"When timbers are joined at an angle other than a right angle the tenon has to be modified in form. If constructed as in Fig. 177, it would be very difficult to work the mortise to receive it; moreover, the long tenon would have a tendency to tear up the joint in case of any settlement of the inclined beam; and further, it would be almost impossible to get the tenon into the mortise when the pieces to be joined formed part of a system of framing.
Fig. 177. Oblique Tenon, bad form.
Fig. 178. Oblique Tenon.
These evils may be remedied by cutting off the end of the beam, as shown at a, Fig. 178.
This is the simplest mortise and tenon for oblique joints, but the only resistance it affords is that offered by the strength of the tenon, which is liable to be crushed, and would in large carpentry works be quite insufficient to meet the heavy strains that might come upon it.1
To remedy this, the cheeks of the mortise are cut down, as in Fig. 179, to the line d b, so that while the tenon is retained to prevent lateral motion, the whole width of the beam itself presses against the abutment a d, by which a much larger bearing surface is obtained.
Fig. 179. Oblique Tenon Joint with Bolt at foot of Principal Rafter.
Fig. 180. Oblique Tenon and Mortise, for Joint between foot of Principal Rafter and Tie Beam.
Figs. 179, 180 show the joint as frequently constructed for the junction of a rafter and tie beam. Tredgold recommends that the depth a d should be greater than half the depth of the rafter, and at right angles to db. It is generally kept shallow from a fear of weakening the tie beam; except for this reason, the deeper a d is made the better, and it is often cut perpendicular to the upper surface or "back" of the rafter, as shown in Fig. 179.
The joint in Fig. 181 is a modification of the last.
Fig. 181. Oblique Tenon Joint at foot of Principal Rafter, with Strap.
Fig. 182. Oblique Tenon Joint with Double Abutment.
Fig. 182 shows a joint with a double abutment. This joint is very difficult to fit with accuracy, and is open to the objections stated at p. 57, but it is sometimes used when the angle of the joint is very oblique, and when there is consequently a large bearing surface.
In putting such joints together they should be left slack at b so as to allow for settlement of the framing.
1 From New-land's Carpenter's and Joiner's Assistant.
As the piece of the tie beam beyond the foot of the rafter would have to be left inconveniently long to prevent its being shorn off, it is relieved of some of the pressure, and the joint is secured by means of a strap or bolt, which also serves to keep the rafter in position. The relative merits of these fastenings are pointed out at page 80. In framing an inclined beam into a post either at its head or foot a tenon joint is used.
It is advantageous to make the head of the post larger (as shown at X in Fig. 183), so as to get an abutment square to the inclined beam.
If the head of the post be not large enough to afford the square abutment, it may be cut as at Y.
The tenon should be made, if possible, the whole depth of the inclined beam, but in cases where the top of the post is cut off close to the back of the rafter, as in some roofs (see Fig. 184), the tenon is necessarily made narrower in order to leave some wood on the post above it to form a strong upper cheek to the mortise.
In all cases the joint should be left a little open at a, so that when the framing settles it may not bear too severely upon the angle at the top of the rafter.
The same remarks apply to joints at the feet of posts. (See the lower part of Tig. 183.)
Fig. 183. Tenon Joints at head and foot of King Post.
Tenon Joints at head of King Post, with Straps.
When the extremities of the post cannot be enlarged the inclined beams may be tenoned into it, as in Fig. 184. It will be seen that this arrangement weakens the post, and reduces the size of the tenon. Whereas the form in Fig. 185 gives a very inclined abutment.
Many other positions in which the mortise and tenon are applicable will be seen in the different examples of framing throughout these notes.