Ascertaining the construction of the external walls of dwellings and their suitability to cope with Categories of
Exposure as defined in BS5628: Part 3: 2001
External material Date of
building
Sound when tapped
Overall
(externally/internally)
thickness (mm)
Likely construction
Suitability to cope
with Exposure
Category
Brickwork in
Pre 1980
Solid
Solid
230–240
bonds other than
stretcher
Solid 1 brick thick wall with plaster Sheltered
internally.
Pre 1980
Solid
Solid
350–360
Solid 1½ brick thick wall with Sheltered
plaster internally.
Pre 1980
Solid
Solid
460–470
Solid 2 brick thick wall with plaster Moderate
internally.
Pre 1980
Solid
Solid
570–580
Solid 2½ brick thick wall with Moderate
plaster internally.
Note: No plain brick
wall of any thickness
and sounding solid
on
both
faces
is
suitable to cope with
exposure
‘Severe’
in
or
the
‘Very
Severe’ categories.
Post
Solid
Hollow
320 upwards
1980
Solid 1 brick wall with insulation on Sheltered
the inside face. Overall thickness
depends on type of insulant, gap
and lining.
Brickwork in any
Late
Hollow
Hollow
170–200
Timber-framed wall, clad
bond; mostly in
1700s
externally with mathematical tiles
header
and early
hung and pointed to look like
1800s
brickwork with lath and plaster
Sheltered
internally.
Late
Solid
Solid
300–310
Solid 1 brick thick wall, clad with
1700s
mathematical tiles bedded in
and early
mortar with plaster internally.
1800s
Sheltered
Late
Solid
Solid
410–420
Solid 1½ brick thick wall clad with
1700s
mathematical tiles bedded on
and early
mortar with plaster internally.
Moderate
1800s
Brickwork in
Pre 1960
Solid
Solid
290–300
stretcher bond
Cavity wall of two leaves of
Moderate
brickwork separated by a 50mm
cavity left unfilled or filled with
injected insulation.
Post
Solid
Solid
275–285
1960
Cavity wall of two leaves – the
Moderate
outer of brick, the inner of
blockwork – separated by a 50mm
cavity left unfilled or filled with
injected insulation.
Post
1980
Solid
Solid
300–330
Cavity wall of two leaves – the
outer of brickwork, the inner of
blockwork – separated by a 50mm
cavity with insulation bats fixed to
the inner leaf.
Moderate
Post
Solid
Hollow
240–300
1940
Cavity wall with brick outer leaf Moderate
attached across a 50mm cavity to a
frame of either timber, steel or
precast reinforced concrete (PRC).
If post 1980, most likely frame of
timber. Roof space check should
determine.
Rendering
Pre 1940
Solid
Solid
260–270
Solid 1 brick thick wall rendered Moderate
externally to improve weather
resistance
and
with
plaster
internally.
Pre 1940
Solid
Solid
370–380 and
Solid 1½ and 2 brick thick walls
480–490
rendered externally to improve
Severe
weather resistance and with
plaster internally.
Pre 1940
Solid
Solid
450 upwards
Solid wall of rubble stonework
Severe
rendered externally to improve
weather resistance and with
plaster internally.
Post
Solid
Hollow
260–320
Cavity wall of two leaves with an
Moderate
1940
outer leaf of cheap bricks or
concrete blocks attached across a
50mm cavity to a frame of either
timber, steel or precast reinforced
concrete (PRC), with a lining
internally of plasterboard or other
boarding. If post 1980, most likely
frame of timber. Roof space check
should determine.
Post
Hollow
Solid
270–280
1990
Solid 1 brick thick wall of cheap Severe
bricks or concrete blocks with
insulation fixed to the outside face
and with plaster internally.
Pre 1980
Solid
Solid
290–310
Solid
dense
construction
concrete,
of
no-fines Severe
rendered
externally and plastered internally.
Only
likely
to
be
found
on
comparatively large local authority
developments.
1950s
Solid
Solid
240–260
”
”
Post
Solid
Solid
190–210
”
Moderate
Solid
Solid
265–280
Cavity wall of two leaves – the
Moderate
1960
Stonework in
Post
regular courses
1920
outer leaf of stone, real or
artificial, the inner leaf of brick or
concrete block – separated by a
50mm cavity left unfilled or filled
with an injected insulation.
Post
Solid
Hollow
290–310
1940
Cavity wall of two leaves – the
Moderate
outer leaf of artificial stone – tied
across a 50mm cavity to a frame of
either timber, steel or precast
reinforced concrete (PRC) and with
a lining internally of plasterboard
or other boarding. If post 1980,
most likely frame of timber. Roof
space check should determine.
1840–
1920
Solid
Solid
450–500
Solid wall of stone with a backing Moderate
of brickwork maintaining regular
courses, plastered internally.
Stonework
Any date
Solid
Solid
500 upwards
uncoursed
Solid
wall
of
stonework Moderate
throughout, plastered internally.
Rendering, but
1600–
always above a
1800s
Solid
Solid
350–900
Solid wall of unbaked earth – i.e.
Severe
clay mixed with straw and water –
stone or brick
rendered externally and plastered
base
internally. ‘Cob or ‘clom’ 600–900,
‘Witchert’ 350–450, ‘Clay lump’
390–410, ‘Pisé de terre’ 350–450.
Location usually determines.
Plain tiles, slates,
Pre 1980
Hollow
Hollow
200–300
Wall of timber frame, clad
shingles or
externally with plain clay tiles,
weatherboarding
natural slates or timber shingles,
Severe
usually above ground-floor level,
or timber weatherboarding
throughout or to ground floor only.
Any date
Hollow
Solid
280–390
Solid 1 brick thick wall of cheap
bricks or concrete blocks, clad with
plain tiles (either of clay or
concrete) or slates (either natural,
stone or artificial timber shingles)
Severe
or weatherboarding (either of
timber or plastic). If tiles or slates
used then usually to upper floors
only, leaving ground floor to be
either rendered or
weatherboarded.
Post
1980
Hollow
Hollow
200–300
Wall of timber frame with
weather-resisting cladding of plain
tiles (clay or concrete), slates
(natural, stone or artificial shingles
of any material) or
weatherboarding (timber or
plastic), backed by a moistureresisting layer and plasterboard or
other boarding internally. If tiles or
slates used then usually to upper
floors only, leaving ground floor to
be weatherboarded.
Very severe
Note:
The table, while covering many forms of construction, is by no means exhaustive and others may be encountered, though less frequently. For
example, neither solid walls of half a brick thickness of around 90mm–115mm nor dense concrete about the same are incapable of coping with
any degree of exposure unless rendered, and then only in ‘Sheltered’ positions. In areas of ‘Severe’ or ‘Very Severe’ exposure, cavity walls to
dwellings from the 1920s onwards may be found where the overall thickness is greater by 25mm–115mm on account of an increase in the
thickness of the outer leaf or an increase of the lower amount in the width of the cavity. Impervious external claddings, with sealed joints, of
glass, metal or plastic may be encountered to cope with ‘Very Severe’ conditions.