Updated
Block Paving Edges Spreading Outwards
The outer rows of blocks have crept outwards, the joints have opened up, and the pattern no longer lines up.
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What's happening?
Block paving works by interlock. Every block leans on its neighbours, and the whole pavement pushes outwards under a wheel, so something has to hold the perimeter. That is the edge restraint: a kerb, an edging course or a concrete haunching bedded and backed with concrete around the entire edge. When it is missing, too shallow, or its haunching has been cut away, the pavement has nothing to push against. The edge rows creep outwards a few millimetres at a time, the joints open, sand escapes, and the loss of interlock spreads inwards from the perimeter. Once the pattern has slipped, the blocks nearest the edge start to rock and chip, and cutting them back in is the only way to close the gaps.
What usually causes it?
- No edge restraint at all, with the outer blocks simply butted against soil, gravel or a lawn
- An edging course bedded on sand or mortar without concrete haunching behind it
- Haunching dug away later for a border, a fence line, a cable or a planting bed
- Jointing sand lost from the perimeter joints, so the outer blocks are no longer locked together
- Vehicles turning or mounting the edge, which loads the perimeter far harder than driving straight over it
- The sub-base stopping short of the edge, so the last row is bearing on unconsolidated ground
- Ground softening behind the edging after prolonged wet, letting the restraint rotate outwards
Is it dangerous?
The immediate risks are minor — opened joints catch a toe, a rocking edge block can tip, and loose blocks near a road or footway can be dislodged. What makes it worth acting on is that this fault does not stay at the edge. Interlock lost at the perimeter migrates inwards, so a strip of movement becomes a spreading area of loose, rocking blocks, and the repair grows with it. It also opens a direct route for water to reach the bedding and sub-base.
Can I fix it myself?
Establish what is holding the edge. Dig behind the outer row with a trowel: you are looking for a concrete haunching against the back of an edging unit. Sand, soil or nothing means there is no restraint. Measure the widest joint at the edge and mark the date, so you can see whether the movement is ongoing. Refill the perimeter joints with kiln-dried sand as an interim measure and keep them full. Do not fill an open joint with mortar to hold the blocks — a flexible pavement is meant to move slightly, and a rigid patch cracks and traps water. Rebuilding an edge restraint properly is a concrete and levels job.
When should you call a driveway and patio installer?
Call a driveway specialist once the pattern has visibly slipped, when the joints at the edge are wide enough to lose sand as fast as you refill them, or when the movement is reaching in beyond the first couple of rows. Make it sooner if the edge adjoins a footway or road, or if a vehicle regularly turns on it. Left alone through a wet winter, an unrestrained edge takes the rest of the pavement with it.
What will the driveway and patio installer actually do?
- Excavate behind the perimeter to confirm whether an edge restraint and haunching exist and how far the sub-base extends
- Lift the displaced blocks and reset the pattern back to true lines rather than filling the gaps
- Install kerbs or an edging course bedded and haunched in concrete around the full perimeter
- Extend and compact the sub-base to the edge where it stopped short
- Cut in closure blocks accurately so the perimeter is tight, and avoid narrow slivers that work loose
- Re-sand and vibrate the joints to restore interlock across the whole area
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