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PAVING & DECKING

Cracked Paving Slab

A cracked paving slab may have suffered impact, frost, poor bedding, settlement or excessive loading.

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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

Voids beneath the slab Unsupported sections flex and crack.
Impact or heavy load The slab exceeds its strength.
Sub-base settlement Uneven support stresses the slab.
Frost and water ingress Moisture expands within defects.
◫ Symptom variations Match the movement, cracking or surface condition. ⌄
  • Single crack across slab
  • Corner has broken away
  • Crack widens under load
  • Water sits in the crack
  • Replacement slabs also fail
✓ Quick checks Safe checks before lifting slabs or boards. ⌄
  1. Check whether the slab rocks.
  2. Inspect neighbouring slabs.
  3. Look for local settlement.
  4. Assess loading in the area.
  5. Check drainage and frost exposure.
Important: Fix the support, drainage or structural cause before replacing surface materials.
1 Step 1 — Lift the damaged slab Make the smallest safe correction and retest. ⌄

Avoid disturbing sound surrounding units.

2 Step 2 — Repair the bedding and base Make the smallest safe correction and retest. ⌄

Remove voids and compact support.

3 Step 3 — Install the replacement Make the smallest safe correction and retest. ⌄

Match level, fall and joint width.

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

Slabs are strong in compression but vulnerable when part of the underside is unsupported.

⌘ Advanced checks Further diagnosis for persistent outdoor surface faults. ⌄
  1. Check slab thickness against loading.
  2. Inspect sub-base compaction.
  3. Assess tree-root movement.
  4. Check bedding coverage.
  5. Review drainage falls.
◇ Tools and information needed Useful items for accurate checks. ⌄
  • Bolster
  • Slab lifter
  • Rubber mallet
  • Spirit level
  • Safety gloves
▣ Parts or materials that may help Common paving and decking materials linked to this fault. ⌄
  • Replacement slab
  • Bedding mortar
  • Sub-base aggregate
  • Jointing material
  • Edge restraint
! When to call a professional Conditions that should end basic DIY diagnosis. ⌄
  • The slab is on steps.
  • The crack affects a large heavy unit.
  • Many slabs are cracking.
  • Vehicle loading is involved.
  • Ground movement is suspected.
? Frequently asked questions Common paving and decking questions. ⌄

Can I fill the crack?

That is usually temporary and may remain visible or fail again.

Why did a new slab crack?

Poor bedding or unsupported voids are common causes.

Should I replace only the slab?

Also repair the support beneath it, or the problem may return.