For those curious why this happens: this building has no diagonal bracing to prevent racking, so over time it can sag and lean more and more as the wood weathers and nails loosen.

Installing the exterior boards at an angle would’ve helped, but usually results in more waste and labour so if you’re a poor farmer with limited supplies you just install them horizontal and it’ll probably last as long as you need it to anyways. Braces on the inside can be annoying to deal with if the building is used to store grain, but a viable option otherwise.