Twistfix

A Professional Guide to Wall Tie Replacement

When wall ties need replacing, the amount of work necessary to do a proper job in restoring the structural integrity of your house can seem overwhelming. It need not be so and, to help you understand the process better, Twistfix has created a handy 6-part guide that explains everything. Below, you can find out about:

  1. The basics of wall ties, including the reasons wall tie replacement may be required
  2. The signs and symptoms of wall tie failure
  3. What happens during a wall tie corrosion survey
  4. How to install wall ties in existing buildings
  5. The different types of retrofit wall ties
  6. Factors that affect the cost of wall tie replacement

1: Cavity Wall Ties - The Basics

Cavity Wall Tie Replacement

For more than century, cavity walls have been the predominant design choice for houses built in the UK.  The structure comprises two thin walls, an internal leaf and an external leaf, with a space between them.  Steel strips or wires, known as wall ties, cavity ties or brick ties, connect the two walls, allowing them to work cohesively together as a single unit. Adequately tied, a cavity wall is just as sturdy as a solid wall.

The purpose of a cavity wall is to protect the inside of the house from damp, improve its resistance to heat loss, and reduce the transmission of sound.  The steel ties function to secure the protective outer wall to the inner structure, which is typically load-bearing, to transfer lateral loads and accommodate differential movements.

The use of mild steel ties, usually with a sacrificial zinc coating, was customary in house construction up to 1981.  Since then, the dominant choice of material is stainless steel, which offers greater longevity and reduced thermal conduc­tivity.

Wall tie replacement becomes necessary in two situations:

  1. Wall tie corrosion occurs when electroc­he­mical reactions between the iron in steel and moisture, in the presence of oxygen cause the steel to rust. The accumulation of rust weakens and erodes the metal, eventually causing the wall ties to fail.
  2. Shoddy construction can result in a brick tie system that is functionally inadequate. For example, the wall may incorporate an insufficient number of ties or the ties may not too short to suit the width of the cavity such that the embedment depth into the mortar is deficient.

In either case, the solution is the same; install a remedial wall tie replacement system to firmly secure the outer wall to the main structure of the building. Re-tying the walls will maintain stability and allow applied loads to be shared and transferred safely.

2: Symptoms of Wall Tie Failure

Cracks due to wall tie corrosion

The different compositions of buildings, including the type of wall tie,  mean that visible indications of problems with wall ties can vary. The two main signs to look out for are:

  1. A pattern of horizontal cracks at regular intervals often suggests an issue with advanced wall tie corrosion.
  2. Bowing walls or undulating external wall surfaces are tell-tale signs that the outer wall has disconnected from the internal wall and thus is free-standing and not supported by the main structure of the building.

Rust occupies a signi­fic­antly higher volume of space than the original metal. As corrosion develops, the accumulation of brittle, bulky iron oxide layers causes the tie to expand such that it becomes several times the thickness of the original steel. When repeated along a whole row of steel strip ties, this expansion can create irresistible forces that combine to lift and split the wall, causing a series of horizontal cracks. These wall-weakening fractures occur along the mortar bed joints incor­porating the rusting ties; typically, positioned 4-8 brick courses apart

Ties made from thin wire, instead of steel strip, usually lack the mass to produce these symptomatic cracks and so can ultimately erode with no visual signs of a problem, until the wall stars to buckle or bulge.

On rare occasions, wall tie failure can be a contributory factor to the collapse of a weakened external wall under gales or storm-force winds.­ The wall areas most vulnerable to high winds are those where there are long spans of brickwork between corners and those where there are no structural returns, such as the brick panels located between window openings and at triangular gable apex walls. So be safe; if you discover any symptoms of wall tie failure get the walls checked out by a profes­sional.

3: Wall Tie Corrosion Survey

Cavity Wall Tie Failure

A wall tie inspection is usually carried out by a surveyor, who will collect and record data about both the construction of the wall and the condition of the wall ties therein. The surveyor uses information collected from the survey to form an objective assessment as to the service longevity of the existing wall-ties.

The 3-stage wall tie corrosion survey involves inspecting the portion of the steel most exposed to oxygen and moisture.

  1. A metal detector locates the position of existing ties. Plotting the locations over a given area of a wall helps to establish whether the number of existing wall ties per square metre meets with current building regulations.
  2. The use of a borescope enables inspection of the tie portion situated in the air-gap between the internal and external walls, subject to the cavity being free from insulation or blockages.
  3. Removing a small amount of mortar in the outer leaf exposes sample tie-ends to facilitate inspection of a section of the steel most vulnerable moisture, enabling the type of wall tie to be correctly established and its embedment depth measured.

The surveyor will be making many observations throughout the process; these generally include noting:

  • The type of masonry on both sides of the cavity, for example, solid brick, perforated brick, concrete block, timber frame etc. will help the surveyor to determine the best type of tie to use
  • Measurement of the cavity-width at various points will establish the required length of replacement ties
  • Any cracks in the walls and whether they coincide with the positions of the wall ties
  • Any bowing, leaning or undulation of wall surfaces
  • The height of the building, its exposure level, wind speed and terrain, as identified in Table 5 of BRE Digest 401
  • Any other defects that may be associated with the wall ties or that may contribute to increased corrosion risk (condition of bed joints, exposure to salts etc.)

The surveyor categorises tie condition, on a scale ranging from 1 to 9 using Table 2 of BRE Digest 401, which designates a classi­fica­tion based on visual corrosion levels. Table 4 of BRE Digest 401, shows both the ‘minimum’ and the ‘best’ measures for the degree of corrosion and the surveyor reports the advice to the house-owner.­  At level 1 (no corrosion present), the Digest suggests carrying out a further inspection within the following ten years; at level 9 (heavily corroded or failed), the recom­men­dation is for immediate action by retrofitting a wall tie replacement system.

4: Installing Wall Ties in Existing Buildings

Installing Wall Ties in Existing Buildings

If the BRE grading and remedial action system call for a wall tie replacement program, an experienced specialist wall tie contractor or a structural engineer can design a compre­­hen­­sive remedial tying and testing package that is best suited to the specific building.

Information about which kind of replacement wall tie is appropriate for use in a general construction type is available from the decision tree in BRE Digest 329. Discovering the most suitable retrofit tie for a specific structure will involve exploring those that have been inde­pen­dently tested, such as wall ties with CE Marking or BBA approvals.

The design package will include:

Selection of Remedial Wall Tie System

  • Helical wall ties
  • Mechanical expander wall ties
  • Resin fixed wall ties

Wall tie spacing – number of ties per square metre based on the fact:

  • That both walls are masonry and are more than 90mm thick - 2.5 ties per/m2
  • That at least one masonry wall is less than 90mm thick - 5 ties per/m2
  • That the inner wall is of timber-framed construction - 4.4 ties per/m2

Tensile load testing

  • Determine the sampling rates for testing
  • Establish the required tensile test load values following Table 5 of BRE Digest 401

Remedial action to alleviate any damage caused by existing cavity tie installation

  • None
  • Isolation of the part of the tie that sits in the outer brick wall layer
  • Complete removal from both inner and outer walls

Making good after insertion of the replacement ties

  • Plug drill holes to colour-match the brickwork
  • Make good the mortar bed joints
  • Repair renders, leaving ready for decoration

A recom­men­dation for any other works required to repair, protect or stabilise the outer wall of the structure including:

  • Repointing
  • Masonry crack repair
  • Lateral restraint
  • Weat­her­pro­ofing
  • Cavity cleaning

5: Wall tie types

HELICAL WALL TIES: What they are

Helical wall ties for retrofit applications

Helical wall ties are work-hardened stainless-steel wires that have been cold-rolled into a cruciform shape before being twisted. They are used to anchor the outer wall of a building to its inner structure and are available in different lengths to suit all cavity widths.

HELICAL WALL TIES: How they work

Lightweight blows, delivered by a setting tool in an SDS hammer drill, drive the helical wall-tie into a small pre-formed pilot hole, causing the tie to corkscrew into the masonry following the angle of its helix.  The work-hardened helical fins undercut threads into brickwork as they advance into the pilot hole, providing a mechanical interlock anchorage that grips the masonry on both sides of the cavity. The helix design provides fortuitous multiple drip points to prevent the possibility of water passing across the tie.

Helical wall-tie systems are quick and easy to fix; they withstand tension and compression loads and are suitable for use in brick, block and concrete structures including buildings requiring a fire rating.

MECHANICAL WALL TIES: What they are

Mechanical wall ties with expanding sleeves

Mechanical wall ties are stainless steel part-threaded threaded studs having an expanding mechanism at either end. The expansion mechanisms typically include a radially expanding sleeve held firmly between nuts and washers. Several sizes are available to suit the most common cavity widths.

MECHANICAL EXPANDING TIES: How they work

A mechanically expanding wall-tie fits into a pre-drilled clearance hole. Turning the torque-nut in a clockwise direction causes the tie-bar to rotate, squashing the neoprene sleeve between the nuts and washers at the far end of the tie. The compressed neoprene sleeve expands radially, tightly gripping the wall of the hole in the internal masonry wall.

At a factory set torque level, the part-threaded torque nut works its way down the bar, radially expanding the neoprene sleeve at the near end of the stud to engage and grip the external wall. Mechanical ties have a small neoprene drip-ring placed in the central portion of the bar; this ring helps to guard against the passage of water across the tie-bar.

RESIN-FIX WALL TIES: What they are

Resin adhesive to bond remedial wall ties

Resin-fix wall ties are pins made from stainless steel; they offer sufficient deformation to create a good bond with resin or cementitious brick adhesives. The deformed ties may take the form of simple course-threaded studs, helically twisted bars or even stainless steel rebars.

RESIN GROUTED TIES: How they work

A blast of compressed air removes all dust and detritus from a pre-formed clearance hole before injecting resin into the inner-leaf bore and pushing the tie-bar into the adhesive. Once the resin has set at the inner wall connection, dispensing more of the bonding agent around the section of the tie buried in the external wall completes the anchoring process.

6: Factors Affecting the Cost of Wall Tie Replacement

How much does wall tie replacement cost?

Stainless steel remedial wall ties themselves are relatively inexpensive but the cost of a wall tie replacement scheme depends on numerous factors. These include:

  • The cost of scaffolding that may be required to work safely at heights
  • The quantity and length of the replacement ties
  • The cost of specialist insertion tools
  • The number of corroding ties that need to be isolated from the brickwork or removed completely
  • The type and extent of cosmetic repairs that will be needed to make the wall look as good as new following the wall tie replacement works

A general builder, or specialist contractor, or even a competent DIY handyman, will find most replacement tie systems quick and easy to fit. It is possible to save a lot of money by carrying out the work yourself, but it is essential to follow all instructions correctly and to verify performance through testing.

Twistfix supplies a professional collection of wall tie replacement kits and we invite you to use the dedicated wall tie calculator on our website which will determine how many retrofit ties you will need for a given area of an existing wall. You can then browse our selection of replacement options to find a high-quality solution at a competitive price. If in doubt about any aspect of wall tie replacement, give our expert team a call for clear-cut, competent advice.