Page 52 - The Building Conservation Directory 2013

5 2 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
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Professional servicesMasonry Structures
Structural investigation and assessment
Ian Hume
There are essentially two approaches
to the investigation and assessment of
masonry structures. The first involves
basic, low-tech methods, while the second
uses much more sophisticated, and therefore
more expensive, techniques. This article
looks mainly at the less expensive methods
available. It also looks briefly at the questions
raised by investigative work and where to go
for further help.
The aim of investigation
Appropriate investigation and assessment
will help ensure that repairs to historic
masonry are carried out where necessary and
that unnecessary work is not undertaken,
minimising the loss of original fabric and
ensuring that money is wisely spent. Not
everything is as bad as it sometimes seems but,
conversely, structures which might initially
seem sound can have hidden problems.
Considering the condition of individual
stones or bricks to estimate their remaining
lifespan is a fairly straightforward part of
masonry investigation and assessment. The
condition of the small elements that make
up a masonry wall rarely affects its overall
condition. Figure 2, which show stones at
Dunstanburgh Castle in Northumberland,
illustrates this point. The much decayed stone
(figure 2, opposite) cannot have much life left
in it, but it could probably erode away entirely
without threatening the stability of the wall.
However, if a whole series of stones decays
then problems will arise, which is why the
repair to the right was carried out.
The same argument does not apply
where loads are greater. In a column,
for example, the serious decay of one
stone can affect the stability of the whole
column (figure 3), with potentially serious
consequences for the entire building.
Simple investigative techniques
The basic techniques described below are used
for the initial investigation and assessment
of most masonry structures. Further, more
sophisticated assessment techniques may
also be useful, but when the assessment is of
a local church, a vernacular house in private
ownership or a small commercial property,
the funds available may only be sufficient for
the basic techniques.
➊ Historic research
Assessing the date of the structure is a
sensible place to start as this will give clues
to the likely construction of the masonry. For
example, medieval masonry is likely to be very
thick but may consist of two well constructed
skins of cut masonry or random rubble with a
rubble and mortar core. The wall core might
be well built but, to quote the Roman architect
Marcus Vitruvius Pollio, it is more likely to
include ‘a lot of broken stone and mortar
thrown in anyhow’.
Georgian and Victorian walls may not
be as well constructed as they might at first
appear because they may contain ‘snap
headers’ (bricks with the short face visible as
if bonding inner and outer leafs, but which
are in fact half-bricks). This was a cost-cutting
measure which enabled cheaper bricks to
be used for the whole of the inner leaf, at
the expense of reducing the effectiveness of
bonding between the face and the main body
of the masonry. If the building is a substantial
one built around 1800 it may contain bonding
timbers, which are prone to decay.
➋ Visual inspection
This will involve looking for cracks, signs of
settlement, leans and other distortions as well
as inspecting the condition of the individual
stones that make up the walls or columns. It is
important to look at and along the face of the
wall seeking out signs of bowing or bulging.
This may indicate movement of the body of
Figure 1: Roof thrust or foundation problems? Investigation is required.