Underwater greenhouse.

Aha, thought that might get you wondering about GG's current state of mental health! My greenhouse has been there for over 15 years and because the land has a high water table the floor (and /or the base) has gradually sunk over the years. In autumn or winter when we get a lot of rain the floor (which is just soil covered with a weed-suppressant membrane with gravel and a few paving flags to walk on) is underwater to a depth of 3"-4". The area outside is just rough grass which also gets wet and boggy so I've nowhere to drain the water off as that's all at the same level as the GH. I need to pave the floor because weeds have seeded in the gravel so would it be best to raise the floor level with sand etc then lay the flags on top of that? When things dry up outside I'll get a spirit level to see if I can find a slightly lower area where I can  drain off any excess water. Any other suggestions are most welcome. 

I do sympathise, my greenhouse is a lean-to and is wood and glass on walls about waist height. It has gone rotten and started to collapse in the gales.  It is like trying to tape back together sawdust. !
Back to your problem.  This probably is no help at all, but might make you chuckle.   How about Hydroponics and Rice.     I'll get me coat.   Mrs Bouquet

Do not pave as it will get worse....paved next to mine last year and discovered one of the run off areas for the septic tank.    Paving now gone.
I have planted water loving plants as the area is damp all year, small trees take a bit.
You can create a drainage areaor several by creating downward trenches, pouring water will show you run off and end in a gravel pit area. Several would help, starting spade depth, ending 4 spade depths if poss.