What is your humidity at night? I can't imagine a g/h this time of the year not dripping with humidity, or at least mine does on dark dreary days and at night. For orchids you need the fans circulating air 24/7. Pea gravel or limestone gravel under your benches will definitely increase your humidity. I have limestone because it is the cheapest around here. When you water your plants, it soaks into the gravel and then into the soil. For two days we had super sunny weather and the g/h temps went to 90 with the humidity at 50%. By 7 last night, the humidity was 85% and the temp was 62. Today we have no sun and it is 75% right now - drip, drip, drip - with the heater running. In the heat of summer, during the day my humidity will go under 30% if I don't thoroughly wet the gravel but at night, it returns to 80%. Add gravel and then you may want ideas to reduce the humidity. Brooke

I'm certainly not an expert, but ANY wood will eventually rot, given the right conditions. Greenhouse interior conditions would be prime for rotting! It may take a really long time for rot to ruin the decking, but if the structure is to be permanent, it might just pay to think about adding a separate floor with a moisture barrier. A drain for run-off would be good, too. Depending on what is under the deck, i.e. gravel, dirt... and whether or not the ground under the deck is level, you'll need to re-direct the run-off from watering so it doesn't pool under the deck, causing more damage. Ventilation and good air circulation should be considerations, not just for moisture but for plant health, as well. I stumbled upon this thread while looking for greenhouse information... I'm considering purchasing a small one, mainly for seed starting, cuttings, and young plants... it will sit on the ground, though! :-)

I worked for a commercial greenhouse that had rough finished concrete walkways in some of their greenhouses. Even with the rough finish the walks would over time become slick with a green slippery moss that grew under the benches, and would eventually work its way into the walks. Periodic washings with a pretty strong Clorox solution would fix it for a while, but...... We were growing orchids/indoor foliage, so were throwing a good deal of water at a lot of plants. Bill

I have heavy clay soil. Zone 7b North Alabama. I put the paver sand down and compacted, then put the pavers on top of that. I think I could of just skipped the sand. https://www.lowes.com/pd/American-Countryside-0-5-cu-ft-Tan-Paver-Base-Sand/50408222 They also make a rock base. https://www.lowes.com/pd/American-Countryside-0-5-cu-ft-Paver-Base/50408224 If you scroll down a little, you can see a picture of how to lay out a paver patio. I did not need anything that solid. I just wanted an easy to walk on floor and no mud. https://www.lowes.com/c/Pavers-retaining-walls-Outdoors If you want rock for drainage, I would think almost anything you could use in a driveway would be fine. They have the same stuff at Home Depot also, You might want to try a few down the middle without anything and see how they do when it rains. Like I said, I wasn't building a patio, so I did cut out quite a few steps that would be needed for one. They have stayed in place for 10 years now. The only thing I wish I had done different is to not have the gap between the paver. It is sized, nominal 12" x 12", is really 11.75" x 11.75", so you can put the small pebbles, groundcover, or polymetric sand for a patio.