I believe your concern about the humid air inside the GH is well founded, because unless it's open to air and allowed dry between soakings, even redwood will rot.

If the GH is planned as a permanent fixture on the deck, you might want to consider protecting the decking by installing a heavy vapor barrier between the GH and the deck, and laying some other material on the inside floor: possibly a low raised floor, just high enough to allow you to install a floor drain, to discharge somewere off the deck.

Will redwood rot?? I know my brother did some timber recovery from old logging bridges in North CA built from redwood. I thought he said that even the stuff below water was fine. I am pretty sure that is the whole point of using redwood.

If the deck is made of boards and is not a solid surface the moisture should just drip through the cracks right? I wouldn't think it would be worth it to put in a separate floor.

Am I missing something?

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! :-)

Kate:

The large dimensional timbers from old logging bridges were cut from very dense, old growth heartwood, and if cut and used after about 1900, very likely treated with a coal tar preservative. In any case, having been deep in the ground below cold water, they would been exposed to far fewer of the live organisims that destroy wood and thereby, resit rot for a much longer time than had they been left out on the suface of the forest floor or- under a damp greenhouse.

Also, much of the redwood sold today has been farmed and forced into faster growth and the faster any tree grows, the less dense the interior structure, and the softer the wood. The same applies to cedar, and why most of the cedar fences and decks constructed and left unsealed to weather naturally in the last few decades are rotting. It's also why the term "hard pine" has become an oxymoron.