Unusual arch support interaction?

[Meliphicent]

I've got a room that needed some support for the floor at the top. Wanting symmetry I went to put an arch on the outermost wall pointing back towards the centre of my structure and was told it is not supported? Arch on the other side no problems. I'm guessing it's because this room was already at maximum extension from an arch support underneath it and the way it's programmed is that outer wall can't have any arch supports attached (to stop a reverse pyramid build).

[Philtre]

The arch support itself needs to be supported, so if you were trying to place it on a wall which already had the lowest possible support, it won't work. I don't think the game cares about reverse pyramid builds, you just need to make sure the wall you are placing the support on is capable of supporting it.