The primary function of a roof is to shed water. The roof is a complex structure of slates, flashings and rainwater goods above timber structural members and modern insulation materials, rainwater goods. When damp is found to the interior of a period property, there are many potential sources for this moisture. This moisture may not be coming from outside, as modern living can provide significant levels of water vapour and humidity within a building from cooking, showering and from uncovered water tanks in attic spaces. However, one of the most common concerns regarding roofs of a certain age is the condition of the slates, if and how they are failing, and when and to what extent should they be replaced.

Natural Stone Slate

Traditional and historic roofs have been covered with a wide range of materials, of which oak shingles, thatch and slates are the most commonly known. Roofing “slates” are not necessarily the metamorphic rock slate, but is an umbrella term to refer to any stone which can be split and used as a suitable roof covering. Sedimentary rocks with fine “beds” were often favoured as the stone could be easily split following these natural lines of weakness to fabricate the roofing slate. Consequently there are a wide variety of stone types found on period properties which were in use until the mid 20th century. Slates, sandstone, siltstone, limestones and a wide range of metamorphic rock types can be found on historic buildings. The few documentary sources we have for 19th building materials reflect a range of “slate” quarries in operation at Clonaiklty and Drinagh in Cork, Portroe in Tipperary, Carnew in Wicklow and near Ross in Waterford. Older medieval buildings with surviving roof elements also retain local materials, with the local schist used for the tower at Doe Castle on the rocky north coast of Donegal. This practical tradition of the use local materials lends character to buildings within particular regions such as north-west Clare where the roofing material  reflects the local landscape. There was also a thriving import trade in slates, with 19th century imports of Welsh “Bangor Blues” and green English “Westmorelands” among others. Consequently, if repairs are needed, it can be difficult to match the local stone.

However, before undertaking widespread replacement, the first question should be what repairs, if any, need to be carried out? The durability of a roofing slate depends on four key factors: the physical durability of the slate itself, how it was cut and shaped, how it was installed, and how or if it has been maintained. Natural stone slates have lifespans ranging from 50 to several hundred years. Sound slates can be “sounded” to give a preliminary indication of condition, however slates typically deteriorate in a limited number of ways. Mineral impurities such as iron pyrites can weather out, leaving a hole in the slate, the edges may flake and scale due to natural weathering, or the uptake of airborne salts such as gypsum combined with the wetting, drying, heating and cooling of exposed roofs can cause the slate to delaminate – as the beds forming the individual slate begin to separate. These failures mean that deteriorating slates cannot be simply flipped over and re-used, but have to be replaced.

Certain areas of roofs may also deteriorate faster than others, shallow pitches and areas subject to concentrated water flows are likely to be the earliest to fail. The “slipped” slates commonly seen to period properties however is typically not due to failure of the slate itself, but to failure of the iron or oak peg holding it in place, or failure of the timber supporting the stone slate. Estimating and carrying out repairs generally require the advice of an experienced slater or other professional. However, short-term repairs using roofing mastic or sealants will fail after a time as they eventually harden and crack.  As a general rule of thumb, if less that 20% of the slates are cracked, broke, missing or loose, it is m=normally less expensive to repair than replace the roof. If more than 20% failure is found, it may be possible to salvage many of the slates for re-use elsewhere on the building. For example, if all the “faces” of a historic roof show failure and the roof needs to be re-slated, it may be possible to salvage enough slate to repair the main faces of the roof with the original material. In this way, significant savings can be achieved by the owner, while retaining the character of the building, and confining repairs in a new stone slate material to the rear or other less visible part of the roof.

Repairs to the roof should not be focused solely on the stone slates, but should also consider corroded and worn flashings, failed gutter systems, and poor ventilation in the attic. Stripping of the roof slates should be undertaken in

Historic stone slates

sections, which allows the roof timbers to be checked for cracking, deflection, twisting or other problems. Periodic checking by the owner can prevent many small repair problems from developing into more costly issues. A pair of good binoculars will allow most owners to check for cracked, broken and missing slates, especially after a storm, and also allow broken and clogged downpipes to be noted. As ever, a stitch in time ……..

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