What is the appropriate size of
natural slate to be specified, and what do we know about the range of
slate sizes available?
If you were to collect together all of the different sizes of slates
that have been available since 1750, you would find that the list would
include 46 imperial sizes, and 33 metric sizes for standard slates, with
additional double and slate and a half width slate, and eaves/top length
slates for each size, some of which were the same as other standard size
If we were to go back to the beginning of the 1700s in
North Wales, the extraction of slates for roofing was very disorganised
with roofing slates having no defined shape and extracted to the biggest
size possible. These were termed rag slates, and each and every one was
a different size. To make things a little easier one or two sides of the
slate may have been dressed, but were still left as big as possible.
In 1750, General Warburton of Penrhyn Quarry introduced
a table of eight rectangular sizes. Each size was named after a female
aristocratic title, starting with Queen for a large size and reducing to
a Lady for a smaller size. This classification was adopted by other
quarries and so began the sizing of slates.
Fairly soon additional sizes were introduced and
further names were added like Small Empress and Broad Ladies, through to
Singles, until there were 25 different named sizes. However some of the
names covered a range of sizes.
With the introduction of steam powered machinery, saws
and dressing machines, the production and transportation of the slates
began to expand from a cottage industry to one of world wide proportions
and the standardisation of slate sizes helped with the process.
By 1933 there were over 40 different slate sizes, and it was decided by
the British Standards Institute to dispense with the aristocratic names
and adopt an imperial inch size from 36 x 20- inch down to 10 x 6-inch.
Regardless of the dispensing of names by British Standards, the names
have lived on, even if they are not officially recognised, and quarries
continued to supply this vast range of sizes. Not every size is
available from every quarry and larger sizes are often not available in
large quantities due to them being more difficult to produce than
smaller sizes. Even diminishing and random slates can be supplied, which
reflects the origins of slating, with every slate being a different
Meanwhile, in other parts of Europe the use of slate for roofing
developed copying the Welsh model but to metric sizes close to the
imperial sizes. As slates began to be imported into the UK from Spain,
so a further range of 33 slate sizes from 600 x 450mm down to 200 x
200mm increased the range of sizes to 79 standard roofing slate sizes
theoretically available, plus slate and a halfs and eaves/tops, raising
that number to above 100 different sizes available in the UK. Many of
these sizes are not held in stock and are only available to order,
therefore need to be ordered well in advance of delivery on site,
especially from small quarries outside the UK. Meanwhile, slate quarries
continue to supply slates in shapes other than rectangular to satisfy
many local traditions, such as German fish scale slates.
If you look at the table of slate sizes you will see that for any given
slate length, the smallest width is not less than half the length, and
the widths increase from 150mm (6-inch) in 20- 50mm (1-2-inch) steps up
to 20-50mm (1-2- inch) less than the length of the slate, and then jumps
to a square slate.
There is one exception: a 20 x 9-inch
slate, which, along with other 9- inch wide slates, was used for damp
proof courses in brick walls. The slate lengths are m a x i m u m 920mm
(36 inch) reducing in 50mm (2 inch) increments down to 200mm long.
With such a wide choice of slate sizes what is the correct slate size to
choose? There are many different parameters available that relate to the
physical size of the roof and the pitch of the roof, but it is the
following rules of thumb that are most commonly used:
- Use a larger slate on a large roof
and a smaller slate on a small roof. Big slates on a small roof look
out of proportion.
- Wider slates perform better than
narrower slates in areas of high rainfall as the distance from the
side lap to the nail hole is further.
- Smaller slates perform better in
very windy locations as there are more nail fixings per square metre
- Large slates cost more per square
metre than medium size slates from the same quarry.
- Small slates require more labour to
install on the roof.
- Small slates are not suitable for
shallow pitch roofs.
- Small slates are easier to install
on a complicated roof with lots of hips, valleys, dormers, and other
- Compare the cost of one square metre
of slate, as opposed to the cost per 1,000 slates. Depending upon
the width of slate and the gauge they are laid at, so the quantity
- Is there an adjacent roof that the
new roof must match, or join into? And what size are the existing
- What is readily available?
It will be impossible to comply with each of the 10 parameters but,
provided the slate size meets a high proportion of the parameters –
especially those with greater importance – so an appropriate slate size
will be determined.
- Wider and longer slates can be
trimmed down, smaller slates cannot be stretched.
- The cost of slate and a halves for
large slates can be three or four times the price of a standard
- Wider slates can be used down to
lower pitches for a given length.
- Use diminishing slates sizes for
curved roofs such as cones.