The roost? Yes, a roost should be more than 3 cm thick - especially as your birds are showing you signs of not being comfortable with it. There is no magic number (though some will quote one), but rather a matter of doing what you have started to do in that you can follow the lead of your birds and let them tell you what is comfortable (after all, people read all sorts of books that give you rules about what chickens do/don't like, etc, but the chickens don't read those books and tend to have their own ideas, lol).
Many people use 2x4 pieces of lumber with either the 2" or 4" side "up" for the birds to be in contact with - but birds will also hapilly roost on larger, flat surfaces such as shelves. There are a lot of options, just a matter of deciding which works for you and, more importantly, your birds.