There currently is a bug that causes pipes, stockpiles and other things to lower the Tier of the room, check if you have any of those in your room and remove them if possible. If that didn't work, you'll need to put in some T3 materials, to increase the tier. This bug is fixed in Update 8.0.
Ah thanks SLG-Dennis, Found the problem eventually, it was a pair of hewn log doors on the opposite wall. Replaced them with lumber doors and got to tier 2 :)