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Replacement Wall Ties

Many house buying surveys recommend getting the wall ties inspected for corrosion, frequently adding that failure of existing wall ties can cause cracking, bowing, or even the collapse of the external leaf. Nothing to worry about there, then!

What Are Wall Ties?

Cavity wall construction originated about the start of the 20th century, although earlier examples are found. The idea is that the air in the cavity itself acts as an insulator, and the cavity protects inside walls from penetrating dampness.

The internal floors are carried on the inner leaf, as is the weight of the roof. The external leaf protects the inner leaf from thermal extremes, and also acts as a wind shield, absorbing most of the suction and pressure of gusts. To stop any major movement of the relatively slim outer leaf, it needs to be affixed to the more rigid inner leaf. During construction, and depending on the period during which the house was built, a series of wall ties would have been incorporated to form this bond.

These ties include iron straps, thin ‘butterfly’ wires, and twisted steel bars, set in to the mortar bed, bridging the cavity from the inner to the outer leaf.  Some constructions are designed to have a longer stone set into the wall at set points to act as a connection between the inner and outer leaves.

How Does It All Go Wrong?

Over time, if the mortar in the outer leaf wears away due to weathering and poor maintenance, the moisture allowed in to the mortar joint can corrode the wall tie. The problems caused by this corrosion depend on the type of tie initially installed. The thicker iron strap tie expands as it corrodes, causing the tell-tale cracks in the pointing or render that can be seen outside. Extreme expansion can cause the edge of the roofline to rise, forming a ‘pagoda’ effect, as the external leaf becomes taller than the internal leaf. Although each tie may only expand by 2-3mm, on a 6m wall this could be a 40-50mm height difference!

Photo courtesy of Twistfix

The thin wire butterfly type corrodes away, but their slimness does not cause forcing of the masonry, or horizontal cracks. This can cause problems in itself, as the structural strength of the ties has gone, but no visible clues exist.

What Does Your Surveyor Look For?

Your surveyor should look at all external visible signs – stepped cracks, horizontal joint de-bonding, roof/gable movement. They will check for bowing or bellying of the walls. A full structural survey is usually recommended if severe distortion of the wall is noted, as rebuilding of all or part of the outer leaf may be necessary.

Photo courtesy of Twistfix

Using a metal detector, the position of existing ties will be determined, to check that, even if in good condition, they are sufficient in number. A hole is then drilled in to the wall and a boroscope inserted to check the condition of the tie. Most surveyors will want to check each elevation, as different weather exposure can cause varying degrees of deterioration.

Photo courtesy of Twistfix

It is important that your surveyor knows what type of tie is in the wall, as removal or isolation of these is a most important part of the job. If these are left in place, they can continue to corrode and cause further cracking, effectively making the new ties unstable (although some thin butterfly ties tend not to do this, and may be left in place). Ties can be physically removed by taking out the adjacent brick and cutting the tie out, or, if the joint is wide enough, it can be raked out and a grease filled sleeve slipped over the end of the tie, thus halting further corrosion.

Don’t go with the cheapest quote – look at the specification that should accompany any quotes, and check that it is ‘like-for-like’ - some companies may make a separate charge for treating existing ties.

So What’s the Treatment?

If it is decided that your ties need replacing, the surveyor will make a decision as to what type of remedial tie to use. This will depend mostly on the type and condition of brickwork. The main types are a mechanical fix, which is either a steel bar with expanding sleeves on each end, or a driven helical bar, or a resin fix which is a threaded, spiral or sleeved bar, with each end set in a resin plug. Some brickwork will require a mix of the types.

Too few ties will give an inadequate bond, and too many can make the outer leaf too rigid, and unable to cope with thermal movement or the suction and compression provided by strong winds. Existing ties will be removed if necessary at the same time as new ties are installed (but not before). A good contractor will ensure the joints are re-pointed to match existing, or render repaired. Remember this when comparing quotations. A properly installed and finished job will give you nothing to look at or show off!