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Slate Manual – Page 17

Standard Specification and Details - Ventilation and Random Slating

Roof Ventilation

Traditionally roof voids covered in natural slates needed no precautions for ventilation since underlay felt was not used. The introduction of these felts, however, together with today’s higher standards of building insulation and the steady elimination of open-flued chimneys means that care must be taken to ensure proper roof ventilation. A lack of ventilation will lead to condensation forming within the roof. With roofs in natural slates the required ventilation is best achieved with a continuous opening at the  eaves on two opposite sides of a roof as well as high level ventilation. The high level ventilation can be achieved by using a dry vented ridge system or by the use of special ventilated ridge tiles and/or ventilator slates. Please refer to BS5534:2014 for more thorough guidance with regards to the amount of ventilation required.

Random Slating

Some natural slates, especially Cumbria or Lakeland green, are supplied and laid in random sizes, either uniform lengths in varying widths, or random lengths in varying widths. In the latter case slates are sorted to provide slating of random width in diminishing courses from eaves to top courses.

Great care should be taken to ensure that each slate has sufficient lap and bond. A fair bond will be obtained by using alternately wide and narrow slates in such a way that the joint between the slates occurs as nearly as possible over the centre of the slate in the course below. Before the battens are set the slates should be graded and sized according to length, and the battening gauge properly calculated.