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PAVING & DECKING

Paving Slab Loose

A loose paving slab usually has failed bedding, poor edge support, washed-out material or movement in the sub-base.

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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

Failed bedding The mortar or laying bed has broken down.
Poor edge support The slab can rock because surrounding support is weak.
Water washout Drainage has removed bedding material.
Weak sub-base The layer below has compacted or shifted.
◫ Symptom variations Match the movement, cracking or surface condition. ⌄
  • Slab rocks underfoot
  • Joint lines crack repeatedly
  • One corner lifts
  • Water collects around the slab
  • Movement worsens after frost
✓ Quick checks Safe checks before lifting slabs or boards. ⌄
  1. Mark which corners move.
  2. Check surrounding joints and levels.
  3. Inspect drainage and nearby low spots.
  4. Lift only when safe to do so.
  5. Check whether the bedding is full or spot-laid.
Important: Fix the support, drainage or structural cause before replacing surface materials.
1 Step 1 — Lift and clean the slab Make the smallest safe correction and retest. ⌄

Remove loose bedding and debris from both surfaces.

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

Compact the base and provide a full suitable bed.

3 Step 3 — Reset and finish Make the smallest safe correction and retest. ⌄

Lay to the correct level and fall, then repoint joints.

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

A slab needs even support across its underside. Voids and weak spots allow rocking, cracking and water ingress.

⌘ Advanced checks Further diagnosis for persistent outdoor surface faults. ⌄
  1. Check sub-base depth.
  2. Inspect edge restraints.
  3. Measure surface fall.
  4. Assess frost damage.
  5. Check whether nearby roots are lifting the area.
◇ Tools and information needed Useful items for accurate checks. ⌄
  • Rubber mallet
  • Bolster and lifting tools
  • Spirit level
  • Trowel
  • Safety gloves
▣ Parts or materials that may help Common paving and decking materials linked to this fault. ⌄
  • Bedding mortar
  • Sub-base material
  • Jointing compound
  • Replacement slab if damaged
  • Edge restraint
! When to call a professional Conditions that should end basic DIY diagnosis. ⌄
  • The slab is very large or heavy.
  • Several slabs are moving.
  • The area is on steps or an access route.
  • Drainage levels are wrong.
  • The sub-base has failed across a wide area.
? Frequently asked questions Common paving and decking questions. ⌄

Can I just add sand underneath?

Only if the paving system was designed for a sand bed. Many slabs need a full mortar bed.

Why does it rock again after repair?

The sub-base or drainage may still be defective.

Should the slab be perfectly level?

It should match the surrounding surface and maintain the correct drainage fall.