Civil Engineering Reference
In-Depth Information
separating walls (for example
chimney breasts or internal
loadbearing walls). Excessively thin
walls have been found where half
brick (112 mm) brickwork rises two,
three or even four storeys with little
restraint.
the interior. There is further
discussion of this problem in
Roofs
and roofing
(24)
.
Rising damp in separating walls
will generally prompt the same
technical considerations as for other
internal walls, but additional
problems may be encountered
because the wall usually separates
dwellings in different ownership and
because floor levels are often stepped
either side of the wall. Where access
is available to one side only of a wall,
the provision of a fully effective DPC
may be difficult to achieve. Where
there is a stepped wall between
dwellings, the wall on the lower side
may need to be considered as if it is
part basement.
Where rising damp has been
diagnosed, the possibility should be
considered that the external wall DPC
is being bridged by a solidly bonded
separating wall.
Older dwellings will often not have
any DPC within separating walls.
Where solid floors are used, the floor
construction may cause bridging of
any DPC which has been provided in
the original construction. Where
steps occur at ground level,
a vertical dampproof membrane
will often not have been provided.
Injection type DPCs are not able to
deal with step conditions and a
vertical physical barrier must
therefore be provided. Where rising
damp exists, rot may be found in
timber in contact with the wall - in
skirtings and stair strings, for
example.
BRE investigators on site have
several times seen separating walls
become external walls when the
adjoining house has been
demolished. It is not easy to provide
for an acceptable degree of
weathertightness without cladding
the whole wall.
their stability upon other parts of the
structure, such as timber floors or
roofs which are not so resistant to
fire.
So far as solid walls of masonry are
concerned, their fire resistance
increases rather more than directly in
proportion to thickness. Hence an
unplastered half brick wall should
give one hour, whereas a one brick
wall should give well in excess of two
hours - indeed, up to six hours.
Sanded plasters give only marginal
improvements, but lightweight
plasters offer considerable
improvement - for example
increasing the resistance of half-brick
walls to six hours. While such a
separating wall might satisfy the fire
criterion, however, it would not
satisfy the sound insulation criterion.
Reinforced concrete separating
walls at least 120 mm thick, with at
least 25 mm of cover to any
reinforcement, should give at least
one hour's fire resistance.
Where a building contains several
dwellings, each must be contained
within fire resisting compartment
walls and floors. When converting a
large property into a number of
smaller dwellings it is important to
ensure that the compartment walls,
together with any supporting
structure, will be able to meet the
period of fire resistance required.
Only limited account should be taken
of old lath and plaster in assessing the
fire resistance of a wall.
A BRE survey of house design and
construction revealed that fire-
stopping at the top of separating walls
is rarely effectively done. If the gap is
not properly fire-stopped, fire can
spread from one dwelling to the next
(Figure 6.6). There is further
discussion of this problem in
Roofs
and roofing
. See also BRE Defect
Action Sheet 8
(202)
.
Internal walls used for
compartmentation (or perhaps earlier
conversion) may not provide the
required fire resistance because the
supporting structure will fail before
the wall. Sealing of all cavities which
bypass the wall (eg floor and roof
voids and around pipes in ducts) is
often overlooked.
Standard fire resistance tests on
Dimensional stability, deflections
etc
Dimensional stability is not often
considered since the separating wall
is covered by the roof. However, see
the case study on page 213.
For coefficients of linear thermal
and moisture expansion etc, see
Chapter 1.2.
Dampness and condensation
Problems may be found in large
numbers of terraced houses where
local byelaws required that
separating walls were built to project
through the roofs. Many houses with
this feature can now be found
'flashed' with cement fillets or
flaunching. There is no realistic
substitute for a metal flashing over a
secret gutter, or soakers, in this
situation. If, however, a cement
flaunching is preferred, it should
include a gritty fine aggregate with
some lime in the mix. In addition, the
DPC may have become ineffective.
Where a pitched roof abuts a
separating wall in a stepped-and-
staggered situation, part of the roof
becomes an external gable, and the
outer leaf of masonry becomes an
inner leaf below roof level. Particular
care must be taken to ensure that
rainwater is prevented from reaching
Fire
Soundly constructed masonry walls
are usually able to provide fire
resistance for periods longer than
required by building regulations. This
inherent resistance to fire is easily
compromised, however, if any
unsealed holes or gaps are left
through a wall. Walls may rely for
Figure 6.6
Lack of fire resistance at boxed eaves
