What is a movement joint in brickwork, and does my wall need one?

A movement joint looks like a gap somebody forgot to fill, which is why so many of them get pointed up by a well-meaning owner or a decorator. It is a designed weakness: the wall is going to move, and the choice is between moving at a joint you put in or cracking at a point it selects for itself, usually at a window corner.

Step by step

1. Understand why brickwork moves.Clay bricks take up moisture after firing and expand irreversibly over the first years, then continue to move seasonally with temperature and moisture. Concrete blockwork does the opposite and shrinks as it dries. Long unbroken runs of either accumulate enough movement to crack.
2. Recognise a movement joint on a wall.A straight vertical line running the full height of a panel, usually about 10 to 20mm wide, filled with a flexible sealant rather than mortar. Often in a corner, beside a window, or at a change in wall height or direction, where it is least visible.
3. Know roughly where they are needed.Spacing is a design matter and differs between clay brickwork and concrete blockwork, with clay generally needing them more often. They are also needed where a wall changes height, thickness or direction, at junctions between different materials, and near openings that would otherwise concentrate stress.
4. Never fill one with mortar.A movement joint pointed up in hard mortar is no longer a movement joint. The wall then moves anyway and cracks somewhere else, and the mortar in the joint crushes and falls out. If you find a raked-out vertical gap in sound brickwork, find out what it is before filling it.
5. Build it properly if you are forming one.A clean vertical gap through the full thickness of the leaf, with a compressible filler board to keep debris out and support the sealant, a backing rod, and a flexible sealant tooled to the face. Wall ties across a joint must be of a type that allows movement.
6. Maintain the sealant.Sealant in a movement joint has a life of years rather than decades. Cracked or perished sealant lets water into the joint. Cut it out and replace it rather than smearing new sealant over the old.
7. Do not add one to an existing wall as a repair.Cutting a movement joint into an existing wall that has already cracked is a structural intervention, not maintenance. If a wall is cracking from thermal movement it needs assessing, because the same symptom is produced by causes that are far more serious.
8. Look for one before diagnosing a crack.A straight vertical crack in modern brickwork in a position where a movement joint should be usually means one was omitted or was later filled with mortar. That is a different and much less alarming diagnosis than foundation movement.

Tips

• Long garden walls need movement joints too, and they are frequently left out, which is why so many long boundary walls have a vertical crack at some arbitrary point.
• Movement joints in a cavity wall must go through both leaves, and are often in different positions in each leaf.
• If a straight vertical gap appears in your brickwork with clean sealant in it, it is almost certainly a designed joint doing its job — not damage.

Questions people ask