In this blog we will look at the signs that one can look for
that distinguish the age of a lead roof. I have come across many lead roofs,
and the most interesting roofs have not had any timber filled lead rolls. Many
years ago I assumed that that lead roofs had always had timber rolls. However
as time has gone by I have realised that this is not the case, and I see many
lead roofs, most of which are 20th century. It is the roofs I have
come across which have not had timber rolls which have posed the most
interesting questions.
A roof which has no timber roll (the
felt is a temporary covering due to lead thieves who stole the lower section of
the roof).
From previous research I have done it would appear that the
timber roll was introduced into the lead roof in the mid to late 19th
Century. The most common detail which followed these was not terribly
dissimilar to the detail we see today. On some roofs we find a date or other
interesting craftsmen’s mark has been left on the leadwork. When there is no
mark, other, more subtle methods need to be employed to establish the date.
It is at this point I must discuss leadwork in general. The
Lead Sheet Association (LSA) provide guidance on all things regarding lead and
have a very useful and helpful advice service on the phone and in their
handbook, mostly for milled lead. For the readers of this who are new to leadwork,
there are some things you need to understand. There are three kinds of lead:
firstly, sand cast lead- this is the mediaeval lead which is still be made
today, a sand bed is made and the molten lead is poured over it by highly skilled
craftsmen, the thickness, size and shape of these castings can vary as
required, while maintaining a very high quality control. This smelting of the
lead allows larger projects, such as cathedrals, to re-cast the original lead
into new sheets, which in conservation terms is brilliant.
The most common type which most people get exposed to is
milled lead. This comes in categorised thicknesses, Code 3 (Green),4 (blue),5
(red),6 (black),7 (White) & 8 (orange). The lead sheet association provides
coloured strips to the lead rolls which are sold and this enables anyone to see
the thickness of the rolls in a simple way. The LSA advise the use of underlays
between the lead and the timber decking, and the type of underlay depends on
the substrate upon which the lead is laid over.
There is a third type of lead, and this is machine cast lead. This
can also be called Rolled lead. Machine cast lead
is produced by dipping a rotating water-cooled drum into a bath of molten lead.
The lead sheet is formed on the immersed section of the drum and peels off at
the top. It is then wound onto spools, before being cut. While
the manufacturing process for this type of lead may be improving, this type of
lead should be avoided; the process can create a variation of the consistent
thickness of the lead work which is produces. For these reasons rolled cast
lead should never be used or accepted as an alternative to either sand cast of
milled.
Anyway, back to point of this blog! It would appear that the
methods of laying a lead roof prior to the use of timber rolls was a standing
seam detail. This was fairly straight forward, however it seems to have been
most effective on roofs with steep gradients These were usually nailed at the
top of the lead sheet only, and this allows for the expansion and contraction
of each individual lead sheet (The fixing details of lead sheeting is very
important and lead should not be over fixed otherwise expansion will make the
sheet fail).
It would appear that the standing seam and the hollow lead
roll came before the timber roll or “solid roll”. Some documents talk about the
timber roll roofs being more resilient to pedestrian access and standing seams
or hollow rolls being thought likely to be flattened by people walking on the
roof. Now, to me this sounds like a fairly weak argument as it’s not difficult
to step over the up-stand of a lead roof, but this is what is described in the
available text.
Photo of a hollow roll lead roof on a
church porch.
The 17th and the 18th century lead
roofs generally had solid boarded roofs beneath, however some also had timber
battened roofs. I recently had a project which had these roof battens and this
was very interesting as it provided the lead with good ventilation on its
underside. I have seen these roofs before with battens where the lead was nailed
at the top of the sheet on the return face of the batten, this allowed the lead
sheet to hang down the roof. In the example which I came across the other day
the lead sheet was not fixed in any way to the roof. Instead the lead sheets
were laid at around 40 degrees, and after looking closer at the roof it would
appear that the use of this angle held all the sheets in place, with only a few
areas where the lead was beginning to move/slide downwards. The lead sheets
were jointed together with a tight hollow roll, which in some areas appeared
more like a standing seam than a roll. When you viewed the lead roof from the
church tower above it was easy to see how the lead had slumped between the battens
and this creates slight shadows of the battens on the roof slopes.
Unfortunately as yet I have not yet come across an unaltered
medieval lead roof as the Victorians seem to have been through most of the
buildings in this country. The roof discussed above, which was laid diagonally
was laid requiring no fixings, I believe, is the oldest lead roof I have come
across, this has been laid without the use of machined timber rolls and as it
is more than conceivable that the early lead roofs were not nailed down, both
of these things could be the sign of an early date. The timber roof structure
is 16th century with some 19th century insertions and
repairs, the roof may be 19th century, with the roofer doing things
his way!
I hope you have found the above interesting, if you have
come across a similar lead roof which has not been fixed down please get in
touch, I would be interested to hear about it.
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