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Wall Ties and Lateral Restraint

Wall Tie Replacement and Isolation

For more than 80 years, the advantages of weather proofing provided by cavity walls has been recognised. As the inner and outer leaves of cavity walls are physically separated, some sort of tying device is needed to give the walls stability and consolidation. This is provided by wall ties, which in early cavity walls could have been anything linking the walls together – including through-stones. Through-stones are also found in rubble filled solid stone walls and have the same function of holding the outer and inner leaves together. More recently wall ties have been metal, usually mild steel in bar or wire form, but these are susceptible to corrosion. Protection of the ties against corrosion has been by coating them with bitumen, zinc (galvanisation) and plastic, but none of these has provided a long term solution to corrosion.

In 1981 new standards for ties were developed by the Building Research Establishment (BRE) www.bre.co.uk and BRE 329 and 401 guideline publications were created for remedial wall tie inspections and works.

Wall tie replacement is now almost exclusively based on the use of stainless steel ties and all new buildings usually have these.

Cavity wall collapse due to wall tie failure

In older cavity constructed buildings with wall ties, corrosion of steel ties is inevitable in all properties, with the durability of ties depending on the type of the tie, its coating, and the chemical processes that might encourage corrosion. For example, corrosion is more likely to occur in walls which are more exposed to the weather (water), where mortars are of black ash type (acid), or in seaside properties where sea spray (salts) might occur.

The condition and type of the existing ties can be established by removing sample ties, removing bricks to expose the ties, chiselling out mortar to expose a tie end, or more usually and with least damage, by the location of sample ties with a metal detector and viewing them in the cavity using an endoscope. Any brown rust corrosion of any tie seen is an indication of the need for wall tie replacement as ongoing corrosion is inevitable. The immediacy of replacement depends on the level of corrosion, but most remedial works become part of the sale/purchase of any property and are therefore immediate, and often a mortgage requirement.   

Removing a brick for wall tie inspection

 

 

Brick removal is sometimes the only way of looking at the condition of wall ties. This applies when the view of the ties is restricted by say, insulation, in the cavity. 

 

 

 

 Brick out and tie exposed

Removal of the brick leaves the tie end exposed. It is particularly useful to be able to see the tie end, as this is where corrosion can be worst, and where the strength of the ties to do its job, is focused. 

 

 

 Corroded tie end

 

 

 This tie end is red rust corroded. The recommendation for this house is that the ties need replacing. 

 

 

 

Building Research Establishment Digest 401 (January 1995), table 4, gives advice on suitable action to take in relation to the findings of a survey, where existing wall ties are found to have red rust on them. Reference 6 in this table sets out the following:-

  1. The classification is suitable for metal ties originally protected by both bitumen paint and galvanised zinc coatings.
  2. The predicted life of such ties is listed as "N/A" - not applicable - i.e. there is no predicted life of the ties in this condition.
  3. The recommended action is divided into two areas of advice:-
    1. Best recommended action - "Fit remedial ties"
    2. Minimum recommended action - "Schedule repairs"

This is a clear guidance for us of what to recommend where ties have been found to be rusting. In addition where ties have red rusting and erosion (loss of the metal in the thickness of the ties), the only recommended action is "Fit remedial ties". The standards to work to are therefore set out clearly in BRE Digest 401, and there can be little room for misinterpretation.

Old wall ties left, new ties right

Remedial treatment of ties involves two processes:-

 - the installation of a new tie system using stainless steel ties (some types on the right of the photo)

 - the isolation or removal of existing metal ties (some types on the left iof the photo).

The new tie system we use depends on the masonry to be tied, but in most cases we have found mechanical ties with expanding neoprene sleeves suitable. We have available resin fixed ties should the former be unsuitable. Our remedial tie supplier, PAM Ties http://www.pamties.co.uk/walltiehistory.htm has further useful information on their website.

 

Mortar bed cracking from wall tie corrosion

Expansion of metal ties results in mortar bed crackingThe existing ties must be dealt with as their continuing corrosion can result in structural damage to buildings. Such corrosion can result in the expansion of the metal embedded in the outer leaf of the wall. This expansion may eventually result in the tie becoming up to four times its original thickness, sometimes splitting the mortar bed joints, causing either lift and/or bowing of the walls and damage to internal plastered finishes. Instability of the wall can result if the wall ties waste away and break completely, resulting in a need to rebuild the wall. Little or no damage to walls may occur where the tie is of the ‘butterfly’ (referring to shape) wire type, but this depends on the thickness of the mortar bed between the bricks and the thickness of the metal wire of the tie.

  

The density of wall ties in a wall should be 2.5 ties per m2, with there being an increase in ties around openings such as windows and doors. These are the points where the wall structure is compromised by the openings. Here the tie spacing should be 225mm from the opening, spaced at 300 mm vertical centres maximum. These tie densities are usually not found in properties built before say, 1981. Although the tie density of 2.5 is usually there, there is usually a lack of ties around the openings. This can be remedied by installing extra ties.

A lack of wall ties in a cavity wall considerably reduces the strength and stability of the wall, in particular resistance to wind loads. While sudden failure is unlikely to occur it would be prudent to rectify the matter once noted as the defect is liable to affect the value and resale prospects of any property.

If symptoms are recognised before damage has progressed too far, the walls may be treated rather than rebuilt. This consists of locating the old wall ties with an electronic metal detector, installing a suitable corrosion-resistant remedial fixing and isolating the original wall ties to prevent further damage to the outer leaf. Where wire ties have been used and they are corroded, new ties will need to be installed, usually without the need to isolate or remove the old wire ties.

Existing Wall Ties

Any existing ferrous metal wall ties left in place may continue to corrode and cause disruption of the brickwork. There are techniques available to overcome this potential problem; removal of the old ties completely, or isolation of the tie end in the outer brick leaf. Complete removal of the wall ties would ensure that corrosion and expansion of the ties could not affect the inner or outer brick leaves in the future. There are, however, disadvantages associated with tie removal.

  1. The extra cost involved in having to remove and replace bricks to allow removal of the ties.
  2. Damage to existing coatings or renders (if present) and consequent cosmetic impact on the outside of the house.
  3. There is a possibility that while extracting the old wall ties some damage might occur to the internal wall plaster, particularly if it is old or in poor condition.

The isolation method involves raking or grinding out the mortar bed joints around all accessible existing wall ties, back filling with a compressible material (foam or fibre insulation) and repointing of this area with sand/cement mortar. This method has a considerable cost saving over complete tie removal, resulting in less disruption to the masonry and no possible damage to the internal plasterwork. The principle of isolation relies on the fact that in almost all cases the wall ties are more corroded in the outer than the inner leaf, due to the outer leaf of brickwork being more exposed to the weather. No one can say with certainty that disruption of the inner leaf will never occur, as corrosion is an ongoing and accelerating process. This should be borne in mind when deciding which system to adopt.

When carrying out wall tie work, we usually carry out the following:-

  1. Erection and dismantling of any scaffolding/towers necessary to complete the work safely.
  2. Location of the existing cavity wall ties using a metal detector. (#)
  3. Installation of stainless steel wall ties to cavity wall sections (not solid wall areas), positioned in accordance with the wall tie manufacturer’s instructions, which is in accordance with current Building Research Establishment (BRE) recommendations.
  4. Random checking of several of the new ties, in order to ensure that correct fixing has been achieved.
  5. Assessment of original wall ties from the most exposed areas of brickwork to be treated, so that the condition of metal in contact with the inner skin of brickwork can be considered. (#)
  6. Isolation of original wall ties over the treated areas, to ensure no damage occurs to outer leaf masonry as a result of further corrosion. The method of isolating original wall ties depends on the condition of the samples assessed in 5 above. Wire “butterfly” type ties do not normally need to be isolated. (#)
  7. Making good (excluding redecoration) of any brickwork, render or mortar disturbed during the wall tie installation or isolation process.
  8. Removal of dust and debris created by the work leaving the adjacent site or garden in a clean and tidy condition. We are unable to avoid some contamination and damage by dust and debris to outside surfaces, including vegetation.
  9. The issue of our 20 year guarantee (insurance backed if required) after payment of our account in full.

(#) Items marked 2,5 and 6 are excluded from the works where installation only is carried out.

Special notes for the installation and isolation of cavity wall ties

The drilling of brickwork, mortar beds, render and other masonry materials causes a lot of dust. Given unfavourable wind conditions this can be carried away from the area of our work to contaminate other areas such as plants, windows, driveways, parked vehicles and paths. Where windows, doors and other openings (e.g. ventilation points, cat flaps etc.) are left open dust can get inside the property. Neighbour’s and your own washing hung outside may also be at risk. All precautions should be taken to minimise the opportunities for such problems to arise and early discussion with neighbours is advised. Where party or shared walls are to be treated it is a legal requirement that you notify and obtain the consent of the adjoining owner under the Party Wall etc. Act 1996. 

Drilling will also cause vibration which can affect hanging items and ornaments inside houses, and sensitive intruder alarm systems both inside and outside houses. These should be removed or in the case of alarms, deactivated, before work is started. Some alarm systems go into “tamper” mode as a result of vibration. Occasionally drilling vibration may also cause minor damage to internal block or brickwork and also plasterwork, particularly if they are in old or poor condition.

It will not be possible to install new ties or isolate existing original ties in areas that are inaccessible. This includes areas covered by or close to guttering, down piping, satellite dishes, external lights, alarm boxes, gas systems or electricity wire runs and other wall fixed items. This will rarely affect the isolation of more than several original ties in any one property and therefore should not be regarded as a significant problem.

Wall tie work is undertaken from the outside of the property. The work is weather dependent as we can not use electrical equipment (e.g. drills and grinders) outside in wet weather. This may mean delays and the need to reschedule the completion of the work.

We require access to a 13 amp, 240 volt electric power and a clean water supply. Our requirement for power can be high for short periods so access to the main electric meter switch board/fuses must be allowed. If electricity is not available then we would hire a generator at the client's cost.

Before attending site, we require that all movable items next to the external walls of the property are moved by the client so that we can gain access to the walls with either our tower scaffolding or ladders. This includes vegetation growing up or bearing onto the walls. The use of this equipment sometimes results in localised damage to flower beds and their contents, though we will do our best to minimise this damage. 

 

Download specwt.pdf specwt.pdf

 

 

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Wall Ties specification sheet This information sheet tells you how we will deal with your existing and new wall ties.

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