It is at this stage which any adjustments caused by the sulphur dioxide tests are applied to the basic thickness. Manufacturers must next guarantee that no slate will be thinner than this value. Because slates are sold on the basis of nominal thickness; if the range variation are great the moderate thickness are much larger than the minimum. If thevariation is small subsequently they will be much nearer. This procedure ensures that slates will be technically satisfactory but there is an alternative route in which it is important for conservation grounds to make use of denser slates. These may be specified based on traditional construction methods. Obviously it would be unwise to use this alternative to specify thinner slates than required to resist the weather loads.