I have just torn up the joists in a room in our 1923 house. The room has 2-3" thick plank walls with a sill made of oak. The sill is placed on larger cut natural stone blocks where the gaps have been sealed with cement by previous owners.
On the south side of the room, the exterior wall is 4 meters long. In the middle of the wall is a window section about 160 cm wide. Under the window, the sill is very poor. You can't stick a knife through it, but you can cut away the surface maybe 2-3 cm with a knife.
On the sides of the window, the sill is superficially soft but otherwise perfectly sound (The impact driver had to work hard to secure a screw into the core during the test.) Towards the corners, it is brand new (hard as stone).
The strange thing is that the 15 cm thick sill doesn't rest on the foundation along its full width but only the last five centimeters on the outside. In cross-section, the sill is shaped like a thick L.
_______
I SILL I
I____ I
I__I
The mistake has been made of plastering with cement from the outside between sill/stone foundation to seal against drafts, thus likely exposing the sensitive sill to extra moisture migration. This has caused the portion against the foundation to still bear weight, but it has also begun to soften.
My first thought was to make a partial repair from the inside by cutting a few centimeters out of the log and inserting a new oak beam. Since the sill is only really bad under the window and this part probably doesn't support the wall, my latest thought is just to remove and insert fresh wood right here. However, I wonder if removing material might weaken the sill as a whole, and it might be better to just leave the bad wood alone? After all, it doesn't support the wall?
In any case, the plan is to drive wedges of oak with a sill paper underneath between the stone foundation and sill to ensure that the sill rests on more than the 5 outer centimeters of the healthy part on the sides of the window. Or is there any reason why the sill only rests on the outer 5 cm?
Clarification of the questions:
Is there any reason at all to remove parts of the log?
Is there any reason why the sill only rests on the outermost 5 cm?
