Why has my block paving sunk in ruts, and how do I re-lay it?

Block paving is a flexible pavement: the blocks are not glued to anything, and the whole surface works because every block is wedged against its neighbours and the field is held in by a restrained edge. Ruts are that system telling you which of its three layers has stopped doing its job. Reading the failure before you lift anything is what stops you doing the same work twice.

Safety

What you'll need

• Block lifters or two bolsters
• Plate compactor and a rubber protection mat, hired
• Sharp sand for the laying course
• Kiln-dried jointing sand
• MOT type 1 if the sub-base needs making up
• Screed rails and a long straightedge
• Rake, shovel, stiff brush
• String line and pegs

Step by step

1. Read what the ruts are telling you.A smooth shallow dip along the wheel track, blocks intact: the sharp sand laying course has been over-thick or was compacted before laying, and has consolidated. A sharp local hollow with cracked or tipped blocks: the sub-base has failed or was never thick enough. Joints opening and blocks fanning sideways: the edge restraint has moved or was never haunched.
2. Probe the depth before lifting.Lift two or three blocks at the worst point and dig down. You are looking for how deep the sand layer is and whether the material under it is firm angular stone or soft ground. What you find decides the size of the job.
3. Mark out an area bigger than the damage.Extend at least 500mm beyond the visibly sunken area in every direction. The material at the edge of a failure is already disturbed, and re-laying up to the exact edge of a rut leaves a joint that fails again.
4. Lift the blocks and keep them clean and sorted.Block lifters, or two bolsters worked into the joints either side. Stack them where they will stay clean — sand and mud on the block faces make re-laying slow and mark the finished surface.
5. Take out the laying course entirely.All the sharp sand comes out, down to the sub-base. It is contaminated and consolidated and it cannot be reused as a screed. Barrow it out.
6. Test and repair the sub-base.Walk it, dig into it, and run the compactor over it. It should be firm, angular and unyielding. If the plate sinks in or you find soft spots, dig them out and replace with MOT type 1 in 75 to 100mm layers, compacting each layer before the next. This is where the rutting came from and it is the step that makes the repair permanent.
7. Deal with the edge restraint if that is the fault.An edging that has tipped or moved has to be reset on concrete and haunched on the outside face to about two thirds of its height. Without a restrained edge the whole field creeps again regardless of what is underneath.
8. Screed a fresh laying course to a consistent 30 to 40mm.Sharp sand, screeded to a loose depth using rails and a straightedge, level to the surrounding blocks minus the block thickness minus the amount the blocks will drop under compaction. Do not compact the screed before laying — the blocks are vibrated into it later.
9. Re-lay the blocks to the existing bond, working off the laid surface.Never walk on the screeded sand. Work forward off the blocks you have laid, butting each up to its neighbours with a nominal 2 to 3mm joint, matching the existing pattern and lines.
10. Compact, joint and compact again.Two or three passes with the plate and a rubber mat, then brush kiln-dried sand into every joint, vibrate again, top up and repeat until the joints stay full. The sand in the joints is what makes the pavement interlock — an unjointed field has no strength at all.
11. Top up the joints for the next few weeks.Sand settles and washes out after the first rain. Brush more in once or twice in the first month, and again after any pressure washing.

Tips

• Photograph the bond pattern before lifting, especially at the edges where the cuts are. Reproducing a herringbone from memory is unnecessarily hard.
• Hire the plate compactor for the whole job rather than for the laying day. You need it on the sub-base as much as on the blocks.
• Keep a bag of matching blocks if you have any. Weathered block paving is very hard to match, and a few new ones scattered in read better than a patch of new in one place.
• Sweep loose sand off the surface before the final compaction pass, or the plate grinds it into the block faces.
• If the drive has always rutted since it was laid, it is a construction fault rather than wear. Deal with the depth of the sub-base, not the surface.

Common mistakes

• Lifting the blocks and adding sand to level them — It looks fixed for one summer. Sharp sand consolidates under load, and a thicker laying course consolidates more. The rut comes back deeper.
• Compacting the screeded sand before laying blocks — The blocks then have nothing to bed into. They sit high, the joints stay open and the surface never locks up.
• Re-laying without the edge restraint fixed — The field creeps sideways, the joints open, the jointing sand escapes and the whole failure repeats from the outside inwards.
• Using building sand for the laying course or the joints — Soft building sand holds water, will not drain and does not lock. Sharp sand for the bed, kiln-dried sand for the joints, and nothing else.

If it doesn't work

Questions people ask