How to build a greenhouse?

[Satras]

So I tried to build a greenhouse, but the game does not seem to recognize it as such. It has a 7x7 field inside, solid blocks underneath. The walls are made out of cobble stone (lowest layer), a pine log frame with glass slabs. the roof has one central pine log beam, the rest are glass slabs. The whole structure is less than 7 blocks high. Where is my mistake?

[Stroam]