I have built greenhouses only for them to get rain and snow, and im sure alot are having these issues. Im glad i didn't buy this last year as this seems to of been an issue for a while now. I just realized it myself, and went looking for something to fix it.
Since there are competitions involving greenhouses, I thought giving this link to this section may help a few. There is a new Stone throw greenhouse, and is available to download.
Please follow the Guru's instructions carefully.
This is from EA themselves, and no Im not saying it works, haven't tried yet, Im in the process of that now.
This thread is to help, nothing more okay???
If you want to ask questions, don't ask here, go to the link and ask there, this is just a guide to help anyone with this issue.