This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
This is a kind of asphalte pasteboard much employed in Denmark; it is laid on close boarding at a very low pitch, and forms a light, durable covering, having the non-conducting properties of thatch. It is sold in rolls 2 ft. 9 in. wide and 25 ft. long, having a superficial content of 7 1/2 sq. yd., at the rate of Id. per sq. ft. When laid, it requires dressing with an asphalte composition called "Erichsen's mastic," sold at 9s. 9d. per cwt., 1 cwt. of the varnish sufficing to cover a surface of 65 sq. yd.; iron wire nails, for fastening down the pasteboard, cost 13d. per 1000, this number being enough to nail down 15 sq. yd. The method adopted in laying the pasteboard is as follows: - The framework of the roof is covered in with dry well seasoned boards, 3/4 in. to 1 in. thick, and not above 6 in. broad. If they are not sufficiently dry, they should be split lengthwise before being laid down, in order to keep them from warping, and every board should be fastened with 3 nails at least to each of the rafters.