106 / 208
1 0 4
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 5
T W E N T Y S E C O N D E D I T I O N
3.2
STRUCTURE & FABR I C :
MASONRY
so moisture inside the building condenses on
the cold surface, adding to the problem.
Externally, saturated walls are also ideal
locations for plant growth and the roots can
cause lifting of bricks and more water ingress.
Over-flowing gutters can wash out mortar
joints, also allowing more water ingress. In
cavity walls with old wrought or cast iron
cavity ties, the development of rust on the
ironwork can lead to jacking up of bricks (see
Structural repairs below).
Freeze-thaw cycles in winter can also
blow the face off any bricks which remain
wet, as the ice expands within the pores.
Furthermore, being wet for long periods can
lead to salts being pulled to the surface of the
wall where the water evaporates, leaving the
powdery bloom of crystals called efflorescence
Salt might be being drawn from the brick or
the mortar or the source might be road salt
splashing against the wall. In coastal areas sea
spray can be another source.
Where crystallisation occurs within
the pores just under the surface of the brick
(‘cryptoflorescence’) you often see a thin skin
of brick bubbling up, often without much
obvious salt staining. This is due to rapid
drying of less soluble salt crystals in the pores
of the brick which cause the surface layer to
deform. This superficial layer can be quite
hard and brittle when dry.
Some salts are also hydroscopic, drawing
water into their crystals and so allowing freeze
thaw spalling of bricks to occur.
Salt migration can be very difficult to
eradicate but encouraging better evaporation
through the mortar joints and promoting
drying out of bricks will help. Broken gutters
and downspouts need to be fixed, and where
road salt is an issue in winter months, any
puddles which collect in the road should be
drained away where possible.
Pointing
Keeping walls well pointed is important.
However, old lime pointing often looks
in worse condition than it is, and is often
replaced unnecessarily. If in some areas it has
weathered back by more than the height of the
bed joint, then it might be right to consider
local or complete repointing.
Samples of mortar should be taken from
deep inside a wall to identify the original
materials. As historic walls may have been
repointed many times in their life, what is
now seen on the face is not always correct. For
preference, the mortar should be matched by
using the same aggregates, which may include
local sand, crushed brick and other material.
If these are no longer available it is best to
achieve a close match without using artificial
pigments which can fade over time.
Old mortar should never be cut out using
a rotary disc cutter as this can easily cut the
bricks and so spoil them for all time, as well
as widening the mortar joint. Chisels and
bolsters should be used with care, and very
tight joints might have to be cut out with a
hacksaw blade. However, where brickwork has
been pointed with a hard cement-rich mortar,
the judicious use of a specialised mortar
cutter (Arbortech or Fein for example) may be
justified, provided the operator is highly skilled.
Portland cement-rich mortars should never
be used to point historic brickwork as these
are far too hard and impervious, forcing water
to evaporate through the brick. Furthermore,
pointing is always shallow; the rigid grip of the
cement is confined to a depth of just half an
inch of brick, so any movement in the wall can
cause the face of the brick to fail.
Lime can be either lime putty or a
hydraulic lime NHL 2 or 3.5, or in the most
exposed locations, NHL 5. (The letters stand for
‘natural hydraulic lime’ and the figures relate to
the material’s compressive strength in N/mm
2
at 28 days.) With soft or damaged bricks NHL
2 might be considered appropriate, while NHL
3.5 or 5 should be used in areas that are very
exposed and wet or with harder brick types.
Hydraulic lime can be mixed with sand and
other aggregates in the ratio of 1:2:5.
Hydraulic, in this context, refers to the
reaction of the lime with water to achieve at
least a partial set. This develops within hours
and the mortar then continues to harden once
it has dried out through carbonation, a reaction
between the lime and carbon dioxide in the air.
Lime putty, on the other hand, is non-hydraulic;
it sets by carbonation alone. This is a much
slower process, and a non-hydraulic lime mortar
is vulnerable to wind and rain in its early stages,
so it needs to be protected for longer.
Lime putty is even softer and more
permeable than NHL lime mortar. This is
regularly mixed with a pozzolanic material
to give the mortar properties similar to those
of a natural hydraulic lime, including initial
setting properties and greater strength, as
well as reduced permeability. The grain size
and shape of the sand and other aggregates
also controls the appearance and strength of
mortars. This can be mixed in proportions of
say 1:3:9 pozzolan : lime putty : coarse sand.
Structural repairs
The repair philosophy is to retain as much
historic material as possible. Once lost, original
material is gone for good, so cutting out old
perished single bricks and even turning bricks
and reusing them back-to-front can be a good
idea rather than renewing bricks. This is easier
to achieve with harder mass-produced bricks.
Structural problems are not uncommon.
Apart from settlement or subsidence which
might need more aggressive repairs, another
common cause of brick failure is deeply
embedded ironwork, rusting and jacking up
bricks. Ferrous metals can expand by up to
seven times their original width when they rust,
exerting a force greater than the compressive
strength of the brick, stone or terracotta that
might surround them. Hidden structural iron
or steelwork such as lintels and tie bars, as
well more visible components such as metal
window frames can all cause problems, often
resulting in long vertical or horizontal cracks
as they corrode. Where a wall contains rows of
early ferrous cavity ties, brickwork between the
rows sometimes bows with the jacking effect.
Repairs will require opening up the structure
to clean and protect the embedded metalwork
from corrosion or remove it altogether.
The use of black ash mortar derived
from colliery waste, particularly
across the north of England, from the
Victorian period through to the 1920s has
contributed to ferrous metals corrosion.
The building of lengths of timber into
brickwork, common in the 17th and 18th
centuries in particular, can lead to movement
as the timber rots out. In larger houses,
interiors were often lined out in timber framing
with lath and plaster finishes. This can make it
difficult to find the timbers that were built in.
Rotting lintels over window and door openings
can also be a local problem.
Sulphate crystallisation can bring problems
to chimney stacks and concrete ground floors.
Introduced into flues from burning coal, the
salts migrate into the brickwork and crystallise
in the mortar, particularly on the side least
exposed to the prevailing wind and rain. The
lop-sided expansion causes the chimney to
lean. When this becomes excessive, the stack
will need to be rebuilt.
In wall bases, sulphate salts are sometimes
drawn from colliery shale, a material that was
often used after the war as hard-core beneath
concrete floors in areas near coalfields. A floor
re-laid in an old kitchen in the 1940s to the
1960s to replace flags or bricks, is often the
site of this problem. Salt migration into the
slab causes the concrete to radically expand,
bowing concrete floors and even pushing the
Frost damage to saturated bricks: the cement
pointing will not have helped.
Surface loss in well pointed and well maintained
brickwork: in this case the problem may simply be that
the brick was underfired and is susceptible to decay.