The felt is applied in overlapping layers to form a barrier two to 4 layers thick. Then, a layer of finely crushed stone is embedded in warm tar over the top to create a incredibly durable and impenetrable roof. A adequately mounted BUR roof can last 20 to thirty yrs. https://learntomore.com/5-contractors-recommended-in-hinsdale-il-top-experts-for-your-project/