Placing the hearth on stone flooring certainly works.
The game just needs to notify the player as to why placing it on anything else isn't going to work.

Originally posted by Neo-Rio:

Placing the hearth on stone flooring certainly works.
The game just needs to notify the player as to why placing it on anything else isn't going to work.

That's very odd, because for ground level, I use earth, and for 2nd level in the house, I use the 4x2 stone walls.

No, you don't need a stone floor, wood is enough if you've built correctly.

First of all, you should straighten and consolidate your construction site (with the hoe).
Then you put round wooden beams in the ground, that's a little tricky but it works. Alternatively, you can also use the hoe to adjust the building floor again afterwards.
It is important that every second floor plate is a beam, since the fireplace is then exactly on the round wooden beam, you can then place it on wood.

Only if the wooden floor is not supported is the stability insufficient for the hearth.

if you are allready in iron age
https://steamcommunity.com/sharedfiles/filedetails/?id=2624608170

Use your hoe and some stone and raise up a spot to place it.
Then you can build your stone floor around it.

The hearth needs to be placed on top of stone or earth and the stone underneath should be supported by either stone or earth. Placing it on wood will immediately break it.

Yea either stone floor or directly on the ground. It's true some clues could be welcome. My very first experience with stone was trying to place a stone floor on a wooden pole, and when it immediatly broke it was pretty obvious. Having that first experience with hearth isn't that obvious though

I did place it on top of earth (leveled with my hoe), but no luck with that.
I'll try placing it on stone later, and see if that works in the context of a wood house.

Originally posted by Neo-Rio:

I did place it on top of earth (leveled with my hoe), but no luck with that.
I'll try placing it on stone later, and see if that works in the context of a wood house.

See if the hearth is actually on top of the earth and not on top of any surrounding items. For example, remove the immediate surrounding wooden floors, and then place the hearth on top of the earth, and put the wooden floors back in after. If it still breaks, then I'm at a loss.