Flexible wall yurts use two basic tarpaulin skins: a canvas treated skin or a poly vinyl weave tarpaulin. Each has advantages and disadvantages.
Two basic weights are used in the poly tarpaulin: a 14 ounce per yard (400 grams per meter) or 22 ounces per square yard (600 grams per square meter). The cost of the 22 oz. weight is approximately 40% more than the lighter weight, and generally lasts about 40-50% longer. However, both are prone to punctures from branches, squirrels and birds, so I recommend the lighter weight, as the price per year will be slightly lower than the heavier weight.
Poly tarps tend to be less pliable than canvas ones, and, in brisk winds, can fray if they are not installed tightly to a frame. On the other hand, canvas tarps are much heavier, and are prone to mould if snow is allowed to accumulate on them, or if they are in high humidity/high rainfall areas.
Canvas tarpaulins are easier to install, yet are a poor choice if you are using an open rafter concept, as they may stretch over a period of several years. However, the authentic and aesthetically pleasing sound of rain or wind on a canvas is unequalled.
For solid or rigid-wall yurts, either canvas or poly tarpaulin skins will suffice. If you are insulating your yurt and the insulation is moisture-proof (rigid insulation or Mylar-coated), condensation may build up between the tarpaulin roof and the insulation. This problem is best addressed by using poly tarps, and treating the skin with a mildew-resistant spray.
Each type should be treated with UV protectant annually, and coated with flame retardant (at minimum, on the inside).
Overall, a poly tarpaulin is a better choice than a canvas skin. Cost of canvas is two to three times that of a medium weight poly, and lifespan (without treatment) is comparable. While canvas is a more environmentally friendly choice, the value of a poly tarp exceed that of a canvas one.