This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
Fig. 57 shows a method of attaching sheet-metal cresting on ridges and hips in such a manner that the nail heads are all concealed. Channel strips a, a of strong sheet metal are bent to the shape shown and nailed on the roof close to the ridge pole. The top course of slates is then pushed in the channel and secured with the nails b, b, which are driven through two thicknesses of slate and two thicknesses of sheet metal. The cresting c is then sprung over the channel strips and the lower edges are closed into them, thus locking them together. The edges d, d are double seamed together to prevent wind from lifting or rattling the cresting. When the ornamentation projects more than 9 inches, it is advisable to nail suitable lookouts on the top of the ridge pole.
The joints in crestings should be made in a manner similar to that already prescribed for cornices.