Cavity Wall Tie Replacement

How to establish wall thickness and cavity width

Caliper Method

Measure the overall thickness of the wall near a door opening or window reveal. this is easiest done using a caliper then measuring the width of the open point in the caliper jaws, take of 30mm from the overall width of the wall and you have your appropriate tie length


Pilot drill method

  • Use a 6mm x 260mm drill bit, from the outside of the wall drill to approximately 110mm depth, you should feel the drill bit pressure drop as it breaks into the cavity. Mark or pinch the drill bit at its abutment with the wall, extract the drill bit, measure the distance, this will give you the thickness of the outer leaf material.
  • Reinsert the drill bit and push it into the wall till you hit resistance. Pinch or mark the drill bit at its abutment with the wall, then extract,
  • The thickness of the outer leaf measurement taken first, subtracted from the second measurement gives you the thickness of the cavity.
  • Both of these processes are very simple and quick to complete and give confidence as to which tie length to use.

Note: it is really not uncommon for cavity width to vary, due to poor construction techniques, on large jobs with multiple walls it is best to check a few walls to be safe.

                         Tie length    Cavity Suitability         Wall thickness minimum
195mm        5mm-45mm                210mm
210mm        10mm-60mm              240mm
220mm        15mm 70mm              250mm
240mm        50mm-90mm              270mm
270mm        100mm-120mm          300mm


Minimum embedment in inner leaf 70mm unless tensile proof tested.

 What size Pilot hole do I need to drill?

People who tell you one size of pilot hole will do the job are not being thorough. Using a standardised pilot hole can lead to underperforming ties or difficulty in installing ties.

Harder materials need larger pilot drills, softer materials need smaller diameter pilot holes.

For example:

  • Drilling through a blue engineering brick might need a 7mm pilot hole for an 8mm diameter tie
  • Fixing into AAC or lightweight block requires no pilot hole, so a pilot drill is only required in the outer leaf.
  • Soft to medium hardness brick inner and outer leaf will probably need a 6mm or 6.5 mm diameter pilot drill in both leaves of the wall.

Wall tie Pull Out Strength

The amount of force required to pull a wall tie out of the wall is relevant as the installed ties are there to prevent failure of the wall due to wind suction loads.

Wind suction loads vary depending on where you are in the county. Coastal and Northern areas have higher wind loads than sheltered inland areas, which leads to higher capacity requirements from the ties, or increased density of ties.

Taller buildings are more exposed to high winds and need even more resistance to wind suction.

The Strucsol Engineering team has been designing cavity wall ties specifications for everything from Sky scrapers to cottages for more than 30 years- we provide advice and consultations for free for one off customers of trade contractors and specifiers. Just get in touch with us either by phone or email and we'll be more than happy to help.

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