October 2016 - last edited October 2016
Steps to Reproduce:
Workaround: Place floor pattern prior to placing round flat/deck pieces.
November 2016
Aparently ,you cant do this.you cant place a round room . Id like to be able to do this but I cant .
November 2016
No, there are no curved or rounded rooms, @WolfieGirlCZ, but there is a curved flat as well as curved deck pieces. This bug has to do with placing floor patterns beneath them and not with the presence or absence of curved rooms.