This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
Notch or Notched Joint is made when two pieces cross each other, and is used largely in letting down bridging and ceiling joists on binders, and purlins on principals, etc. A rectangular, or triangular, or other more complicated piece is cut out, leaving what is called a notch, which fits into another notch, or else down upon the uncut arris or side of another timber. Fig. 50 shows a common rafter in elevation and a section through the purlin, cleat, and principal. The purlin is notched and fitted down upon the uncut back of the principal, whilst the common rafter is notched and spiked upon the uncut arris of the purlin. Fig. 51 represents alternative modes of notching joists for spiking down to wall plates. There are other less simple varieties, but they more resemble forms of halving than of notching. Figs. 34 and 52 show struts notched into posts and beams. As a rule, in heavy work, struts should not be notched to horizontal timbers, but should bear upon cleats bolted so as to present an end grain for the seat of the strut, the two being connected by means of a rough tongue of boiler plate or stout dowel to prevent any lateral sliding at the joint.
Notch and Bridle Joint is another name for a bridle joint.
Oblique Joint is formed at the junction of timbers or pieces which meet at an angle greater or less than a right angle. Oblique mortise joints are easily made by truncating the tenon and joggling the cheeks of the mortise, but in designing joints of this description, besides considering what may be the effects of the weather, lodgment of water, and the attacks of rot, it is well to remember that whereas the tensile strength of good fir is about one-sixth that of wrought iron of equal sectiona area, its power to resist that description of shearing which forces the fibrous to slide over the vascular plates is only 1/120, or thereabouts, of the shearing resistance offered by an equal-sized section of the metal in question. Hence mortise joints at the ends of really good timber exposed to oblique thrusts are generally considered weak, whilst iron straps, bolts, etc, present an admirable means of strengthening them. As a rule, the greater the obliquity of the joint the greater will be the shearing stress upon it.
This occurs when the parts are not in contact. It is corrected in trusses, etc, by tightening up at the proper time with cotters or screws. If the joint happens to be exposed, and is caused to open and close by pressure and its removal, as is sometimes the case in roofs, timber bridges, etc, moisture will paaetrate and caus e decay, but a little management and skill displayed in weathering the joints and in obliterating all lodgments for water, so that none may flow into the joints as soon as opened, will go far to reduce the danger to a minimum Lapping beams in exposed situations where practicable is better than scarfing, since there is less chance of the former kind of joint rotting from alternate dryness and moisture induced by the ingress of wet.