord and Mars Ultor, 23°; of Fortuna Virilis and the Pantheon, 24; and, of more modern date, the slope of the roof of St. Paolo fuorì le murà was 23o. 2050. We shall now give the reader the table above mentioned. This ingenious theory is taken exception to by P. Waterhouse in his Essay on Pediments, &c., 1886. "There is no article," says Ware in his Body of Architecture," in the whole compass of the architect's employment that is more important or more worthy of a distinct consideration than the roof. The great caution is," continues our author "that the roof be neither too massy nor too slight. Both extremes are to be avoided, for in architecture every extreme is to be shunned, but of the two the overweight of roof is more to be regarded than too much slightness. This part is intended not only to cover the building, but 43 24 46 8225 to press upon the walls, and by that bearing to unite and hold all together. This it will not be massy enough to perform if too little timber be employed. so that the extreme is to be shunned. But in practice the great and common error is on the other side; and he will do the most acceptable service to his profession, who shall show how to retrench and execute the same roof with a smaller quantity of timber; he will by this take off an unnecessary load from the walls, and a large and useless expense to the owner." B 2031. We shall now proceed to a popular view of the strains exerted by the timbers of roofs, referring the reader back to the section on BEAMS, PILLARS, &c., for a more extended and scientific view of them. Suppose (fig. 680.), in the simplest form of roof, the rafters (shown by dotted lines) A B, CB to pitch upon the walls Aa, Cc. Let the rafters be supposed to be connected together at B as by a hinge, as also similarly connected with the walls at A and C. Now if the effective weight of the walls be not sufficient to resist the thrusts of the rafters, as respects the height, thickness, and situation of the centre of gravity of such walls, taken as solid masses and moveable on the points X and Y, it is manifest the rafters by their own gravity will descend, and the walls will spread and be thrown out of an upright, as in ab and cd, and the rafters will take the places shown in the figure. It has already been shown (par. 1622) that the horizontal thrust of a pair of rafters thus meeting each other. is proportional to the length of a line drawn perpendicularly from the rafter's foot until it intersects a vertical line drawn from its apex. As the roof therefore becomes flatter, the length of the perpendicular increases. Hence, if AB and BC be the rafters, and their weights be represented by their lengths, the weight or power of thrust exerted by the rafter AB in the direction of its length will be represented by BO, and the horizontal thrust by AO; AO being perpendicular to AB. To secure, then, the walls in their perpendicularity, which the thrust of the rafters tends to derange, a system of framing becomes necessary. Thus, in fig. 681., a beam AC, which from the office it X Fig. 681. Fig 680. the direction of their length, and hence, that for such especial purpose, if they be prevented from sagging or bending, a small size or scantling will be sufficient, for we have already seen that the cohesive power of timber is very great in the direction of its length. To take care that the tie beam thus introduced should be strained only in the direction for which it is used, we are now led to another expedient. The beam by its own gravity, especially in a large opening, would have a tendency to sag or bend in the middle, and the more so if its scantling be simply proportioned to its office of a tie. To prevent this a fresh tie is introduced called a kingpost DB (fig. 682.), by which the beam is tied or slung up to the apex of B Fig. 682. the principal rafters; and this combination of a pair of rafters, a tie beam and a kingpost, is called a truss, and is the most important of the assemblages which the carpenter produces. When the rafters are of such length that they would be liable of themselves to sag down, supports aa are introduced at the points where such failures would occur, and these supports are called struts, because their office is to strut up the rafter, which they should do as nearly as the case will admit in a direction perpendicular to the slope of the rafters. 2032. It is clear that out of this last case a fresh system of trusses may arise as in fig. 683., for from those points procured by the struts against the rafters new rods may SS be slung for increasing the stiffness of the tie beam ad infinitum in theory, but not in practice, because the compressibility of the fibres of timber is considerable in lines perpendicular to their direction, and the contraction and expansion of metal places a limit to its use. This compression of tim Fig. 683. We may lay down as a rule 1 ber deserves great attention on the part of the architect. in respect to it that the more the weights or pressures act in the direction of the fibres, the less will be the compression. 2033. To exemplify this, fig. 684. shows in No. 1. the principal rafters of a roof butting in an ordinary roof, against No.1. No.2. Fig. 684. the shoulders AB, CD of the kingpost, whose fibres, being vertical, are compressed by the pressure against it, on each side of the rafters, whereby they approach each other, causing the whole figure of the roof to suffer a change. For by the action of compression and its consequence the kingpost must descend, and with it, consequently, the tie beam which is slung up to it. To remedy the inconvenience in roofs constructed of fir, the kingpost is often made of oak, which is less compressible, a practice which should be observed in all roofs of consequence. But cast iron kingposts are the best substitute where the expense can be justified. In No. 2. the end is accomplished much more economically by housing the rafters in the head of the kingpost at the angle in which the rafters meet, by which the fibres of the rafters butt against each other, bringing the compression nearer to that which takes place in a post according as the rafters are less inclined to each other, and the beam is then literally suspended from the vertical planes of the rafters at their junction. queen B E D 2054. When a roof (fig. 685.) is trussed by two upright suspending posts, which be come necessary in increased spans, such posts, AB, CD, are called posts, and the piece between them, BD, is called a collar, which acts as a straining piece to prevent the heads of the queen-posts moving out of their places towards each other. It will on mere inspection be seen that Fig. 685. this roof has three points of support, B, E, and D; for by means of the struts AE, EC, a new suspending point is gained from E for sustaining the tie beam between the points A and C. It is also to be observed that the collar or straining piece BD performs in this assemblage an office exactly the reverse of that which it does in fig. 681. B C D It 2035. The Mansard roof, so called from its inventor's name, and with us called a Curb roof, frequently used for the purpose of keeping down the height of a building, and at the same time of obtaining sleeping or other rooms in it, is shown in figs. 686 and 687. may be considered as primarily consisting of four pieces of timber connected by hinges at the points ABCDE. If these be inverted, they will arrange themselves by their gravity in such a manner that when returned to their first position they remain in a state of equili brium, which, however, in practice is but a tottering one, and requires additional expedients to prevent the whole assemblage thrusting out A the walls; and, moreover, to prevent the upper rafters from acting by their thrust to displace the lower ones. To obtain these ends the first object is to introduce the tie AE; and, secondly, the tie BD. It is to be understood that means are to be used, when needed from their length, to prevent these beams from bending, similar to those already directed in the cases of simple trusses. Fig. 686 is an example selected from Krafft, (Art. de la Charpente, fol. 1805), having an arched ceiling to give additional height to Fig. 686. 15.0 Fig. 687. some large or public room. This form of roof has been frequently adopted in the palaces of France and Germany. Fig. 687 is a king-post Mansard roof, affording a wide space over the tie beam available as an apartment. Fig. 698 is an example of the principles adopted for a much wider spanned roof. 2035a. We have thus far endeavoured to explain in the simplest way the conduct to be pursued for obtaining stability in the construction of a roof; but before we proceed to the scantlings of the timbers to be employed, the reader must be informed that the trusses to roofing, with whose nature he has now become acquainted, are placed only at certain intervals (which should not exceed 10 feet) apart, and are thus made to bear the common rafters and the weight of the covering, as well as to perform the office of suspending the tie beam by which the walls are kept together. Hence the rafters so framed in a truss are called principal rafters; and by the means of a purline A (fig. 688.), which lies horizontally throughout the roof's length on the principal rafters, they are made to bear all the superincumbent load. The purlines are in various ways made fast to the principal rafters, and upon it the common rafters are usually notched down. Their bearings are thus lessened, and less scantlings suffice for them. Fig. 688. They are received at their feet on a piece of timber (B in the figure), which runs longitu dinally along the sides of the building. This piece of timber is called a pole plate, from being the uppermost plate in a building; at their summits they abut against a ridge piece D. When a roof slopes each way, the space enclosed between the intersection of the slopes is called a hip (fig. 689.); and the longest rafters in it, which are those at the angles, are Fig. 689. called hip rafters, and the shorter ones are named jack rafters, as A, A, A, &c. 2036. We have, at the beginning of this section (2007.), observed, that the use made of bolts must be always in a direction as nearly as possible counter to the strain which the pieces exert; the method, therefore, of introducing them will, on due consideration, be sufficiently obvious. Before proceeding to lay before the reader some few examples of roofs suitable to different spans, as well as of some of magnitude which have been executed, it may be as well to complete this portion of our labour, by giving some information on the scantlings of timber for roofing, in which a medium, founded on our own practice, is introduced between ignorant overloading, and fanciful theory. 2037. For roofs whose spans are between 20 and 30 feet, no more than a truss with a king-post and struts will be necessary, in which case the scantlings hereunder given will be sufficient. For a span of 20 feet, the tie beam to be 9 in. by 4 in.; the king-post, 4 in. by 4 in., principal rafter, 4 in. by 4 in.; struts, 4 in. by 3 in. For a span of 25 feet, the tie beam to be 10 in. by 5 in.; the king-posts, 5 in. by 5 in.; principal rafter, 5 in. by 4 in. ; struts, 5 in. by 3 in. For a span of 30 feet, the tie beam to be 11 in. by 6 in.; the king-post, 6 in. by 6 in., principal rafter, 6 in. by 4 in.; struts, 6 in. by 3 in. 2038. For roofs whose spans are between 30 and 45 feet, a truss with two queen-posts and struts will be required, and a straining piece between the queen-posts. Thus For a span of 35 feet, the tie beams to be 11 in. by 4 in.; queen-posts 4 in. by 4 in. ; principals, 5 in. by 4 in.; straining piece, 7 in. by 4 in.; struts, 4 in. by 2 in. For a span of 40 feet, the tie beams to be 12 in. by 5 in.; queen-posts, 5 in. by 5 in.; principals, 5 in. by 5 in.; straining piece, 7 in. by 5 in.; struts, 5 in. by 2 in. For a span of 45 feet, the tie beams to be 13 in. by 6 in.; queen-posts, 6 in. by 6 in.; principals, 6 in. by 5 in.; straining piece, 7 in. by 6 in.; struts, 5 in. by 3 in. 2039. For roofs whose spans are between 45 and 60 feet, two queen-posts are required, and a straining piece between them; struts from the larger to the smaller queen-posts, and struts again from the latter. |