How close to the edge of a brick can I put a fixing?
Why a fixing needs a complete cone of sound material behind it, what happens when an edge or a neighbouring anchor removes part of that cone, and what to do when the distance is not available.
- Difficulty
- intermediate
- Time
- 12 min
- Read
- 3 min
Short answer
An anchor is only as strong as the cone of masonry spreading behind it. Too close to an edge and part of that cone is missing, so an expansion anchor splits the edge off. Too close to another anchor and the cones overlap so neither reaches its rating. Where the distance is not available, use resin, which applies no outward pressure.
Manufacturers publish minimum edge distances and spacings for every anchor, and they are the least-read numbers on the data sheet. They exist because the fixing's capacity does not come from the fixing at all — it comes from the volume of masonry behind it, and geometry decides how much of that volume is there.
Step by step
- Picture the cone.Load pulling on an anchor is resisted by a roughly conical volume of material spreading outwards from the deep end of the anchor towards the surface. If that cone is complete and in sound material, the anchor performs. If part of it is missing, the anchor does not.
- Keep clear of edges.Near the arris of a brick, the end of a course, the edge of a slab or lintel, or an unfilled joint, part of the cone is simply not there. What is left is a thin lip, and the outward force of an expansion anchor levers it off.
- Keep anchors apart from each other.Two anchors set close together share the same cone, so both are weakened. Increasing the spacing usually gains more than increasing the anchor size.
- Keep clear of the mortar joints for heavy work.A cone that is mostly mortar is weaker than a cone that is mostly brick, even where it is geometrically complete — and old lime mortar has very little strength at all.
- Read the specific figures for the anchor you are using.Minimum edge distance, minimum spacing and minimum member thickness are published for every branded anchor, and they vary considerably with type. Where they are not published, that itself is a reason to be conservative.
- Use resin where you cannot find the distance.A bonded anchor exerts no outward pressure as it sets, so it will not split an edge. It still needs a sound cone to resist load, but it removes the splitting mechanism entirely.
- Move rather than compromise.Shifting a bracket 40mm to bring a fixing into the middle of a brick is almost always possible and always cheaper than the alternative.
- Watch for the invisible edge.The back face of the material matters too. An anchor drilled almost through a thin lintel, a paving slab or a single-skin wall can crack it from behind, and you will not see it until it fails.
Tips
- A crack radiating from an anchor to the edge of a brick is a warning about the whole installation, not a cosmetic problem with one fixing. Change the fixing type.
- The centre of a brick is the strongest place to fix into a brick wall, and it is usually only a few tens of millimetres from where you first marked.
- On block walls the same logic applies but the material is weaker throughout, so both distances and depths need to be larger.
Common mistakes
- Putting several anchors in a line into the same brick — The cones overlap and the brick itself becomes the weak element. Spread the fixings across several bricks, or use a batten to distribute the load.
- Fixing into the very end of a brick at a reveal or a corner — This is the least supported material in the wall, and an expansion fixing there frequently pops the corner off.