A Professional Guide to Wall Tie Replacement
When wall ties need replacing, the amount of work to make sure that a proper job is done and structural integrity is restored can seem overwhelming. It needn’t be so and, to help you understand the process better, Twistfix have created a handy 6-part guide that explains everything. Below, you can find out about:
- The basics of wall ties, including the reasons wall tie replacement may be required
- The signs and symptoms of wall tie failure
- What happens during a wall tie corrosion survey
- How to install wall ties in existing buildings
- The different types of wall tie available
- Factors that affect the cost of wall tie replacement
1: The Basics
A large proportion of buildings feature cavity walls, which comprise two single, separate wall layers that are connected together with strips of steel known as wall ties. The outer layer faces the outside of the structure and is usually made from brick or stone. A cavity separates this from the inner wall which may be constructed from stone, brick, timber or concrete blocks and is typically load-bearing.
During the construction phase, the layers of the cavity wall are strongly locked together using wall ties; this allows the two independent walls to work cohesively together as a single unit. Properly tied, a cavity wall construction such as this is just as strong as a solid wall.
Wall tie replacement becomes necessary in two situations:
- The existing wall ties have become corroded due to the chemical reactions between the metal tie and moisture, in the presence of oxygen. A buildup of brittle iron oxide begins to weaken and erode the wall ties, causing them to fail.
- In instances of defective design or construction, where the built-in brick tie system has been fitted incorrectly. This can occur where ties have not been embedded to a sufficient depth or where an inadequate number of ties has been incorporated.
In either case, the solution is the same; a remedial wall tie replacement system must be installed 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
The different compositions of buildings, including the type of wall tie originally used, mean that outwardly-visible signs of problems with wall ties can vary. The two main signs to look out for are:
- A pattern of horizontal cracks at regular intervals. This often evidences an issue with advanced wall tie corrosion
- Bowing and/or undulating wall surfaces. This is a telltale sign that the outer wall is not properly tied back to the main structure.
Rust has a significantly greater volume than the original metal. Therefore as corrosion develops, the buildup of brittle, bulky iron oxide layers causes the tie to expand such that it is 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 in which the rusting ties are embedded; typically 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 erode completely with no visual signs of a problem, until the wall stars to buckle or bulge.
The wall areas most vulnerable to resulting wall failures 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.
On rare occasions and typically when coupled with gale-force winds, old cavity walls that have been weekend by cavity tie corrosion have been known to collapse. 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
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 information collected from the survey is used to form an objective assessment as to the service longevity of the existing ties.
The wall tie corrosion survey takes the form of 3 stages
- A metal detector is used to locate and plot the position of existing ties over a given area of wall. This helps to establish whether the number of existing wall ties per square metre meets with current building regulations.
- A borescope is then used to view randomly selected ties within the cavity. Note that this stage is subject to the cavity being free from insulation or blockage.
- The removal of localised areas of mortar in the outer leaf. This exposes sample tie-ends to allow inspection of the their most vulnerable section, allows the type of wall tie to be correctly established and permits measurement its embedment depth.
The surveyor will be making a number of observations throughout the process, these generally include noting:
- Masonry type on both sides of the cavity, e.g brick, perforated brick, concrete block, timber frame etc. This will help determine the best type of tie to use
- Cavity width at various points. This will establish the length of tie required
- Cracks in the walls and whether or not these cracks 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 band and terrain, as identified in Table 5 of BRE Digest 401
- Any other defects that may be associated to the wall ties or that may contribute to increased corrosion risk (condition of bed joints, exposure to salts etc.)
After inspection, the ties will be categorised according to Table 2 of BRE Digest 401, which is designed to aid correct classification based on corrosion levels. Classification is marked on a scale from 1 (no corrosion present) to 9 (totally corroded and failed). Potential remedial measures, based on tie condition as presented in Table 4 of BRE Digest 401, will be identified, showing both the ‘minimum’ and the ‘best’ measures. At level 1, it will be recommended that another inspection need not be carried out for a ten-year period; at level 9 classification it will require immediate action in the form of a full wall tie replacement system.
4: Installing Wall Ties in Existing Buildings
If the BRE grading and remedial action system calls 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.
Establishing suitable tie types can be achieved using the decision tree in BRE Digest 329. Whilst preference should be given to remedial wall ties that have been independently tested, such as those with CE Marking or BBA approvals, onsite testing should be performed to verify load capacity since the natue and quality of masonry can vary widely.
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:
- Both walls being masonry and greater than 90mm thick - 2.5 ties per/m2
- At least one masonry wall being less than 90mm thick - 5 ties per/m2
- The inner wall being timber framed - 4.4 ties per/m2
Tensile proof testing
- Sampling rates
- Required tensile test load values taken from Table 5 of BRE Digest 401
Remedial action to alleviate damage caused by existing tie irons
- Isolation of the part of the tie that sits in the outer brick wall layer
- Complete removal from both inner and outer walls
- Plugging drill holes to colour-match brickwork
- Making good mortar bed joints
- Making good renders, leaving ready for decoration
A recommendation for any other works required to repair, protect or stabilise the outer wall of the structure including:
- Masonry crack repair
- Lateral restraint
- Weather proofing
- Cavity cleaning
5: Wall tie types
HELICAL WALL TIES: What they are
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
A small pilot hole is created, which the helix-shaped tie is driven into; this provides a mechanical connection that grips both the inner and outer layers of the cavity wall structure. The process of driving in the tie causes it to corkscrew into the masonry, its helical fins undercutting the bricks and providing an expansion-free anchorage that will withstand tension and compression loads. The constant helix design provides multiple drip points to prevent water passing across the tie.
MECHANICAL WALL TIES: What they are
Mechanical wall ties are stainless steel part-threaded threaded studs that have been fitted with an expanding mechanism at either end. The expansion mechanisms are usually in the form of a neoprene sleeve which is held strongly between nuts and washers. A number of sizes are available in order to suit the most common cavity widths.
MECHANICAL EXPANDING TIES: How they work
Firstly, a clearance hole is drilled into the hosting masonry before the mechanical tie is inserted. Torque is then applied to a part threaded torque-nut which turns the bar; this in turn forces the nuts and washers at the far end of the stud closer together and squashes the neoprene tube.
This squashing action causes radial expansion of the neoprene sleeve, As the tube is squashed it expands radially, tightly gripping the wall of the hole in the inner-leaf masonry. At a factory set torque level, the part-threaded torque nut works its way gradually down the bar, radially expanding the neoprene sleeve at the near end of the stud to engage and grip the outer wall. Mechanical ties have a small neoprene drip-ring placed in the central portion of the bar and it is this ring that will guard against the passage of water across the tie.
RESIN-FIX WALL TIES: What they are
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
Once again, a small clearance hole is drilled through the outer wall and into the inner leaft to a depth of around 70mm. Compressed air is used to make sure all drill dust is removed before resin is pumped into the far leaf, followed by insertion of the wall tie; more resin is then pumped around the portion of the tie in the outer leaf. The resin will bond the deformed tie to both brick walls as it sets.
6: Factors Affecting the Cost of Wall Tie Replacement
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 in order 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
Most replacement tie systems systems are quick and easy to fit and can be successfully achieved by those competent in DIY as well as general builders and specialist contractors. Obviously, you can save a lot of money by carrying out the work yourself but it is essential to correctly follow all instructions and to verify performance through testing. If you choose to contract the work out to a builder, you may well find that the installation process itself costs much less than things like scaffolding, isolating/removing corroding ties and cosmetic repairs which in combination may form the major cost elements.
Twistfix supply 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 ties you will need for a given area of 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, competent advice.