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:
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:
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.
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:
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.
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
The surveyor will be making a number of observations throughout the process, these generally include noting:
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.
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
Wall tie spacing – number of ties per square metre based on:
Tensile proof testing
Remedial action to alleviate damage caused by existing tie irons
A recommendation for any other works required to repair, protect or stabilise the outer wall of the structure including:
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.
Stainless steel remedial wall ties themselves are relatively inexpensive but the cost of a wall tie replacement scheme depends on numerous factors. These include:
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.