I've searched high and low and can't find an answer to this. Why are deck beams cut on an angle at the ends? Is it cosmetic? Is so no sharp corner sticking out? Does it have something to do with reducing rot?
See image attached below.
I'm not aware of any reason for the corner of deck beams to be clipped like that other than for cosmetic purposes or clearance from something else.
My rafter tails have a lovely curved end and a little decorative notch in them, however, my deck beams are not cut back at all. The rafter tails are exposed & nicely decorated (as are most of them in our older - late 1800s to early 1900s - town), but I'm about to sister them up with square ends to hang soffit & fascia boards to cover them all up.
Most of the deck ends I've noticed around town are cut square and not clipped at all.
Figured I should make an answer out of this
I also believe it's for cosmetic reasons.
Sometimes facia boards are angled at the end of a roof so as to maximize the drip-off distance from the building.
Fence boards are also sometimes tapered or angled at the bottom to drip away from the decking.
The question to ask about these beams is then is why is it longer on top? In other words why is it shaped __/ and not __\ ?
Apart from a cosmetic "up" or "awake" look, that design in general provides a function too.
Have a look at these joist ends:
The support provided by the cantilever requires a pivot or fulcrum further in than the roof edge. This is why functionally the top can be made longer. Also, as mechanical loads taper off towards the end, material can be saved. For this same mechanical reason, airplane wings taper toward the tip.
And since such function has made the look ubiquitous, we could surmise that the cut in your picture is a mix of "familiarity" and -well- virility.
Image from gardenholic.com