We have been making tipis since 1970. In the beginning, we used both untreated cotton army duck canvas and 13 oz. treated army duck canvas, which held up very well. A 10-year lifespan for the cover was standard and expected. However, since the ban of heavy metals such as mercury in the use of fabric finishes, we have seen our tipi covers last considerably less when used all year around. We are aware that ultraviolet radiation from the sun, increasing air pollution and proliferation of mold and mildew are the most destructive factors to the longevity of cotton fabric, combined with a decrease in effectiveness of the fabric treatments themselves. In today’s world, a cotton tipi cover made from the sturdy 15 oz., 100% organic cotton can last from 4-5 years in a moderate, mostly dry climate if you choose not to use additional protective finishes.