Slab filling

Addressing these voids early prevents the concrete from cracking or collapsing under its own weight. Professional slab filling involves injecting specialised materials into these gaps to stabilise the foundation, often saving thousands of pounds compared to a full floor replacement.

What Does the Work Involve?

• Site Inspection: A builder uses ground-penetrating radar (GPR) or simple "thump" testing to map the extent of the void and locate underground services like water or gas pipes.
• Access Preparation: Small-diameter holes (typically 12mm to 20mm) are drilled through the concrete slab at strategic intervals to create injection points.
• Port Installation: Injection packers or ports are fitted into the holes to ensure a tight seal for the high-pressure filling equipment.
• Material Injection: A cementitious grout or a high-density expanding structural resin is pumped into the void until the cavity is completely filled.
• Monitoring: Builders use laser levels or dial gauges to monitor the slab in real-time, ensuring the void is filled without accidentally lifting the floor too far and causing wall cracks.
• Making Good: Once the material has cured, the ports are removed, and the drill holes are filled with a matching colour-coded mortar to leave a clean finish.

Typical Costs

The cost of slab filling depends heavily on the volume of the void and the material used. Resin injection is typically faster but more expensive per litre than traditional cementitious grout.

Prices in 2025/26 are influenced by the accessibility of the site. If a pump needs to be run through a house to reach a rear conservatory, expect labour costs to sit at the higher end of the scale.

How Long Does It Take?

• Small Voids (e.g., Garage Corner): 3 to 5 hours from arrival to cleanup.
• Standard Domestic Room: 1 full working day, including setup and monitoring.
• Large-Scale Subsidence: 2 to 3 days if multiple rooms or deep injection is required.
• Curing Time: Expanding resins cure in minutes, while cement-based grouts may require 24–48 hours before heavy furniture can be replaced.

DIY or Professional?

Slab filling is strictly a professional job. It requires specialist high-pressure pumping equipment and precise monitoring tools that are not available for standard tool hire. Attempting this as a DIY project carries a high risk of "slab heave," where too much pressure is applied, causing the floor to lift unevenly and damage the internal walls of the property.

Professional builders carry the necessary indemnity insurance to cover structural movement, which is essential for maintaining your home's long-term value and insurance validity.

Choosing the Right Tradesperson

When looking for a builder to fill a slab, you should look for specialists in structural repairs or ground engineering rather than general handymen.

• Check Qualifications: Look for membership in the Federation of Master Builders (FMB) or the Property Care Association (PCA).
• Insurance: Ensure they have specific Public Liability and Professional Indemnity insurance that covers structural works.
• Red Flags: Be wary of tradespeople who suggest filling voids without first checking for the cause (e.g., a leaking drain) or those who don't use monitoring equipment.

Questions to ask:

• Will you be using GPR or a CAT scanner to locate my water and gas pipes before drilling?
• Which material is better for my specific soil type: cementitious grout or expanding resin?
• Do you provide a guarantee against the fill material shrinking or degrading over time?
• Can you provide a post-work report showing the volume of material injected?

UK Regulations

• Building Regulations Part A: If the void filling is being done to rectify structural subsidence, you may need to notify Building Control.
• Health and Safety: Contractors must follow COSHH regulations when handling chemical resins and ensure dust suppression is used during drilling.
• Waste Management: Any chemical waste or large amounts of spoil must be disposed of via a licensed waste carrier in accordance with UK environmental laws.

Common Problems

• Service Damage: Accidental drilling into underfloor heating pipes or drainage lines is a major risk if the area isn't scanned first.
• Over-filling: Excessive pressure can cause the slab to lift, resulting in doors jamming and cracks appearing in the plasterwork above.
• Material Escape: If the void is connected to a drain or a neighbour's property, the filling material can travel where it isn't wanted, leading to expensive blockages.
• Ignoring the Root Cause: Filling a void caused by a burst pipe without fixing the leak first will only lead to the problem returning within months.

Frequently Asked Questions

How do I know if there is a void under my floor?

The most common signs are a hollow sound when walking, visible gaps appearing between the floor and the skirting boards, or unexplained cracks in floor tiles.

Is resin injection better than concrete grout?

Resin is cleaner, faster, and less invasive, making it ideal for finished homes. Grout is often cheaper for very large voids but introduces significant moisture and requires larger drill holes.

Will slab filling fix my sloping floor?

While the primary goal is to fill the gap, "slab jacking" can sometimes be used to lift the floor back to level. This is a more complex process and should be discussed with your builder beforehand.

Can I stay in the house while the work is being done?

Yes, usually. The process is relatively non-invasive, though there will be some noise from drilling and the hum of the pump located outside.

How long will the repair last?

Modern structural resins and grouts are designed to be permanent. Most reputable UK contractors will offer a guarantee of 10 to 25 years on the material and workmanship.