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.
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