105 / 208
T W E N T Y S E C O N D E D I T I O N
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
1 0 3
3.2
STRUCTURE & FABR I C :
MASONRY
porous. Originally it was probably made from
lime putty mixed with coarse sand and other
aggregates, but in many parts of the country
the lime was impure enough to be more
‘hydraulic’ in nature.
Lime mortar remained in use until the
beginning of the 20th century. Whether
hydraulic or not, these traditional lime
mortars allowed some movement in the
brickwork without showing signs of cracking
under normal seasonal conditions.
Solid brick walls in domestic two-storey
buildings tend to be 9–13½ inches thick, which
equates to a depth of 1–1½ brick lengths. In
taller buildings base brickwork might be
18 inches thick or more, reducing in thickness
as you rise up the building. In the absence of
a cavity, solid walls simply rely on the mass
of the wall to keep moisture out, a principle
that works well as long as the wall is kept in
good condition. Rain would soak part way
into the structure but would then evaporate
away again. Walls constructed of porous brick
and lime mortar and which are plastered with
lime plasters are said to be ‘breathable’, as are
brick or flag floors. In the winter months, the
combination of large open fires and draughts
from ill-fitting windows and doors kept a flow
of warmed air running through a building
and so enhanced the evaporation of moisture
from walls and floors. The end result was a dry
building with little sign of damp internally. It
worked well, particularly when you consider
that damp-proof courses in wall bases and
under-floor membranes were only common
from the mid-19th century.
The brick bond, seen in the wall as its
horizontal pattern, can help in identifying
the construction of the wall. In a solid brick
wall of two leaves, header bricks are those laid
at right angles to the face of the wall to bond
inner and outer leaves, while stretchers are
those laid parallel with the face. In English
bond the courses alternate between headers
and stretchers. In English garden wall bond,
every fourth course or so is of headers, while
in Flemish bond stretchers and headers
alternate in the same course, and there are
many variations on these themes. The type of
bond used was often a trade-off between cost
and strength.
The appearance of header bricks can help
to distinguish solid brickwork from cavity
walls, which first began to appear in the mid-
19th century and became common in the early
20th. The outer leaf of the cavity wall is often
a half-brick thick, so it cannot have headers.
However, cavity walls of 15 or 15½ inches are
quite common, and look for all the world as
though they are solid. These can have a 9 inch
outer skin and an inner skin of 4½ inches
separated by a two-inch cavity. Some early
examples of cavity construction in the mid- to
late-19th century also used specially made
bricks, not iron wall ties, to join the two leaves
together, which appear as headers in the facade.
Just to make life even more difficult,
Flemish bond brickwork is sometimes found
where ‘snapped headers’ have been used. These
are half-length bricks rather than full length,
allowing a 4½ inch outer leaf to masquerade
as a solid wall. Snapped headers were also
sometimes used in solid walls, leaving the
outer skin poorly tied into its inner leaf.
Buildings can also deliver unexpected
problems. Larger buildings and terraces from
the late-17th to the mid-18th century saw some
poor building practices. Sometimes a high
quality brick outer skin was built by a different
team from the rest of the structure and was
inadequately tied into a poorer quality inner
skin. Despite the evident thickness of the wall
and the appearance of well-bonded brickwork,
the outer leaf is liable to move outwards
away from the inner. Frequently, wall plates
and so roof weights were supported off this
poor quality inner leaf. Such problems can be
expensive to put right.
The best bricks tended to be used for the
front of a building but as you move towards the
back, more often ‘commons’ or ‘stocks’ were
used, which were cheaper but not as regular
in shape. Internal bricks were usually of the
poorest quality and often under-fired as they
did not have to withstand the elements.
In the north of England there are tens of
thousands of houses with 9-inch thick walls,
usually at the rear, constructed with common
brick in English garden wall bond. These walls
were cheap to build, with header bricks every
five to seven courses to hold the wall together,
and the stretcher courses were not always fully
mortared. They must therefore be kept well
pointed to reduce the risk of water ingress.
In addition to solid wall construction,
an early use of brick was for infilling timber
frames. In houses, the frame and infill was
usually plastered on the inside and sometimes
rendered on the outside too, to draught-
proof the construction. Thinner walls in old
buildings, say 8 inches thick, that feel solid
when tapped, might well indicate a brick
infill to a timber frame. The thickness here is
controlled by the size of the timber posts and
beams used in construction, which remain
structural in performance.
In view of the wide variety of brick
construction methods, once the wall thickness
has been measured and the brick bond
inspected for clues to its construction, try
to get a view into the core of the wall from
around window reveals, or perhaps where a
gas meter has been built into an outer wall.
Only when you have fully understood what
you are dealing with can you start to consider
an appropriate repair.
BRICKWORK PROBLEMS
Problems with brickwork can be categorised as:
• inherent defects such as inadequate
firing, poor design or bad craftsmanship
• aging defects such as weathering
and settlement
• maintenance defects such as open
joints, plant growth in masonry and
saturation from leaking gutters.
Water plays a significant part in many of the
most common problems found in brick walls.
All brick is porous to some degree. Where
brickwork is well pointed, most rainwater is
shed from the relatively uniform surface, and
any moisture absorbed by the brick or the
mortar quickly evaporates. However, leaking
gutters and downspouts can lead to saturation,
causing major problems with any wall, brick
or otherwise. Constant water running down
walls will soak through most thicknesses of
brick wall, eventually leading to the decay
of any timbers it supports. Dry brick acts as
insulation, but saturated brick conducts heat,
Detail of late 18th century English bond brickwork
with five courses of stretchers between header courses
Late 18th century terraced houses in Welbeck Street,
London: the ground floor and basement are stucco-
rendered, while the brickwork above is London stock
in Flemish bond.
Detail of late 19th century Flemish bond brickwork in
St Michael’s Street, Shrewsbury, with light-coloured
headers used to decorative effect