Although they are architecturally not very prominent from the exterior, they are recognizable from the rooftops by their pierced domes and vaults which usually covered the main chambers.[63] Walls are coated with Tadlakt to smoothen their surfaces and to resist the high humidity.[3] The warm and hot rooms were heated using a traditional hypocaust system just as Roman bathhouses did, with furnaces usually located behind the hot room. Fuel was provided by wood but also by recycling the waste by-products of other industries in the city such as wood shavings from carpenters' workshops and olive pits from the nearby olive presses. This traditional system continued to be used even up to the 21st century.[63]