snails in the plastic greenhouse....

anybody got any reliable methods of keeping snails from entering a plastic greenhouse (aldi type)?

i went down at 1.30am this morning to see if any were about doing their deadly deeds in my greenhouse, sure enough they were.  my plants were on the top shelf, and they were helping themselves
(how they manage to climb up and get on those thin wire mesh shelves i dont know), these were huge snails.
round the bottom inside and outside i had put a layer of sawdust thinking that they would not be able to crawl over it, (how wrong i was!)

what methods do others have? and do they work

I don't necessarily keep them out, but there's a blue nasty surprise when they get in!

 

Im having the same problem. Dont see so many slugs but the snails seem to really like it in my greenhouse. (not just babies either but snailzillas too).
The only thing Ive found to do is keep checking all the pots and trays morning and night. Ive lost a few seedlings but have some back ups on the windowsill if I need them.