help with designing greenhouse

[moh dana]

hi I'm building a greenhouse for the first time and i don't have any experience with framing and i wanted to upload some pics and so you could tell if there is anything wrong with it(especially the framing):
i want to build something like these:


here are my designs so far:

1.jpg
4.jpg

i'm not sure about the roof do you think this would be strong enough?am i doing it right?
2.jpg

and here:
3.jpg

and I'm building it on our roof!

[moh dana]

BTW since it'll be built on the roof i don't know what to do about foundation

[Kent Adams]

I own a greenhouse and have some experience with them. That slant design would seem to me to be a waste of space because the slanted end won't allow you to plant anything that would require vertical trellising. The best greenhouses have vertical walls, unless you're aim is for a poly tunnel type that has only flats and small plants in them. The airflow in that may be a problem as well as would be the heating and cooling. How is it vented? What is the reason for the slant wall out of curiosity?

[Warren Wilson]

Hi Moh:

I've built a few greenhouses and a couple of houses and would offer a few words based on that, rather than on formal training.

First of all -- nice work in Sketchup: anyone who can do what you did there is more than ready to build a simple greenhouse!

Second, I'd re-think your ridge and rafter roof design. I doubt it would provide adequate structual strength -- mostly against the wind load. I would recommend that if you want to have that asymmetrical design you consider it like a truss set. It is easier to erect for a one-person operation, and using truss reinforcing plates (cheap at a building store) will give you better structural strength than nailing into a ridge pole. This also goes for the join of your roof to your vertical wall. Using "gussets" there (or diagonal bracing) will increase resistance to racking.

An aditional benefit to using trusses is that you can very significantly increase the strength with bottom chords -- which then act as very handy members for hanging baskets: increasing your gardening space inside the greenhouse. We grow a LOT of cherry tomatoes in our hanging baskets! (Admittedly using a design like that suggested by Kent above).

In your current design, there is no structural member to prevent the entire greenhouse from falling over like dominoes. In house construction, the plywood or OSB sheeting adds significant strength. In greenhouse design, the poly sheeting adds no structure. Bracing of some sort will be required, or a lot of effort will fold up.

As for the issue around installing it on the roof, I'd think there are three aspects: weight, moisture control and anchoring. Dirt certainly helps wood to compost. Moist warm soil eagerly embraces cellulose. You might want to use thick poly sheeting to keep the moist soil from both your wood studs and from your existing roof. Weight, however, might be the most important consideration. Your house was probably not built to a code that anticipated lumber and moist soil sitting on the roof -- it's worth googling the idea of "point load" so you can understand how your greenhouse structure and roof structure will work together.

Obviously the topic requires sitting down with a beer!