Building or sheathing paper is now placed over the sheathing to keep out the weather and to cover more completely the joints in the boarding. This paper must be tough and strong as well as waterproof, and must be easy to handle and put in place. It must not be easily broken as any hole in it is an opening through which the weather will surely find its way. There are a number of different kinds of building paper on the market, prepared with various chemicals to render them as nearly waterproof as possible. Tar is sometimes used for this purpose. The paper should always be laid with a good lap.