Here is an interesting, downloadable 16 page PDF document about stone roofing techniques that aren’t discussed very often. Of particular interest to me was the use of “shadow” slates underneath heavy stone shingles, where thin slates are utilized in a manner similar to bib flashings on heavy stone roofs. I have seen this technique used on only one slate roof — a graduated roof with massive lower slates where thin, 3/16 inch slates were inserted under the vertical butt joints of the largest slates in order to fortify the water-shedding ability of the roof. Horsham Stone Roofs, among other things, illustrates a technique similar to this. This booklet was developed during a series of meetings and discussions with people experienced in the use of Horsham stones and it describes the state of the art.
Excerpt: “There are two systems of roofing with Horsham stone — the ‘normal’ double lap system and single lapping which is unique to Horsham stone roofs. Both systems use random sized slates which are arranged on the roof with the largest laid at the eaves and gradually diminishing to the smallest at the ridge. Traditionally, in both systems, each stone-slate was top hung with wood pegs on split wood laths. Top hanging was essential when thin split laths were used, but the usual modern practice is to nail them to substantial sawn battens. It is conjectured that the earliest Horsham roofs — predating the 19th century — always used the ‘normal’ double lapping system, where course three overlaps course one, four over two, etc. This is the system which is almost always used for sandstones, limestones and slates throughout the UK. In the Horsham single lap system, the slates only overlap the slates immediately below — course two overlaps course one and so on. This leaves a gap along the perpendicular joints. This is weathered with pieces of metamorphic (Welsh) slate, known as shadows or shading, and these and the Horsham stone slates are bedded in mortar” [see illustrations below].
Contact: Terry Hughes, Stone Roofing Association
Ceunant, Caernarfon LL55 4SA 01286 650402