A dado joint, Fig. 266, No. [25], should not be so deep as to weaken the supporting board.
A tenon should not be so large as to weaken the mortised piece.
Pins or other fastenings, Fig. 267, Nos. [38] and [39], may weaken rather than strengthen a joint if they are so placed or are so large as to shear or crush their way thru the timber.
9. Place each abutting surface in a joint as nearly as possible perpendicular to the pressure which it has to transmit.
Illustrations of this principle are as follows: the angle in a strut joint, Fig. 266, No. [62], should be equally divided between the two beams.
The thrust joint, Fig. 268, No. [63], in a bridge truss, is exactly at right angles to the pressure.
It is on account of this principle that a spliced joint for compression, Fig. 264, No. [4], is different from a spliced joint for tension, No. [5]; and that a housed braced joint, Fig. 269, No. [66], is better than a plain braced joint, No. [65].
A joint to resist vertical cross strain is stronger when scarfed vertically than horizontally.