There’s a lot going on in this picture, and much of it is not good.
The center of the picture is a girder* with joists on both sides framing in. (The joists on the left are mostly hidden below the plywood.) The fact that we see the tops of the joists inset past the edge of the girder shows that they are half-lapped in place – a connection similar to a mortise and tenon, except that the tenon is the top half of the supported joist rather than the middle third, and the mortise is open on top. An electrician** has notched the top of all the joists on the right. The top of the girder and some of the joists are damaged – possibly through long-term exposure to water, possibly by insect infestation, possibly by both.
The half-laps reduce the section of the girder, so structurally it’s smaller (and weaker) than it looks. The cuts also serve as stress concentrators, leading to more cracking in this type of girder. The water/insect damage has removed the top of the girder and some of the joists, but it can also continue further down into the wood. After this picture was taken, we had some fun digging in with a screwdriver. The electrician’s notches are not big enough to reduce the strength of the joists, but are indicative of a bad attitude: when I see a spot where someone was attacking structure with a saw, I start to look for where else they might have been busy.
There are no great lessons here besides the obvious ones: old wood structure is often poorly designed and is subject to all sorts of degradation over time. Losing the tops of the beams may be a rare form for the degradation to take, but it’s still a problem.
* In certain circles, this girder is known as a summer beam.
** Or maybe a plumber. Depending on when those notches were cut, they may have been to lay gas pipe for lighting rather than electrical conduit for lighting.
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