1500 sq. ft. 2-story house built between 1872-1877. Vertical plank construction, meaning no stud walls (interior or exterior).
I've slowly been working down in the basement, removing all the PO's garbage, nasty old insulation, doing wiring/plumbing, etc. The PO, who owned the house since 1955, had "boxed in" a large beam in the basement using 1/4" plywood, so as to conceal the beam. I decided to remove the plywood, only to discover there is extensive insect damage to the beam. The beam is basically a hand hewn log, approximately 8x8.
Forgive me if I use the incorrect terms here. The item in question used to be the rear "sill beam" which rested on the stone foundation before a small kitchen addition was tacked onto the back in the 1930s and the basement was later dug out beneath this addition (1950s).
The house's floor joists run parallel to this beam, so it is not carrying any floor load. It does, however, carry the weight of the 3 layers of 1-by planks that run from sill to eave on the back side of the house (gable end), and whatever weight the addition is imposing on the back wall of the house.
At one point, a triple 2x10 beam was added which runs underneath, and is perpendicular to, this damaged beam. This newer beam is supported at both ends by masonry structures and is VERY solid.
Now I guess my main question is, is the beam with insect damage really something I need to be concerned with? Nothing else seems to be failing/sagging due to the damage.
Here is a photo to show the extent of the damage. Yeah it's bad. It feels like touching a sponge, and I can stick a screw driver straight into it (the large hole was from old wiring which I removed).
And here is a rough and not-to-scale drawing of what I'm trying to describe above.
I know nobody can really say "for sure" and I know assessing structural issues is a sensitive topic, but I appreciate any input!