Hiya

I hope this beautiful day is treating you fine.

I live with my parents in the city, but we like to have some green around the house. We have a garden with some berries , roses, herbs and a prune tree. But we're out of space in the garden since there also is a terrace. My mom agreed with the idea to put a small greenhouse on the roof. I'd like to use it to sprout plants and maybe grow some herbs.

It's a 3m*5m roof, gets about 6 hours of sun during winter and has bitumen roofing.
We live in Belgium so we get frost throughout the winter.

For the greenhouse itself I'm thinking a wooden frame with either glass or those double layered plastic sheets.
Our big concern is protecting the roofing. Should we use some kind of foam for protection of the roofing and also for insulation?
I'm also wondering about the efficiency of small greenhouses. Will I need extra warming? I was thinking keeping a waterreservoir in the borrom of the greenhouse, insulated to all sides but in contact with the bottom of the soil. So if the greenhouse gets to cold I can pour some warm water in there to get them through the night.

I'm wondering if you have any references or info about accomplishing this idea.

Thanks

The first thing you should do before even considering a greenhouse is figure out if your roof can structurally support a greenhouse on it. In regions that don't get snow, flat roofs aren't always strong enough for the possible addition of weight to the roof. In regions that get lots of snow, the roofs should be able to handle it, but in those 1 in a 100 year storms, that drop much more snow than usual, roofs do collapse and that would just be an added weight.

If none of your family members have the experience to judge it, you may have to hire a contractor to take a look. I don't know if online there are resources that can help you with judging how much additional weight it can support.

Now after having seen if it's feasible, the idea of making a greenhouse out of wood & glass or plastic sheets if more than doable.

I found this article on a website that looks useful to your endeavor. DIY Greenhouse - Glazing Comparisons

As far as keeping the roof leak free, I think the best idea would be to simply use as large of a material as possible, windows can only leak at their joints, so larger windows = less joints, large clear sheets of plastic make for a fairly rainproof screen until they're punctured. And remember, when sealing for water silicone just might be your best friend if dealing with glass.

On heating. The vast majority of greenhouses do need extra heat in cold areas. The alternative to heating is building a greenhouse that heats itself with the sun, and most roofs can't handle the extra thermal mass you would need to guarantee the functionality of the system, so unless your experienced family member or contractor tells you otherwise it's most likely a no go. I unfortunately don't have experience in heating greenhouses, and would just like to remind you that whatever way you go, you should be wary of the weight. The idea of keeping water in the greenhouse should also be carefully decided based on how much extra weight you are adding.

As far as the actual building goes, If you don't mind the added complexity, you might want to think of building a geodesic dome, the shape helps in preventing cold and hot spots forming along the edge, and they can be a universe unto themselves if done right. Otherwise standard wood construction should suffice. You need to be wary of rotting in the wood, and while commercially inclined people paint on a toxic compound that pretty much kills everything on it, most of the folks here at permies would probably agree you would be better inclined by treating it biannually with linseed oil.

If you get past all of the structural stuff and it turns out that this project will proceed it will be important to protect the existing roofing. Contact a roofing contractor and have them place sleepers against the asphalt material. They will remove any gravel or other large items which are likely to puncture the material. Sleepers could be set in a bed of tar. Everything that you add will rest on these.

I have built many greenhouse structures and would never fiddle with a geodesic dome. Too many joints, weird angles, almost impossible to find recycled glass in the proper shape. I have demolished a couple geodesic dome structures. They generally failed due to water infiltration.

Rain and snow which falls on the greenhouse needs to be dealt with properly. A single plane would be easiest to deal with. Generally one roof should not simply be allowed to pour onto another as this will erode the material in the lower roof. It's best if the greenhouse can drain directly onto the ground or through its own gutter system. A dome built on the ground doesn't really need a gutter system but on top of another roof any concentration of water flow or snow accumulation may be problematic.

A rooftop greenhouse can be used as a powerful exhaust fan for the rest of the home in summer. If it were connected to a skylight or to some other vent in the roof, opening vents at the top of the greenhouse would facilitate plenty of ventilation without electrical cost.

Good luck and be sure to check out everything the other poster mentioned about structure before proceeding with any of this.