The use of a tile roof is expensive but very desirable, as its duration of wear is practically unlimited. However, its cost is so much greater than that of a slag or plastic slate roof that the latter, from the standpoint of economy, must generally be preferred. Both of these kinds of roofs are easier to repair than any metal roof, and are much preferable. The use of heavy tar paper, laid on top of the finished slag or plastic slate roof, serves to protect it from damage due to walking upon it, and is more effective than gravel, which is used for the same purpose.