BUilding a greenhouse on top of a deck

Hey everyone. I have been looking for a spot for a greenhouse that is further in on my property . I have an old hot tub that came with the house that I want to get rid of and I would free up alot of space on my decg which is about 6X10. I would still be able to anchor it to the wood and I was just thinking of putting slabs for a walkway and smalle pebbles on the side for the flooring. Any issue you can thing af not to do it on a deck...???Z

Unlike most greenhouses, you will have to consider the floor. Not only is it exposed to the elements (read cold air), it isn't going to last forever like dirt and rock. Nevertheless, I assume it's attached to the house which, IMHO, is a great advantage (my DW might not agree). My first thought (after a thorough and presumably successful structural inspection) would be to find materials that will:
1. Protect the wood from rotting from the top,
2. Insulate the floor (see below);
3. Allow for installation of some kind of top material like pebbles or crushed rock.

One thought I had was to possibly use the room under the deck for energy storage. By enclosing and insulating this area, and adding rock or water-filled barrels (etc), you have the start of a very efficient greenhouse. The challenge is how to take the heat of the day and store it below. That done, the storage can radiate all night from below. You could move the hot tub down there.

Stressbaby will surely have some good ideas for you too.

Dave.

Yeah, heat. Are you going to use it all winter?

THe deck is only a foot or so above the ground.( probably should have said that in my first post) I was just going to put some heavy duty plastic sheets down first, mabey somehow have a drainage hole for clean up... And yes I would like to use it all winter!!!... Z

If you put plastic down for a floor, it will be slippery and dangerous unless you put like gravel on top.
Carol

I had already thaught of putting some larger flat step rocks in the center and smaller ones on the side

That sounds good then. What kind/brand of greenhouse are you thinking of?

I think you have three issues. One is heat retention. This could be addressed with 1" rigid foamboard insulation, either over the deck or underneath between the joists. That will give you an R value of 6 or 7 and decrease your heat loss through the floor to negligible levels. Even just 1/2 inch will give you about R-3 which will be twice as good as twinwall PC. If you only have 1 foot of space underneath the deck, installation there would be difficult. So you may be left with putting the foamboard on top of your current deckboards.

The second issue is protection of the deck. Wet wood will be slow to dry out and prone to rot, so you will want to keep it from getting wet every time you water.

The third issue is drainage. At some point you will probably want to hose everything out. Even if you don't do that, you will have the issue of drainage of water from the pots. To the extent that you allow for drainage, you may increase air infiltration and lose heat.

Your deck should, if properly built, have a slight slope away from your house. You may be able to use this to your advantage. My first choice wouldn't be rocks or stone. This would give you thermal mass in the floor, but would be irregular to walk on, harder to get in (or get out if you decided you didn't want it), and might damage the foamboard underneath. I might consider a layer of plastic, then 1" foamboard, then another layer of plastic, then 1/2" Durock concrete board. Then on the side away from the house (presumably the south side of the GH) I would put some drainage holes which you could plug when not needed to prevent air infiltration in the winter...or even put a flapper valve of some kind on it.

This idea of building a GH on a deck is often discussed...yet I cannot remember ever seeing a finished deck GH or hearing about successful solutions to these problems. So if you figure it all out, post back!

I like Digital Dave's ideas about collecting heat during the day and using that heat at night. That's the whole key, isn't it? My next GH will have a solar panels with which I'll heat water during the day. Then using a second loop I'll cycle water through the raised beds of the GH at night.

SB, you are right. I wasn't thinking. Gravel or rocks sure won't work. You have some great ideas tho. I will just keep my mouth shut and watch the progress with awe.

YOu know I was thinking of building an insulated base to put the GH on top of so mabey I can figure away to creae on nice drainage hole and see if I can get the base to slope into it... Could get interesting but I have a tendancy to over plan everything. As far as a GH I have a few ideas but I would enjoy a wooden one.

Here are assembly photos of a nice Cedar-Built Greenhouse on a deck.

http://cedarbuilt.homestead.com/rowland1212lt.html

Great link, HDCochran!

They say that it is insulated, but it is hard to tell whether they insulated from below or above. The "well" idea for the floor, filled with marble chips, appears to have been scrapped for a more walkable rubber mat flooring.

Thanks for the post.

Zappa1 if it's only about a foot to the ground why don't you just open the deck up where you want the greenhouse to sit and build it on the ground? You'll rid yourself of any potential rot problems and reduce the heat problem. The rest of your deck can stay where it is. Save the boards you remove and if you ever decide to get rid of the greenhouse you can easily just put your deck back. Wherever you have to cut into the joist just fasten the ends to a cross board and support those.

MollyD

zappa1 you may want to think of bulding that green house out of pcp pipe or a light metal one . i know you can get a lot of snow at times where you are id all so make a portable base for it to then you can anchor it down with heavy duty cords. just my thought . i have a small one on my deck. paul