William Fenn Roslansky, AIA:

At the reference of David J. Dauerty we can inform you of repair
techniques we used on historic 19th century mortice and tenon barns
in northern New Jersey  (High Breeze Farm, Wawayanda State Park) in
the early 1990's.

1.  We could use no epoxy consolidants or patches to structural
members due to the unreliability of such treatments in very hot or
very cold environmental conditions.

2,  We respected the existence and the appearance of the original
frame, including particularly long tie beams spanning multiple bays.

3.  Whereas we found the greatest damage at joints rather than
stretches of wood members, we opened up exterior siding and installed
stainless steel "X" and "T" reinforcement on the hidden (exterior
side), thereafter covering it up.  The remaining historic fabric
contributed some, but relatively little to the structural value of
the reinforced joint.

4.  We installed stainless steel "L" or inverted "T" shapes on top of
the tie beams, preserving appearance from the floor.  Exterior
junctions were reinforced by angles again installed on the top side.
These could be seen, but not intrusively.

Our approach leading up to these conclusions was to replace defective
material with new wood stitched in in  the manner of 19th century.
Our respect for authentic material and its being seen unencumbered
drove the final decision...in the end quite satisfying.

I don't know how far gone your frame is, and supplementing old
structure with new, preserving the original, is not a bad approach,
but I hope you can find an adequate but least intrusive solution.

James W. Rhodes, FAIA
Partner
Beyer Blinder Belle
Architects and Planners
New York, NY