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John Lee and I under the tutelage of the internationally renowned engineer Tim Macfarlane did a great deal of work about 10 years ago on this sort of problem. We were looking at attaching a glass prosthesis to a conserved structural member that had lost length among other things.
Let's go back to basics. You know empirically that the 4x10 of original construction adequately carried the load for a number of years until it deteriorated. Therefore creating a component equal to or exceeding the original will suffice. Second a cantilever requires tension across the top and compression at the bottom. Therefore fancy joints are unnecessary if you connect the top to provide tension and the bottom abuts in a way that transfers the compressive load from the new to the old. Therefore a butt joint between a sound or consolidated square cut end of the original 4x10 and a new piece of matching dimensions and grain orientation will suffice with a tension connect at or near the top.
So how accessible is the top and how much remains beyond the wall of the original member? If you have 6 plus inches of original member and full top access you can do some elegant carbon fiber connections otherwise a post-tension threaded rod connection is probably best. I can give you some rule of thumb dimensions we used but if you have an engineer who is not afraid of existing buildings especially one that has deteriorated you should have them calculate the necessary embedment and diameter of the threaded rod or rods. As I remember we determined that this approximate condition would take a 5/8" diameter threaded 3+ inches into sound material. I like redundancy and would use two rods set about 6" into the original. We also found that pullout for a threaded rod in a snug hole (not treaded) that was flushed out with a penetrating epoxy and the the rod coated in a full bodied epoxy and inserted before either epoxy set would fail somewhere in the wood beyond the epoxy penetration, so threading in is unnecessary. I would place the center of the rods at the 1/3rd points relative to the width and that same distance down from the top. The least complicated arrangement would have the rods run out the end of the attachment and be countersunk. I would use a 1" washer and snug the system together but not try to compress the fibers. The space around rods should be filled with epoxy so it is distributing the load throughout its length. I would also have an alignment pin toward the bottom of the joint. Epoxy would not be inappropriate in the butt joint both sealing the joint and providing full compression transfer but not structurally necessary. In fact the joint can be loose at the bottom. For aesthetics and keeping moisture out plug the countersunk rods. Also consider flashing the top.
If you have experience to work out an IKEA style connection that can be done about 6 to 8 inches back from the joint and not run the rods all the way out. Also it is not necessary to square cut the end if you want to preserve as much of the original as possible the new piece can be fit to whatever remains of the original.
If you have 6+inches of the original to work with I'd be glad to walk you through using carbon fiber.
One last thought. It is almost never a good idea to wrap exterior wood in fiberglass. One usually ends up trapping moisture and accelerating rot and failure. The same with epoxy. It should not be a coating for wood. It is a great consolidant, adhesive, and casting resin but wood moves too much for epoxy to be used as a coating unless you customize it to be flexible.
I don't know anything about Mad Dog Primer. But I think its adhesion will only be as good as the bond between the old paint and the substrate.
Edward R. Acker | Emeritus AIA | LEED AP
VP Education, Preservation of Historic Winchester, Inc.106 Clevenger CourtWinchester VA 22601703.635.8968EdAcker39@comcast.net