Timberframers call theis a "brace".
Look at the fourth page of this document (upper left). It illustrates brace
joinery which meets your need to install into a preexisting T-shape. The
bottom mortise for the brace is extra tall. The brace it inserted into the
lower mortise, and lifted straight up so that the upper tennon inserts into
the cross-member mortise. A block is then inserted below the lower tennon to
fill the oversized mortise.
The document has a few other approaches to brace joinery that might also
meet your needs.
If you need specific pointers on how to to the layout for these, just ask
I've done a "barn-load" of them.
I’m in progress with an external structural timber project.
Part of this is a number of 150 x 150mm (6” square) treated softwood,
which support roof beams.
The posts will have 45 degree struts coming off them aka Gallows
brackets. Again out of same size stock.
The leading edge of the 45 degree joint is to be cut into the
vertical posts, effectively giving it a stop to rest against (and
transfer load I assume) point, the design calls for this stop to be
‘let in’ 20mm (~3/4”)
Sketch of joint is on: http://tinyurl.com/6nr2jp
Is there any technique for doing this type of joint (don’t even know
what they are called), some posts will have one gallows bracket –
others will have a pair .. in both cases there are galvanised steel
plates through bolted providing structural strength.
Sketch of double one is at : http://tinyurl.com/574dhz
I do not have the option to move posts form their now vertical
permanent position, so I am looking for technique for cutting in-situ,
maybe it’s just a case of getting in there with a chisel, but thought
I would ask.
Intent is to glue the joint as well with quality polyuretane glue ...
but mainly to prevent water ingress, steel plates will provide the