When large surfaces have to be covered with boarding, the pieces should be as narrow as possible, in order that the shrinkage in each, and the consequent opening of the joints, may be reduced to a minimum.
Such shrinkage, however, there will inevitably be, and several arrangements are adopted for preventing cold air, dust, etc., from penetrating through the opening thus made between the boards, also in order to prevent the shrinkage from injuring the appearance of the joints; and further, to counteract the tendency of an ill-seasoned board to warp,1 twist, and raise its edges above the general plane of the surface.
Plain or Butt Joints are explained and figured at p. 135. The opening caused by the shrinkage of such a joint is, of course, very apparent; and there is nothing to prevent a board from twisting its edges above the surface.
Grooved and Tongued Joint is explained and figured at p. 136.
Match Boarding consists of boards put together with the last-mentioned joint, one edge of each board being beaded, as in Fig. 489. It is so called because the groove is formed with one plane and the tongue with another to match or correspond, so as to fit the groove.
Plough Grooving is so called because the groove is formed with a peculiar plane called a "plough."
Cross Grooving is the same as the above, but that the groove is cut across the grain of the wood.
Slip Feathers are detached tongues or strips of iron or of hard wood cut across the grain for strength; in using them both boards have to be grooved and the tongue inserted, as shown in Fig. 296. Tongues are generally of hoop iron, and slip feathers of wood cut across the grain.
1 Sc. for warped is Thrawn.
If a slip feather is cut with the grain - that is, if the grain runs parallel to the length of the slip - and it is glued tight into the grooves, it is very liable to split longitudinally when the boards it unites commence to shrink. This cannot happen with a cross tongue - that is one cut across the grain.
Ploughed and Tongued - Grooved and Feathered - are terms applied to boarding prepared with tongues or slip feathers.
Dovetailed Slip Feathers are of a double dovetail shape in section (see Fig. 488), and must of course be pushed into position endways from the extremity of the boards. They are very seldom used.