Cavity wall ties are an essential part of any building. Their job is to tie together the visible protective outer cavity of brickwork or blockwork of a building to it's structural and load bearing inner masonry skin.
As you may be able to tell from the above, if your property is suffering from wall tie failure then this can mean that both skins become independent of each other. The building can then become structurally unsafe and a danger to those that may be in it or around it.
In respect to the danger factor, if you are in any doubt about the condition of your wall ties it is a good idea to get a fully qualified structural engineer or expert in wall tie examination to inspect your property and produce a full report on their condition as they will have access to specialist equipment such as a Borescope (small camera that can be inserted into the cavity to inspect ties) so that a physical, visual inspection can take place.
You may be thinking this sounds expensive and to some degree you're right, it probably will be but a professional inspection will tell you the exact condition of your wall ties and from that you will know exactly what you will need to do to fix the problem, saving in the long run from any future fixes due to incorrect diagnosis and repair measures.
In this project guide we will deal with wall tie failure, how to replace wall ties and also how to ensure that all wall ties are spaced the correct distance apart so that your outer cavity wall is stable without the possibility of it falling or blowing over. Additionally, if after reading this project you are still a little unsure and would like some more help and advice in diagnosing wall tie issues please contact Property Repair Systems on 01626 336180 and they will be please to supply additional, expert assistance.
A potted History of Wall Ties
Wall ties have been in use within housing construction since the early 19th century and became commonplace in construction during the 1930's. They have now become standard in any property featuring a cavity wall. When it comes to the construction aspect, the purpose of the cavity wall was to split the more traditional 9 inch think solid wall into two parts to improve it's weather resistance, producing one 4 1/2 external weather-shielding wall and one 4 1/2 inch internal load bearing wall.
This new cavity also stopped any water from getting from the outside skin to the inside and also stopped an extremely cold surface outside being transferred to the inside.
Wall ties effectively tie the two walls together, improving stability and, to a very minor degree, the load sharing capabilities of both walls.
How do I Know if I Have a Cavity Wall?
There are a few ways that you can tell this. One method is to use a long masonry drill to drill through the outer wall or your property. If during the drilling process the drill goes slack after around 90mm and then starts to bite again this is a good indication that you have a cavity as the drill will stoop biting when it reaches the cavity gap and then start again when it hits the inner cavity wall.
Another method is to look at the pattern of the brickwork in the wall and how the bricks have been laid. If they have all been laid lengthways along the full width of the wall this is a positive sign of a cavity.
If they have been laid in such a way that you have one brick laid lengthways and then next to it you have one brick that has been laid width ways then this is a good sign that the wall is solid.
Visual Signs of Damaged Wall Ties and Wall Tie Corrosion
Below follows a quick check list of tell-tale signs that may indicate wall tie damage or failure:
- Vertical or horizontal cracks in brickwork
- Bulging Brickwork
- Cracks or separation of window reveals
- Lintels sagging
In most cases the visual damage is caused by the following:
- Vertical or horizontal cracks in brickwork – Normally caused by expansion of corroded wall ties. As the tie corrodes, the rust or metallic oxide that is produced takes up more space than the original tie did and due to this the bricks above and below the tie are forced apart cracking the mortar bed. If this problem is wide-spread within your property you will normally see this occurring every six horizontal course of brick (450mm) if the original ties have been spaced correctly.
- Bulging Brickwork – This can occur when the corrosion of wall ties and the resultant expansion in their mass forces the outer wall skin either up or down. If there is no where for it to expand into then the brickwork itself will have no choice but to bulge outwards.
- Cracks or Separation of Window Reveals – This issue normally manifests itself on the internal walls of a property. When expansion and movement is caused, gaps and cracking can appear between the window frame and the window reveal. Additionally the depth of a window reveal can also appear as inconsistent.
- Lintels Lifting or Sagging – Again, due to expansion movement, lintels can either be pushed upwards or downwards. This can also be the same for window sills.
Wall Tie Failure and How This can Happen
Here follows a breakdown of the two most common causes of wall tie failure:
Corrosion of Wall Ties
In most cases this will simply be age. Over time the mortar bed between bricks or blocks that each wall tie is set into changes on a chemical level due to carbonation. Through this the mortar itself begins to attack the wall ties and cause a build-up of iron oxide (or rust as it's more commonly known). As each tie oxidises, layers of rust are formed and expand outwards, pushing apart the mortar beds and causing visible cracks.
The visible cracks are more common when twisted or sheet steel wall ties have been used in a more sturdy and solid mixed mortar bed.
In other situations where the mortar bed may be a bit more flexible, any expansion in the wall ties due to corrosion may not be visible on the outer wall, apart from the possibility of bulges and you may have no idea that the wall is in fact essentially flapping in the wind.
Wall Ties Incorrectly Fixed or Installed During Construction
Aside from age and erosion, another common failure point for wall ties is incorrect installation. This occurs during the construction stage where they may not have been correctly inserted in the mortar beds between brick or block courses, the incorrect length ties may have been used meaning that they are too short and do not sit far enough into each mortar bed, they may also not have been placed at the correct spacing e.g. too far apart vertically or horizontally and in some cases they may not have been added at all!
Different Types of Wall Tie
Over the years there has been a great many different materials used in making wall ties. In most cases the age and location of the property will dictate how its wall ties have been constructed; some common types are as follows:
- Metal – More recently, stainless steel is the chosen type but historically mild steel, copper, wrought iron and cast iron have been used. Wrought and cast iron ties tend to be quite heavily constructed and if found to be corroded will in most cases need to be removed. Copper ties are quite rare mostly down to their cost but are in fact very hardy and long lasting
- Brick or Terracotta – Found rarely and normally isolated to specific regions. They also have several disadvantages in that they have been found to create cold bridges to inner walls allowing moisture and cold to travel along them.
- Stone or Slate – Again, not commonly found and come with some disadvantages that include a tendency to snap and crack when movement is present. However, they are quite resistant to corrosion and if featured in a property where there is no evidence of movement should last a good long while. Once again however, this type of tie can transfer moisture and cold to the inner skin.
In most properties dating from the 1930's onwards, mild steel is the material of choice when it comes to wall tie construction and in normal circumstances the mild steel used was either left as-is (uncoated) or coated in a bitumen type substance. As technology and awareness of rusting and corrosion developed, zinc and galvanised became the coating of choice providing the ties with a water and moisture resistant outer layer to further their longevity and lifespan.
Today, stainless steel has taken over from zinc coated or galvanised steel due to it's increased resistance to rusting and corrosion.
In terms of actual wall tie design, there are two common types:
- Strip Ties - aka vertical twist or fishtail. These are the most common type of wall tie
- Wire Ties - aka double triangle or butterfly
The images above represent the more modern wall tie design but depending on the age of your property it may have more traditional type wall ties such as the below:
Types of Retrofit and Remedial Wall Ties
There are many different types of retrofit and remedial ties available today and most of them fall under one of three different types:
- Resin or Grouted – These are normally pushed into a pre-drilled hole that has been filled with resin or grout. Once seated in place more resin or grout is pumped down through the tie to fill around the end in the inner cavity wall. Once the resin or grout sets it will bond with the surroundings and anchor the tie in place. These types of tie are ideally suited to buildings and properties where the condition of bricks or blocks it not that great
- Mechanical – These types of tie are inserted into a pre-drilled hole and normally feature a sleeve that expands when the tie is screwed up. Normally these ties will be supplied with a setting tool that can be used in a combination-type drill. Due to their expanding nature it is recommended that they are only used in properties where the brickwork is in good to excellent condition. More can be found about these types of ties over on the Property Repair Systems website
- Helical Screw – These ties look very similar to a long corkscrew and feature quite a course, wide thread. In a similar fashion to the above types, a pilot hole is drilled (normally around 6mm in diameter) through the outer cavity wall and into the inner to the required depth. Using a special setting tool inserted into a hammer drill, the tie is screwed into the walls in much the same way as a traditional wood screw would be screwed into a piece of timber. These are generally regarded as being safe to use on any condition brick or block walls
Life Expectancy of a Wall Tie
Any wall tie that is made of mild steel will eventually corrode. This, unfortunately, is a fact! Due to the materials used for wall ties in properties constructed before 1940 it is almost impossible to say how long these will last.
However, properties constructed between 1940 and the mid 1960's can be roughly estimated as follows:
- Wire Ties – between 15 and 30 years
- Strip Ties – between 30 and 60 years
For properties constructed between the mid 1960 and early 1980's:
- Wire Ties – between 13 and 26 years
- Strip Ties – between 23 and 46 years
How to Replace and Install Cavity Wall Ties
Before we commence with the "how-to" stage of this project it is important to mention if your property is suffering from visual cracks due to wall tie corrosion this may have gone past the point of being a DIY job. You will to consult with a qualified structural engineer so that he can assess and recommend next steps. In this situation it is almost going to be the case that the corroded wall ties will need to be removed and this is a very skilled job that will need to be carried out by someone with prior skill and knowledge so that the removal of the damaged ties causes no further structural damage to the property
What Type of Wall Ties do I Have?
This can be a tricky one to establish. In reality the only real way to tell is to firstly locate a wall tie in the wall using a metal detector (more information on these can be found on the Property Repair Systems website – see the image below) and then either using a Borescope (can be hired or supplied in most cases with a full inspection from a professional) drill a small hole and insert the camera end to identify the tie.
In the absence of specialist equipment such as this, again, locate the tie using a metal detector and remove a brick (see our project on removing and replacing bricks for more information) so that you look inside the cavity either visually or using a camera or phone camera and take a picture.
If you are going to be installing a retrofit type wall tie then this stage may not be necessary as most types and designs can be installed regardless of the existing types.
One thing that may dictate the type of retrofit tie that can be used is the type of bricks you have and also if you have cavity insulation. These points are dealt with next
What if I have Cavity Insulation Installed?
If you have cavity fill insulation is it important that you don't use any retrofit or remedial ties that involve resins that need to chemically react in order to set as this may affect your insulation.
Most remedial solutions will involve you having to drill a small hole in order to insert the tie into the wall. You should also ensure that you use the ones with the smallest insertion hole possible, as this will reduce any loss in the thermal efficiency of the insulation to the absolute minimum.
What Type of Bricks do I Have?
This is another very important question that will need answering as you will have to establish if the bricks or blocks used in your home are solid or perforated as this will dictate the type of retrofit tie you can use.
There are two ways to do this – if in your loft space you are able to access and view the tops of the walls you will be able to see what type of bricks you have.
If however you are unable to do this you will need to get the drill out! You are going to need to drill at least one hole (you may need to drill two or three just to be sure) in each one of the walls of your property. This may be in at least four walls in the property is detached or two or three if the property is terraced or semi detached.
If the drilling rate is consistent e.g. the drill bit is in a constant state of "cutting" then you have a solid bricks or block construction
If drilling is not consistent e.g. the drill bit is cutting through the brickwork and then goes slack and then starts cutting again then is it a good bet that the bricks or blocks are perforated.
If you have had to drill holes please ensure that you fill them with a suitable filler or mortar mix that will provide a waterproof seal. If left unfilled, this will provide the elements with a direct route into your cavity!
Which is the Correct Type of Wall Tie I Should Use?
Firstly, you will need to have completed the above step to identify what type of brickwork you have. Once you have established this, you can identify your required wall ties from the four options below:
- Solid Bricks or Blocks – For this type of construction you have a choice of three tie types. You can use Resin Grouted ties, Mechanical ties or Helical screw in ties
- Perforated Brick or Blocks – For a property with perforated bricks or blocks you can use either a Resin Grouted tie in a sleeve or the Helical screw type tie
- Properties With Fire Requirements – If may be that your property is subject to certain fire regulations e.g. it may be a guest house or hotel etc.... If so you will need to use the Helical screw in tie as these have a minimum of 30 minutes fire resistance (check with manufacturers specifications)
- Cavity Wall Insulation – If you have cavity insulation you will not be able to use any ties that feature a chemically reactive resin, you will only be able to use Helical screw ties or Mechanical ties
If you require mechanical ties then we work closely with Property Repair Systems that supply a good range, see them here.
What is the Correct Wall Tie Spacing?
The correct spacing of the ties in your walls is critical! Too far apart and your protective outer cavity wall will not be securely tied into the structural inner cavity wall and may collapse!
The general rule when constructing a cavity wall is that ties should be spaced at the rate of 2.5 ties per square meter. This equates to at least one tie every 900mm along the horizontal and at least one tie every 450mm vertically.
These rules change slightly when openings such as windows and doors and gable ends are present. In this case you should have at least one tie every 300mm vertically and no further than 225mm away from the opening horizontally.
This can be quite difficult to visually imagine so a good way of thinking about it is to picture the ties in a diamond pattern.
- Horizontal – Spaced no more than 900mm apart
- Vertical – Spaced no more than 450mm apart
- Openings (windows, doors and gables – Spaced no more than 300mm vertically and 225mm horizontally
The above spacing's are based on standard width bricks or blocks that are 90mm thick. If widths are anything less than this then spacing rates should increase from 2.5 ties per square meter up to 5 tie's per square meter.
Additionally if your property is constructed around a timber frame with a brick cladding ties should be spaced at a rate of 4.4 ties per square meter.
- Less Than 90mm Thick Brick or Block Work – 5 ties per square meter
- Timber Framed and Brick Clad – 4.4 ties per square meter
When it comes to retrofit wall ties, when installing you should stagger them around any original ties using the measurements and pattern described above.
How Many Wall Ties Should be Used?
The number of wall ties that you will need, will really be dictated by the size of your property and the number of openings.
You can however estimate the number of tie's you are going to need by calculating your total wall space in square meters and then dividing this by the number of ties required for your particular construction type. For example:
- Wall One Square Meters – 10m x 2.5m = 25m2
- Wall Two Square Meters – 8m x 5 = 30m2 (this will be a gable end so need to calculate apex area)
- Wall Three Square Meters - 10m x 2.5m = 25m2
- Wall Four Square Meters – 8m x 5 = 30m2 (this will be a gable end so need to calculate apex area)
- TOTAL m2 = 110 square metres
If the property is solid brick or block you will need 2.5 ties per square meter so if we multiply 110 by 2.5 we get the grand total of 275 ties required.
Obviously this has not taken into account openings and gable ends so these will need to added also. As a general rule 7.5% is added for this and also the wastage factor bringing the actual total up to around 296.
What is the Correct Length of Wall Tie I Should Use?
The length of the ties you will need to be set by the width of your cavity so this will need to be measured accurately. This can be achieved several ways:
- Measured From the Loft Space - If you can access your loft space and at the very edges can see down into the cavity then you will be able to measure the width from here
- Removing a Brick – You can also remove a brick from the outer wall and measure the cavity internally. This is quite and easy job to do and further information can be found in our removing and replacing a brick project
- Drilling – Using a long masonry drill bit (at least 250mm), drill through the outer cavity wall and push the drill bit through until you can feel it touch the inner cavity wall. Using a small piece of tape, place it round the bit so that it is flush with the edge of the outside cavity wall and withdraw the bit. Now, measure from the tip of the bit up to the start of the tape and then subtract the width of brick or block work used and this will give you the width of your cavity
Now that you know how wide your cavity is you can work out the length of ties required.
It is generally recommended that each end of a wall tie should protrude a minimum of 50mm into each leaf wall to create a secure anchor. You will also have to take into account the centering of the tie and also tolerances. With this in mind you should be looking to allow somewhere between 65 and 75mm of embedding in each wall leaf:
|Width of Cavity||Recommended Length of Wall Tie|
|50 – 75mm||200mm|
|76 – 100m||225mm|
|101 – 125mm||250mm|
|126 – 150mm||275mm|
|151 – 175mm||300mm|
|176 – 200mm||325mm|
|201 – 225mm||350mm|
|226 – 250mm||375mm|
|251 – 275mm||400mm|
|276 – 300mm||425mm|
Installing Wall Ties into a Wall
This will very much depend on the type of tie you can use taking into account the advice above.
In most cases it will involve drilling a hole through the outer cavity wall into the inner wall to the set depth and then inserting the tie and fixing it as instructed by the manufacturer's guidelines. Normally this hole will be either around 6mm for mechanical or screw-type ties or around 12mm for some of the resin fixed ties.
It is also a good idea to ensure that the tie slants downwards and towards the outside wall a slight amount as this will ensure that any moisture that collects on the tie will run towards the outer wall as opposed to running onto your inner wall and creating damp or cold spots. Again, you may have to check with manufacturers guidelines concerning this.
Once all your ties have been inserted it may then be necessary to fill any holes in the outer wall to prevent any possibility of water ingress into the cavity void.
Other Points to Think About
One point to note is that by adding remedial wall ties, this will solve any immediate structural issues you may with your property but it won't solve the issues that any corroding ties have caused e.g. cracking and expanding mortar etc.... as if these ties are allowed to remain then the more they breakdown the worse the cracks will become.
In this case you should consult with an expert with the aim of removal.
As part of any repair you should also consider full repointing of any cracks as this will also aid in structural stability and prevent any water ingress into the cavity area.
As mentioned at the start of the project, if you do need any assistance with diagnosing wall tie issues, what replacement wall ties you should be using or even how they should be installed give Property Repair Systems a call on 01626 336180.