“Look at this old roof, whose rafters are framed (Fig. [15]): the wall plates, A, are cut from small trunks, the heart being in the centre. It is the same with the rafters B, the tie-beams C, the collars D, the king-posts E, the foot-pieces F, and the foot-posts G; all these pieces, therefore, have preserved their rigidity, and none of them has been bent, because they were used dry and were unsplit trunks. Observe, on the contrary, this purlin, H, placed on this truss, I, of recent date; it is bent not so much on account of the weight of the rafters it supports as because it is split and the carpenter has unadvisedly turned the heart on the inside. If he had done the contrary,—if the heart had been placed next to the rafters,—this purlin would probably not have bent, perhaps have even become more rigid—that is to say, it would be convex on the outer side. Carpenters, however, are but men, and they do not care to give themselves trouble when they think they can avoid it. The man that put this purlin here found it more convenient to place it on its sawn side than to turn it the other way with the flat under the rafters.
Fig. 15.
“Considering this quality of wood, and of oak especially (whose internal fibres are harder and closer than the outer layers), when we have to place a piece of wood horizontally on two points of support or posts, and wish to give it all the strength possible to bear a weight acting on its centre, we saw it in two lengthwise, and turning the flat faces outside, bolt these two pieces together, as shown here (Fig. [16]). Then as the heart-wood is outside, and the two pieces tend to become bent, forming two convex surfaces, as you see at A (Fig. [17]), if they are firmly held by bolts furnished with good heads and nuts, they must remain straight; the tendency to curvature in the one neutralizing that in the other, these two opposing forces tend to make the piece more rigid, so that, if you take a piece of timber that is slightly bent naturally and then place these two pieces with their hollow downwards,—that is, after having placed one upon the other, putting the tail of one against the head of the other,—you will have given this piece of wood all the resisting power of which it is capable.
Fig. 16.
Fig. 17.
“It is in this way that clips and all coupled pieces should be placed. Here, for example (Fig. [18]), you see a pair of clips where the sawn faces have been turned outside to replace a decayed tie-beam. We call clips those pieces of wood which, in pairs, usually clench two or more parts of a framing. These clips, A, hold fast by means of notchings, the blades B, the king-post C, and the two struts D. Iron bolts with screw-nuts tightly hold the notchings of the clips, like a pair of jaws, against the timbers which have to be kept in their place. But this is enough for to-day, and you will have plenty to do to make a fair transcript of this lesson in carpentry between now and this evening.”
