Home > English Heritage Transactions volume 9: Stone Roofing > Excerpts
Saving England's Stone Slate Roofs Susan McDonald, Terry Hughes, Chris Wood, Pat Strange. 
Criteria for roof conservation
The appearance of a stone slate roof is an amalgam of many features: the texture, colour, size and format of the slates; the pitch of the slopes and the treatment of their intersections, and of the eaves, ridges, gables, verges, dormers and chimneys. Each of these features should be carefully recorded before restoration work commences, and modern techniques and materials should only be substituted where there is a sound technical reason to do so. The same traditional techniques and styles should be applied to new construction, extensions, and alterations. Guidance on the correct techniques for reroofing is provided by the conservation departments of Derbyshire County Council and the Peak Park Joint Planning Board. Currently, this guidance does not include criteria for the selection of replacement or additional stone slates. The following are suggested in order of importance to the roof’s appearance.
Rock type: sandstone should never be replaced by metamorphic slate or limestone. Aesthetic considerations apart, limestone placed above sandstone can be very damaging to the latter. Limestone dissolved in rainwater will percolate into the sandstone interstices, where it recrystallizes as the roof dries, disrupting the sandstone and leading to spalling and early failure.
Size range and mix: traditionally, these would have been a consequence of the characteristics of the rock and the policy in the delph. As far as possible, the same sizes should be obtained as were originally used. Because slates deteriorate at their top edges, reclamation and removal of delaminated top edges leads to a reduction in length. It is very important to obtain extra slates of the maximum length required in order to make up for this deficiency. Ideally, the pattern of reducing courses should be conserved, although there is a limit to what can be achieved without wholesale renewal of the slates. The length of exposed slate margins should be recorded before dismantling a roof. If the larger slate lengths can be obtained to make up losses due to delamination, then the diminishing coursing will usually work out to a close approximation of the original. It is important to realise that, in random slating, the head lap is not constant and must be increased at changes of length to ensure adequate resistance to driving rain. This influences the length of the slates’ exposed margin but it must take priority over the need to conserve the roof’s appearance.
Thickness: this characteristic is very important because the edges give the authentic stone roof its bulky appearance. Its visual effect is influenced by the way in which the edges are dressed and the angle at which the edges spall during manufacture. Stone slates are sometimes turned over when being reinstated. The argument for this is that they have often settled into the roof, becoming curved. When replacing them, they have to be positioned with the concave face downwards to avoid the tails kicking up. One disadvantage of turning reclaimed slates is that the edges will be bevelled the wrong way. This not only spoils the appearance but tends to hold water rather than shed it.
Surface texture and grain size: surface texture may vary from completely flat and featureless to rippled and twisted. Besides the obvious effect grain size has on the appearance of the surface, it will also affect the build-up of surface deposits. Further, if the slate has an open structure, it will tend to absorb and hold more water, making it easier for lichens and other vegetation to become established.
Colour and visible mineral: colour can be more significant than surface texture for the smoothest and flattest slates. When newly laid, the colour of slates is very evident but diminishes as dirt, soot or lichen growth build up. For this reason and because pigmenting minerals may be leached out, the colour of most old roofs is quite different to the rock from which they are made. Some minerals (such as mica) will give the surface of new slate a very distinctive appearance but this may alter with time. Little is known about how the colour and surface of stone slates change over the years and it may be that the soot-coloured appearance of many old roofs is becoming a thing of the past.