The somewhat unsaid case for slopes.
Just to clarify for those many among the readership who have never cut a timber joint – “Slope” is the going term for an angled cut into the backside of a mortise which receives an adjacent member which joins it at an angle, these typically being the common wind or “knee” braces typical to every timber framed structure.
Most of those who cut timber joinery fall into either of two camps, those who cut them and those who don’t. The “don’t’s” have reason – A slope cut at an angle even a single degree too low, will simply not allow for full insertion. This added to there being in most instances, little, if any return on the time invested in (laying out – the cutting is simple) cutting them.
The other reason often held up, is historic precedent, and this is certainly there. Most Brace Mortises in North America lack any slope – Though I am going to qualify this somewhat slightly, by suggesting this, only most, and not all.
I have come upon historical examples, and of two types – Early Scribe Ruled frames do sometimes have slopes. This would be because both the exact placement of the spine of the brace is a known, (this is not true of Square Ruled frames) and the exact angle that the brace joins the mortised stick is also a recordable known, and with that information, it is easier to chop out a slope with a Corner Chisel than it is to bore out empty space with a Tee Auger.
The other instance in which I have seen this is in bridge joinery, and though such is most often scribed, the reasoning for the trouble taken to cut them was not about the information to cut them accurately being a known. It is about not unnecessarily removing “section” – It is for reason about not wasting out wood and unnecessarily weakening the mortised timber any more than need be.
Some of this is driven by that constantly considered aspect of wooden bridge design, this being “Dead Load” – Timber, particularly in later examples, is sized to the smallest section possible, to make the trusses as light as was possible to minimize “Dead Load”. (though then as now all is /was ticked up in size with a bit of a “failsafe”)
A knowing and careful Carpenter / Bridgewright, is and was cognizant of this, and did / does not let the joinery he is cutting unnecessarily weaken the stick he is joining.
Some of this thinking necessarily goes to the proximity of the joints being cut – Would two mortises in close proximity (should they even be in close proximity?) be better off were one or both cut with slopes?
As a bit of an aside, the lack of any slope leaves a void. Without exception, in every brace mortise I have ever opened up, something had moved in, and had brought with it, the stuff and sluff of life. The shredded whatnot for its bedding or its bathroom. This “stuff” often serves as an unintended sponge holding moisture, when that someday comes and an inconvenient leak points water at the spine of a brace, and funnels it into the mortise. This “Sponge” is not infrequently the secondary genesis of a huge problem…
I avoid this void in cutting new barns, (again, both mice and bats bring their stuff to these voids in every barn frame I’ve explored) and in frames subject to weather, like porches and pavilions, and of course – in Bridges