At Steamrail in Victoria, a new roof was done with canvas. There is a particular grade of canvas that is used, it's not the same as the canvas used for shade awnings or tents, it's a lot thicker and coarser. It was possibly called Jute, I'm not sure what the differences are.
The canvas was glued down with contact adhesive, then a waterproof membrane called 'Envelon' http://www.dominionplas.com.au/envelon.htm
was used, I assume it was Envelon 452. At least 2 costs were applied and this wold soak into the canvas. Be warned, its solvent is fairly potent, in this day and age read the MSDS.
After that, the topcoat was enamel based paint with added linseed oil, don't know the percentage of oil added. Topcoats were applied on an annual basis to keep the canvas nourished with oil.
It's been quite a few years since I've done a roof, don't know what the process is these days. Envelon is an Australian product, you may be looking for a German supplier of a similar product. Over the years there have been many alternative suppliers of products claimed to be up to the task, but the roof of a wooden bodied vehicle has to put up with flexing, wet weather and in Australia, a lot of UV exposure. All these take their toll on a roof, not all products have proved themselves over time.
A shed is the best roof covering of all