| Wall tie corrosion is affecting many properties
in the UK, especially in coastal and exposed locations. The
mild steel or galvanised wire ties embedded into the mortar
eventually start to corrode. In its advanced stages, the corrosion
will cause horizontal cracking and eventually loss of stability
between the two skins of brickwork.
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This photograph is taken using the Allen
Inspector Scope attached to a Sony Mavica camera. Ideal
for inclusion within Structural Engineers reports. |
The design of cavity walls invariably incorporates
the use of metal wall ties, and this system has been in use
since the beginning of the nineteenth century, to prevent
rain penetration and to improve thermal insulation. Cavity
Wall-ties are an integral support in walls constructed with
a cavity between the inner (load bearing skin of masonry),
and the outer (weather shield), skin. The outer skin is normally
very thin in relation to its height, and without support,
would be subject to damage by extreme weather conditions.
Research by the BRE (Building Research Establishment), has
found that wall ties corrode faster within the mortar joints
of the external leaf of brickwork, where rust builds up around
the ‘fish tail’ ends of the ties. This expansion
will cause blistering of the mortar joints (at tie courses)
creating consequential structural instability
Over recent years, it has become apparent that a high number
of properties, built prior to the early 1980s are suffering
from cavity wall tie failure, due to corrosion of the metal.
This is principally due to poor quality galvanising standards
prior to that time. There is also evidence that many properties
have been constructed with a lack of density of the original
cavity wall tie system, particularly around reveals. Visual
evidence of corrosion can be seen by horizontal cracking to
the mortar joints of external wall surfaces, or hairline cracking
to external render. If corrosion is severe, or neglected for
an extended period, this may lead to bulging of the wall.
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Horizontal cracking of the mortar joint |
However, it is important to note that a property does not
need to be in excess of 100 years old to require remedial
wall tie work! Over the past few years, we have experienced
the need to install a supplementary wall tie system when insufficient
ties have been found in relatively ‘new build’
property, only discovered when changing the window casings!
The location of a property will have a significant bearing
on the state of corrosion, and this will inevitably vary around
the country. For example, on the south cost of Kent and Sussex,
exposed southern facing elevations deteriorate quicker than
the remainder of the property, requiring only partial replacement
(but to the complete affected elevation). The opposite may
be the case in the North West and North East of the country.
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A wall tie in the flesh. |
WALL TIE REPLACEMENT SPECIFICATION
The following specification is the our specification for full
wall tie replacement. Unless stated otherwise in our inspection
reports, allowance will have been made in the specification
for the following works:
• Erection and dismantling of any ladder access necessary
to complete works safely. Any requirement for additional access,
such as scaffolding or towers, will be at additional cost.
• Location of the existing cavity wall ties irons using
a specialist metal detector.
• Installation of stainless steel wall ties spaced and
positioned in accordance with current BRE329/401 recommendations.
• Random ‘pull out’ testing of several new
ties using a hydraulic tension tester, in order to ensure
that a correct fixing has been achieved, and that the most
suitable model of tie is being used.
• Where necessary, the structural isolation of remaining
ties over affected areas, in order to ensure no further damage
can occur as a result of corrosion. (This method of immobilizing
of the original ties is dependent on the condition of any
samples which are first removed and inspected).
• Structural isolation of the original wall tie system
is not always necessary. Where wire (butterfly) ties have
been used, isolation would not normally be required.
• Occasionally full removal of the original system is
necessary, particularly where internal cracking has resulted
from corrosion. This is rare and will usually be the result
of a Structural Engineers recommendation.
• The making good of any brickwork, render or mortar
disturbed during the installation process. This does NOT allow
for specialist types of render finish, pebbledash or plastic
coatings. External redecoration will NOT have been allowed
for. We will NOT accept responsibility for disturbing pre
existing faulty or blown render.
• Removal of any debris created by the works, leaving
the adjacent site or garden in a clean and tidy condition.
IMMOBILISATION (ISOLATION):
Apart from the installation of new stainless steel wall ties,
the most important aspect in wall tie replacement is the correct
immobilization of the original cavity wall ties.
When cavity wall ties corrode they expand, this expansion
causes characteristic horizontal cracking which appears in
external walls, at ‘tie courses’. This expansion
can eventually lead to serious damage occurring. This may
then require extensive rebuilding. In view of this it is essential
that the original ties are properly immobilized by structural
isolation (see para 5 above).
Wall tie replacement is a structural repair. We employ our
own trained installation technicians and do not employ ‘labour
only’ contractors to do wall tie work. During the course
of repairs the interests of property owners are protected
by the maintenance of a £5,000,000 public liability
insurance.
GUARANTEE:
We are registered installers and a member of the Wall Tie
Installers Federation (WTIF) and are approved to issue WTIF
Insured Warranties on completed works. The Warranty is valid
for 25 years and is transferable to any new owner of the property.
For avoidance of doubt the obligation to indemnify extends
to all damages, costs expenses and liabilities arising as
a result of any financial loss incurred by our customers pursuant
to the wall ties and workmanship failing to perform their
intended function.
Some banks and building societies prefer to have this sort
of structural repair work supervised and the completed workmanship
certified by Consulting Engineers or Chartered Building Surveyors.
We can arrange for works to be independently supervised and
the completed workmanship certified, however independent supervision
does involve additional costs in line with the engineer’s
fees.
THE NEW TIES:
We only use Remedial Ties from corrosion resistant ‘18/18’
stainless steel. Our technicians use sensitive hydraulic ‘pull
out’ equipment to test every job done, thus ensuring
complete quality control. No one make or model of wall tie
is right for all buildings. We are not connected or ‘tied’
to any manufacturer of wall ties, and will install the tie
most suitable for the property. Random testing is carried
out using the Hilti ’Pull Test’ kit to 1.5kn (or
to a strength specified by a structural engineer, which would
normally be 1.5kn).
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