Timber joinery exposed

Butt cog joint
Last time we saw one of these we were
watching the Amish barn-raising in "Witness."
110 years ago this joint was probably tighter.
There are also a few dove-tail joints to keep the
building from spreading apart.

Jennifer's subfloor was pulled off today, revealing century-old main floor timbers, 

with their custom hand-hewn joints binding the grid together. The massive interlocking beams are still in reasonable condition, no rot, but they have warped somewhat over the years, making them less than ideal for supporting a new floor. Square nails held the tongue-in-groove floor boards in place. . . the supporting beams rely on gravity and ingenious joinery.
Square or "cut" nails  gave way to round nails before World War I, according to Wikipedia.
Progress continues on Friday with a small gang of volunteers expected on the job site.
Read all about Historic American Timber Joinery at this link. You'll find techniques described there that we found in Jennifer's floor. Very cool.

Midway through the removal of the subfloor. The ladder protrudes where the floor mounted furnace once hung.
To save labor, it was common to hew only one side of the timber when it was to be used as a floor joist.