Page 108 - The Building Conservation Directory 2013

1 0 8 t h e b u i l d i n g c o n s e r vat i o n d i r e c t o r y 2 0 1 3 T w e n t i e t h a N N i v e r s a r y e d i t i o n
3.2
Structure & Fabric :
Masonry
the essential requirement was to arrest the
water penetration. It was found that the sky
surfaces of the enormous coping stones were
in perfect condition; even the tooling marks
were as crisp as the day in the 1840s when
they were cut. The water was just getting in
through the open joints between them. Where
the copings had been previously covered with
lead weatherings, these had caused problems
with differential run-off and consequent
algal soiling on the facades (illustrated
above). Where weatherings had not been
used, the wetting and drying capacity of the
exposed copings had moderated and evened
out the run-off onto the facades. It was
therefore decided to reverse this alteration,
keep the sky surfaces exposed and fill the
open joints with molten lead (illustrated
opposite) which would resist thermal
movement, UV degradation and frost action
better than mortar or poly sulphide joints.
There was much debate about whether the
areas of friable stone should be treated with
a stone consolidant such as an alkoxysilane.
However, in this case consolidation was
felt to be unnecessary because the original
worked face had already been lost and, once
the cause of saturation had been rectified, the
porosity and sacrificial nature of a mortar
repair would be more effective at desalinating
the stonework. Had the upper surfaces of
the copings shown signs of delamination or
of becoming more porous, then the use of a
consolidant might have been justified, but
metal cramps need to be removed, is to cut a
square opening across the connected stones,
replace the cramp, and cover with an indented
section of stone. However, this method causes
significant conservation issues, particularly
where many repairs are required, as it can
lead to the loss of the original jointing pattern
over time as original stone sizes are reduced
or rationalised. In some areas of St George’s
Hall (see illustration top right) the approach
had already resulted in the introduction of
more vertical joints, repairs which bridged
the original vertical joint entirely, and
miss-matching of stone colour or weathering
patterns in the indent. The result was a
patchwork appearance, with greater loss of
original material than was necessary to effect
the repairs.
The enormous number of cramp repairs
required for the new repair programme raised
significant concerns both over the supply
of new stone and the visual and structural
prior to conservation fractured piece released to be reinstated
corroded cramp and sound stone behind fracture resulting cramp repair on right hand side
Repairing cramp damage at St George’s Hall
impact of the work. This led the architects to
develop a repair technique that differed from
the normal rectangular indent, retaining the
original joint lines and as much of the original
stone as possible, as illustrated on the left. The
bottom right illustration in the series shows
the final repair on the far right of the view
as well as a more conventional but angular
indent to the centre which was adopted where
the fractured stone was in more pieces and
could not be pinned back in situ once the
cramp been replaced. Also visible in this
image is a small grey rectangle on the corner
of a stone. This is an example of a hydraulic
lime repair which was used for minor edge
damage and to consolidate the areas of decay
around the cornice joints, after first dressing
back the friable stone to a sound surface.
Water ingress
Where decay around the joints of the cornices
required ‘plastic’ repairs with lime mortars,
Algal staining down the facade at St George’s Hall
prior to conservation
Indents gone mad at St George’s Hall prior to conservation