424. There is no part of carpentry where defective joints are attended with such serious consequences as in ties, nor are there any other joints so often ill constructed. It is not easy to make a good tie-joint, from the very nature of timber, and therefore it is always desirable to avoid the use of wooden ties. "Where they cannot be prepared in one piece the maxim cannot be too strongly urged, as we have stated in Art. 395, " that dovetail joints should seldom be used in carpentry," and never when the joint is intended to hold the parts together.
For let A, Fig. 152, represent the angle of a building, where the wall plates are joined by a dovetail joint; the part a b being the crossway of the wood, will shrink in drying; and as the other piece is the lengthway, its shrinking will be insensible; therefore a very small degree of shrinkage will allow the joint to draw considerably, as shown by the dotted lines, and it acts with the power of a wedge to force off the end of the piece. A joint made as shown in Fig. 153 avoids any danger of giving way from the shrinking of the timber, and is better than any dovetail joint whatever.
Dovetail joints, or dovetail tenons, have been used in various parts of carpentry; such as the collar-beams of small roofs, the lower end of king and queen posts, and for joining plates, and the like. In all these cases they are the worst kind of joints that can be used. The carpenter's boast, as described in Nicholson's ' Carpenter's Guide,' must also be classed as a dovetail joint, and equally defective.
425. Fig. 154 shows a method of notching a collar-beam, C, into the side of a rafter R, which is far superior to a dovetail joint, though also defective, owing to the manner the rafter is weakened by being cut into.
A stout pin, of tough but straight-grained oak, is an excellent addition to a tie-joint, and is more economical, where numbers are required, than an iron bolt, though, of course, not so strong. The excellence of wooden pins is fully shown by their extensive use in ship carpentry, where they frequently form the chief connection of ship timbers (see Art. 135, Sect. II.).