Replace a floor joist.

Replacing or repairing joists is a critical maintenance task for UK homeowners, particularly in older period properties where damp-proofing may have failed. Addressing these issues early prevents localized damage from spreading into a costly whole-house structural problem.

What Does the Work Involve?

• Inspection: Assessing the extent of the damage and identifying the root cause, such as a leaking pipe or rising damp.
• Support: Installing temporary Acro props and "strongboy" supports to take the load while the structural timbers are removed.
• Access: Lifting existing floorboards or removing ceiling plasterboard from below to expose the joist run.
• Removal: Cutting out the defective timber sections and clearing the wall sockets (pockets) of debris.
• Installation: Fitting new C24-grade pressure-treated timbers, often using galvanized joist hangers or "sistering" new beams to existing healthy ones.
• Treatment: Applying fungicidal or insecticidal spray to surrounding timbers to prevent the reoccurrence of rot or infestation.
• Reinstatement: Replacing floorboards and ensuring the surface is level and squeak-free before tidying the site.

Typical Costs

Prices are heavily influenced by access; ground floor joists with a crawl space are often cheaper to fix than upper floor joists that require ceiling removal and replastering. All prices include VAT at 20%.

How Long Does It Take?

• Single Joist Repair: 4 to 6 hours.
• Small Room (e.g., Bathroom/Hallway): 1 to 2 days.
• Large Living Area or Full Floor: 3 to 5 days.
• Drying Time: Allow an extra 24 hours if chemical damp treatments or new screeds are applied.

DIY or Professional?

This is strictly professional territory. Floor joists are structural elements; an incorrect installation can lead to floor collapse or significant damage to the building's envelope.

Note: While a DIYer can lift floorboards to inspect damage, the actual cutting and seating of structural timber must be done by a qualified carpenter to ensure compliance with UK Building Regulations.

Choosing the Right Tradesperson

• Look for NVQ Level 2/3 in Carpentry & Joinery or membership in the Institute of Carpenters (IOC).
• Ensure the contractor holds Public Liability Insurance of at least £1 million.
• Red Flags: Quotes that don't mention timber grading (e.g., C24) or contractors who suggest "patching" rot rather than removing it.
• Ask: "Will you be using pressure-treated timber?"
• Ask: "How will you ensure the new joists are level with the existing floor?"
• Ask: "Are you familiar with Part A of the Building Regulations?"

UK Regulations

• Building Regulations Part A (Structure): Any work affecting the structural integrity of a floor must comply with these standards.
• Timber Grading: Replacement joists should typically be C24 grade for strength, though C16 may be used for shorter spans.
• Notification: Major structural changes may require notification to your Local Authority Building Control (LABC).
• Paperwork: Retain all invoices and timber treatment certificates for your home's "Gold Command" file, as these are vital when selling the property.

Common Problems

• Hidden Services: Accidentally cutting through gas pipes or electrical cables hidden within the joist notches.
• Inadequate Bearing: Failing to seat the joist deeply enough into the wall (usually 100mm minimum) or using poor-quality hangers.
• Ignoring the Source: Replacing the timber without fixing the damp or leak that caused the rot in the first place.
• Incorrect Notching: Weakening new joists by cutting holes for pipes in the wrong zones (notches should only be in the top 1/8th to 1/4 of the span).

Frequently Asked Questions