29/04/2017 by Martin James
Cavity Wall Tie Failure
Understanding cavity Wall Tie Failure
Prior to current building methods and today's technology, houses built with cavity walls for decades were built using mild steel to tie inner and outer skins. Different types of wall tie were commonly fish tail which were a thick mild steel and butterfly also know as wire ties which was a thin steel.
Both types of ties were used to cross from inner to out leaves of brick work, when the fish tail tie corroded it tended to expand caused by rusting, the build up of corrosion opened the mortar beds causing horizontal cracking, the cracking is more visible around window and door openings and higher up the building where there is less weight from the structure.
The other type of tie known as butterfly or wire tie would snap when corroded due to it's thin gauge, either tie when corroded can cause problems. If the problem is identified at an early stage the cost to remedy is more cosmetic and remedial compared to having to resolve more structural where as there are bulges due to the outer leaf of brick work becoming unstable.
If cosmetic, face brick work to remedy involves replacing the existing ties, repairing damaged bricks which may be in random areas and re-pointing the mortar joints. If there is a rendered finish the work would involve hacking off the render, replace the existing wall ties, render back in 2 coat finish.
Work involved if more structural and the outer leaf has bulged becomes more significant. You would need to prop and shore the internal to stable and reduce the weight of the structure, take down the outer leaf and reconstruct, in essence you are partially rebuilding the property.
What causes the wall tie to corrode? Prevailing weather conditions can cause water ingress, the wall ties dampen and the rust process commences. The rust process is sped up when the properties are more exposed to salt laden atmospheric conditions, another factor to consider is the mortar type, pre war mortar was commonly made from industrial waste know as black ash, properties within the mortar contributed to firstly allowing water ingress and adding to the rust process.
So in the early days when fish tail type ties were produced and were thick and strong and the mortar beds were and still are around 10mm thick, there is not much room with a wall tie of this type in the bed when average thickens of the wall tie was 3-5mm , when it corrodes it's not long before the tie expands and opens up the mortar joints causing the cracking.
After this era of wall tie they began to manufacture the butterfly or wire tie which was much thinner however galvanised, when the galvanised coating is breached the tie still corroded however in this case the metal disintegrated causing the inner and outer skins to part, this type of tie was more prominent in contributing to bulges on the outer leaf of brick work.
Today they manufacture stainless steel wall ties, how things have changed over the years and always are.
To replace the existing walls ties the old are detected with a metal detector and marked to indicate their location, this is the first process, the second is to install the new stainless steel tie which is fixed by drilling all the way through the outer leaf of brick work and half the depth a brick on the inner, dust from the drilling is then blown out to ensure proper and sufficient contact and fix with the new replacement tie, the positioning of the new tie is done in between the old which have been marked to identify their location. It should indicate a diamond pattern which should be 900mm apart horizontally, 450mm spacing vertically, it form a diamond pattern, the lower of the building the tie should commence 300mm up from dpc level, 450mm away from corners and 300mm on any openings such as window and doors etc.After fixing new ties, using a Hydrajaws Portable Tension Tester the new ties are tested for adequate grip to both skins of the cavity, the next process is to chase around the old tie on the outer leaf of brick work breaking it's contact with the brick and mortar, the old ties are then covered with a grease filled cap prior to rendering to isolate and prevent the tie from causing further damage.
At Greenleaf construction we can take care of either scenario with our competent work force who will get the job done right first time. If you are a property owner in need of this type of remedial work, ensure you are employing the surfaces of a competent contractor.
When your choosing a contractor, especially if it's more structural make sure it's reputable one who knows how to comply with Construction design Management (CDM) regulations. This means the work is notifiable to the Health & Safety executive and an F10 form has to be displayed on site for the duration of the work. It is the clients responsibility to ensure they have employed a competent CDM coordinator along with a contractor. At Greenleaf construction we use the services of and Health & Safety consultancy who can provide this service and guide you through the legislation ensuring you are up to date and on track. If your in need of our services then please visit our web site www.greenleafconstruction.co.uk and contact us from there for any enquiries.
Kind Regards all
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