thickness of stone slates varies between geological sources and from quarry
to quarry. The loading on a roof is also dependent on the range of sizes
of a set of slates and the head lap used to lay the roof
slates in common with other random slates are normally sold by weight with
an indication of the area which can be typically covered by a given weight
assuming a given head-lap - normally 75mm. To anyone unfamiliar with the
system this is a confusing and cumbersome method of deciding how much slate
they need. It is also a long standing problem. In 1695 the anonymous author
of The Art of Building or an Introduction
to Young Surveyors in Common Structures complained that 'their proportions
in covering houses are uncertain, so I cannot assign a certain number,
unless the builders shall prescribe a true proportion for the Slate'.
is difficult to be precise about coverage because there are many imponderables.
Firstly the thickness of stone slates varies considerably. Then there is
the mismatch between the number of slates of each length in a consignment
and the width of the building. It is the roofers responsibility to make
the mix of slates fit the shape of the roof but inevitably there will be
too many slates of some lengths to complete a whole number of courses exactly
with a consequent waste of slate and a reduction of coverage. Further,
for technical reasons, the head lap must be altered whenever there is a
change of slate length but the extent by which this reduces the coverage
cannot be estimated without knowing how many courses there will be of each
S et al 2003 English Heritage Transactions 9 p17.