There are many reasons why you may need to consider installation of remedial cavity wall ties. The existing ties may be corroding. This can cause cracking and weakness, eventually leading to collapse. Poor supervision during original construction can lead to ties being omitted, or cavity width may vary from design, resulting in poor tie embedment. Whatever the reason, this operation needs to be carefully planned and the quality monitored if the work is to be a success.
Which remedial cavity wall tie?
It is vital that the proposed remedial tie is chosen properly. Building materials vary and a wall tie suitable for a timber framed house may not work in a house built from brick, or concrete.
Remedial ties come in several basic types:
1. Mechanical Ties (which work by expansion)
2. Resin bonded or grouted ties (non-expansive and usually a combined mechanical and adhesive fix)
3. Friction fix and screw in types (particularly suitable for timber and concrete/clinker blockwork)
4. Grouted sock anchors (for hollow units) and,
5. Specialised 'starter' type ties for installation into exposed walls prior to building another leaf to form a cavity.
Some remedial wall ties offer a combination of the above to accommodate different cavity construction.
Before selection the vital information you need is:
- The materials used for the inner and outer skins of masonry (or timber frame)
- Cavity width and the width of each skin
- Any special considerations like exterior coatings, existing cavity wall insulation or enhanced fire resistance.
How many wall ties are needed?
This depends on the cavity width, the width of the masonry and the wind exposure of the building. Guidance is contained in BRE digest 401 (1995) and BRE 329 (2000). If in doubt get advice from a qualified wall tie specialist or a structural engineer.
It’s not just about the wall ties
All remedial wall ties require equipment and tools to install them correctly. If you try to ‘make do’ with the wrong size drill bits (even 1mm diameter under or over will lead to failure with some ties), or you omit to clean holes destined for a resin fix – you will have problems. Failure to factor in the correct ‘setting tools’, ‘drive tools’ or ‘blow-out’ pumps at tender and planning stage is a common cause of failure
Testing is key.
Whichever wall tie is chosen it is advisable to proof test the desired type first. This can be done by using a tension tester to ‘pull’ a known force on the ties when they are installed. If this is successful, then work can proceed. Use the same methods applied when the test ties were installed. Random ‘pull-out’ tests on the work as it proceeds is essential.
Remember that installing cavity wall ties in a building as it is built, is much easier than working with remedial cavity wall ties. This is because the inner leaf is not visible; everything is happening either in the cavity or masonry, with not much to see. Only ongoing testing will assure you of success.
This is just a basic guide, for more information please visit PreservationExpert blog.
By Bryan Hindle
Remedial wall tie specialist