Floor joists are the backbone of your home’s structural integrity. When they rot, your entire floor system becomes compromised. This guide shows you exactly how to identify, assess, and repair rotten floor joists safely and effectively.
Quick Answer: Repairing rotten floor joists involves removing damaged sections, treating the cause of rot, and either sistering new timber alongside existing joists or completely replacing severely damaged ones. Most repairs require Building Regulations approval and professional structural assessment.
Floor Joist Rot: What You’re Dealing With
Floor joists are horizontal wooden beams that support your floorboards and distribute weight to the walls. In the UK, most homes built before 1980 use traditional timber joists, typically 150mm x 50mm or 200mm x 50mm sections.
Rot occurs when three conditions meet:
- Moisture content above 20%
- Temperature between 5-40°C
- Oxygen present
Types of Wood Rot Affecting Floor Joists
Wet Rot (Coniophora puteana)
- Most common in UK homes
- Causes timber to become soft and spongy
- Wood darkens and develops a musty smell
- Affects only damp timber
Dry Rot (Serpula lacrymans)
- More serious but less common
- Spreads through masonry
- Causes timber to crack in cube patterns
- Can affect dry timber once established
Signs Your Floor Joists Need Attention
Visual Indicators
- Bouncy or sagging floors
- Gaps appearing between floorboards
- Visible fungal growth on timber
- White cotton wool-like growth (dry rot mycelium)
- Dark staining on joists
Physical Tests
- Push a screwdriver into suspected timber
- Healthy wood should resist penetration
- Rotten wood feels soft or crumbles
- Check moisture levels with a digital meter
Professional Assessment Required
Contact a structural engineer if you notice:
- Cracks in walls above affected areas
- Multiple joists showing damage
- Any signs of dry rot
- Floors dropping more than 25mm
Tools and Materials You’ll Need
Essential Tools
- Reciprocating saw or handsaw
- Drill and bits
- Spirit level
- Moisture meter
- Safety equipment (goggles, gloves, dust mask)
- Crowbar
- Measuring tape
Materials Required
- Treated timber (same dimensions as existing joists)
- Joist hangers and screws
- DPC membrane
- Timber treatment solution
- Coach bolts or structural screws
- Metal restraint straps (if required)
Building Regulations Compliance
Most floor joist repairs require Building Regulations approval. Contact your local authority before starting work. You’ll typically need:
- Structural calculations
- Approved drawings
- Building control inspection
Step-by-Step Repair Methods
Method 1: Sistering (For Minor Damage)
This technique involves attaching new timber alongside damaged joists.
Step 1: Prepare the Work Area
- Remove floorboards above affected joists
- Clear the area of insulation and debris
- Ensure adequate lighting and ventilation
Step 2: Assess the Damage
- Mark the extent of rot with chalk
- Measure the span and load requirements
- Check adjacent joists for similar damage
Step 3: Install the Sister Joist
- Cut new timber 600mm longer than the damaged section
- Position alongside the existing joist
- Ensure proper bearing on both ends (minimum 100mm)
- Bolt together every 600mm with M12 coach bolts
Step 4: Secure the Connection
- Use galvanised joist hangers where appropriate
- Install metal restraint straps if specified
- Check levels with a spirit level throughout
Method 2: Partial Replacement (For Moderate Damage)
When more than 30% of a joist is damaged, partial replacement is often better.
Step 1: Support the Structure
- Install temporary supports (Acrow props)
- Support the floor above with timber bearers
- Never remove structural timber without adequate support
Step 2: Remove the Damaged Section
- Mark cutting lines 300mm beyond visible damage
- Cut square using a reciprocating saw
- Remove the damaged timber completely
Step 3: Install New Section
- Cut replacement timber to exact length
- Treat all cut ends with preservative
- Connect using steel plates or timber splice joints
- Bolt connections with minimum M12 fixings
Step 4: Restore Load Paths
- Ensure proper bearing on supporting walls
- Check alignment with string line
- Install any required bridging or blocking
Method 3: Complete Replacement (For Severe Damage)
When a joist is more than 50% compromised, complete replacement is necessary.
Step 1: Structural Calculations
- Obtain engineer’s calculations for new joist sizes
- Confirm load requirements and spans
- Check building regulations compliance
Step 2: Temporary Works
- Install comprehensive temporary support system
- Support both the floor above and below
- Mark all services before cutting
Step 3: Removal Process
- Disconnect all services attached to the joist
- Remove in sections if access is limited
- Clean the bearing areas thoroughly
Step 4: Installation
- Position new joist accurately
- Use appropriate joist hangers or built-in bearings
- Connect to adjacent structure as specified
- Install bridging and restraints
Treating the Root Cause
Repairing joists without addressing moisture sources guarantees future problems.
Common Moisture Sources
- Leaking pipes or radiators
- Poor ventilation under suspended floors
- Rising damp through walls
- Condensation from lifestyle factors
- External water ingress
Ventilation Solutions
- Install airbricks in external walls (one per 1.5m of wall length)
- Ensure 150mm minimum void under joists
- Remove any debris blocking airflow
- Consider mechanical ventilation in problem areas
Damp-Proofing Measures
- Install DPC under new timber bearings
- Treat all new timber with approved preservative
- Ensure 50mm gap between timber and masonry where possible
- Use treated wallplates where direct contact is unavoidable
Building Regulations and Safety Requirements
When You Need Approval
- Structural alterations to floor systems
- Replacement of load-bearing elements
- Changes affecting fire resistance
- Work in buildings built before 1980 (potential asbestos)
Key Standards to Meet
- BS 5268: Structural timber design
- Building Regulations Part A: Structural safety
- Building Regulations Part C: Resistance to moisture
Professional Requirements
Consider hiring professionals for:
- Structural calculations
- Building control liaison
- Complex repairs affecting multiple joists
- Any work involving dry rot treatment
Cost Considerations and Planning
Typical UK Repair Costs (2024)
| Repair Type | Materials Cost | Labour Cost | Total Range |
|---|---|---|---|
| Minor sistering (per joist) | £30-50 | £150-250 | £180-300 |
| Partial replacement | £50-80 | £200-350 | £250-430 |
| Complete replacement | £80-120 | £300-500 | £380-620 |
| Professional survey | – | £300-600 | £300-600 |
Factors Affecting Cost
- Access difficulties
- Number of joists affected
- Requirement for temporary works
- Building regulations fees (£200-400)
- Additional treatments required
Money-Saving Tips
- Combine repairs with other renovation work
- Source timber from local suppliers
- Hire plant equipment daily rather than purchasing
- Group multiple repairs together
Prevention: Keeping Your Joists Healthy
Regular Maintenance Schedule
Annual Checks:
- Inspect visible timber for staining or damage
- Check moisture levels in vulnerable areas
- Ensure ventilation remains unobstructed
- Look for signs of pest activity
Five-Year Maintenance:
- Professional moisture survey
- Check condition of DPC and ventilation
- Assess effectiveness of previous treatments
- Update insulation if required
Early Warning Systems
- Install moisture alarms in high-risk areas
- Monitor heating bills (increased costs may indicate structural movement)
- Keep records of any minor repairs
- Photograph any concerning changes
Common Mistakes to Avoid
Structural Errors
- Removing load-bearing timber without adequate support
- Undersizing replacement timber
- Poor connections between old and new elements
- Ignoring Building Regulations requirements
Treatment Mistakes
- Treating symptoms rather than causes
- Using untreated timber in damp conditions
- Inadequate preparation of bearing surfaces
- Rushing the drying process
Safety Oversights
- Working without proper PPE
- Inadequate temporary support
- Not checking for services before cutting
- Disturbing asbestos without proper precautions
When to Call Professionals
Immediate Professional Help Required
- Any signs of dry rot
- Structural movement or cracking
- Multiple joists affected
- Work involving party walls
Warning Signs You’re Out of Your Depth
- Calculations become complex
- Building control raises concerns
- Access requires specialist equipment
- Insurance claims are involved
Choosing the Right Professional
- Look for RICS or CIOB membership
- Check insurance and qualifications
- Request detailed written quotations
- Verify Building Regulations experience
Frequently Asked Questions
How long do floor joist repairs typically last?
Properly executed repairs using treated timber should last 50+ years, provided the moisture source is eliminated and adequate ventilation maintained.
Can I repair floor joists myself?
Minor repairs are possible for competent DIYers, but structural work typically requires professional expertise and Building Regulations approval.
How do I know if rot has spread to other joists?
Conduct systematic moisture testing of all timbers in the area. Wet rot typically stays localised, but dry rot can spread extensively.
What’s the difference between wet rot and dry rot treatment?
Wet rot treatment focuses on moisture elimination and local repairs. Dry rot requires extensive investigation, specialist treatment, and often affects multiple rooms.
Q: Do I need to move out during floor joist repairs? A: Minor repairs usually allow continued occupation. Extensive work may require temporary relocation, particularly if floors become unsafe or services are disconnected.
Can floor joist damage affect my home insurance?
Gradual deterioration is typically excluded, but sudden damage (like burst pipes) may be covered. Report significant structural damage promptly.
How can I prevent future joist rot?
Maintain adequate ventilation, fix leaks immediately, monitor moisture levels, and conduct regular inspections of vulnerable areas.
What timber should I use for replacement joists?
Use C24 grade timber, preferably treated with preservative. Engineered timber (I-joists or LVL) may be specified for longer spans.
Conclusion
Repairing rotten floor joists is a serious undertaking that directly affects your home’s structural integrity and your family’s safety. While minor repairs may be within reach of experienced DIYers, most floor joist work requires professional expertise and Building Regulations compliance.
The key to successful joist repair lies in three critical areas: accurate assessment of the damage, addressing the underlying moisture problem, and executing repairs to current structural standards. Cutting corners on any of these aspects typically leads to recurring problems and greater expense.
Remember that prevention is always better than cure. Regular inspections, prompt attention to moisture issues, and maintaining adequate ventilation will protect your floor joists for decades. When problems do arise, early intervention usually means simpler, less expensive repairs.
If you’re dealing with rotten floor joists, start with a thorough assessment of the extent of damage and the root cause. Don’t hesitate to seek professional advice – the cost of proper repairs is insignificant compared to the potential consequences of structural failure.
Your home’s floor system is designed to last a lifetime with proper care. By understanding how to identify, repair, and prevent joist rot, you’re protecting both your investment and your family’s wellbeing for years to come.