Why does my floor rafters bow upwards without any support under them?

Question

I live on sandy soil . one block from R R tracks. house is over 100 years old.

Answer 1

Because that is the way they were designed
If you look at the cross cut of the rafter, you will see that they were all placed in the same way - look at the rings (annual growth on the wood)
When the wood dries the "inside deisgn" makes the beam to bend upwards - the ends are fixed to the walls and any furniture will help counteract that "bowing". At the same time the furniture will lean safely against the wall instead of falling over (as would happen if the floor sagged in the middle)

Looks like the carpenter who built your house knew his job

Answer 2

If your house is that old, I imagine it was built with rough sawn lumber, as the weather changes it can cause these floor joists to bow, this is typically caused by a moisture change in the wood. or the heaving of the ground as the ground freezes and thaws

What type of foundation do you have?
are you sure it is the floor joists and not the sub floor that is bowing upwards.

Answer 3

there are a couple of reasons for this. the beams could have been cut and installed with a slight bow so that when weight is placed on them they would level out. the most likely reason is that they have dried out and just bowed due to the fiber makeup of the wood.