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PAVING & DECKING

Paving Slab Sunken

A sunken paving slab usually means the bedding or sub-base beneath it has compacted, washed out or been disturbed.

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Make trip and fall hazards safe first

Loose slabs, rotten decking and slippery surfaces can cause serious injury. Stop using elevated or structurally soft decking until it has been assessed.

MOST LIKELY CAUSES

Start with movement, support and drainage

Sub-base settlement Poorly compacted material has compressed.
Water erosion Drainage has washed away bedding or fines.
Ground movement Soft soil or roots have changed support.
Heavy loading The paving was not built for the imposed weight.
◫ Symptom variations Match the movement, cracking or surface condition. ⌄
  • Puddle forms over one slab
  • Trip edge has developed
  • Joint cracks around the slab
  • Area sinks further after rain
  • Several nearby slabs are low
✓ Quick checks Safe checks before lifting slabs or boards. ⌄
  1. Check the extent of the low area.
  2. Measure levels and fall.
  3. Inspect nearby drainage.
  4. Look for soft ground.
  5. Check whether vehicles or heavy loads use the area.
Important: Fix the support, drainage or structural cause before replacing surface materials.
1 Step 1 — Lift the affected slabs Make the smallest safe correction and retest. ⌄

Remove them carefully and keep reusable units safe.

2 Step 2 — Rebuild and compact the base Make the smallest safe correction and retest. ⌄

Replace weak material and restore the correct depth.

3 Step 3 — Relay to proper fall Make the smallest safe correction and retest. ⌄

Set the slabs evenly and repoint once stable.

⌁ Why the problem happens How support, water and movement affect paving and decking. ⌄

Paving follows the support beneath it. When that support compresses or washes out, the surface sinks.

⌘ Advanced checks Further diagnosis for persistent outdoor surface faults. ⌄
  1. Check sub-base thickness.
  2. Assess soil bearing condition.
  3. Inspect drain leaks nearby.
  4. Review edge restraints.
  5. Check load classification.
◇ Tools and information needed Useful items for accurate checks. ⌄
  • Slab lifters
  • Compactor
  • Spirit level
  • Shovel
  • Rubber mallet
▣ Parts or materials that may help Common paving and decking materials linked to this fault. ⌄
  • Sub-base aggregate
  • Bedding mortar
  • Jointing compound
  • Replacement slabs
  • Edge restraints
! When to call a professional Conditions that should end basic DIY diagnosis. ⌄
  • The sinking affects a large area.
  • The patio is beside the house or steps.
  • A drain may be leaking.
  • Vehicle loading is involved.
  • Ground movement is ongoing.
? Frequently asked questions Common paving and decking questions. ⌄

Can I pack the low corner only?

That often creates uneven support and another failure.

Why does it sink after rain?

Water may be softening or washing out the support.

Should I replace the slab?

Only if it is cracked or damaged; the base is usually the real issue.