Floor Levelling Explained: An Expert Guide
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The right floor levelling method depends entirely on your property’s structural subfloor. In this expert guide, we break down how we level both suspended timber joists (common in Victorian and Edwardian homes) and concrete subfloors.
Neven pine floorboards on suspended timber joists in a Victorian home before structural subfloor levelling and hardwood installation
How to level an uneven timber subfloor (Suspended Joists)
What causes sloping & wonky floors in older UK houses?
Most Victorian, Edwardian, and Georgian homes in London were built without deep modern foundations. Because many sit on clay or former marshland, ground settlement over time causes brickwork and timber joists to shift.
Additionally, internal partition walls added during past renovations are often overloaded onto unreinforced joists, causing them to bow significantly under the weight
Diagram of a Victorian brick foundation showing simple construction methods that make the building prone to ground settlement and subsidence.
Diagram of a modern foundation with deep footings, damp-proofing, and insulation designed to prevent ground settlement and subsidence.
Levelling workflow
When addressing uneven suspended timber joists, our primary approach is always to use custom timber firrings rather than sistering the joists. While this master carpentry method requires a higher degree of precision and takes longer to execute on-site, it adds significantly less weight to historic joists and drastically reduces long-term timber shrinkage.
Below is an the step-by-step overview of the process we follow:
Step 1 of floor levelling: removing old carpets and floor coverings to fully expose the original softwood pine floorboards underneath
Step 1: Expose the original subfloor
Removing all existing floor coverings—such as carpets, laminate, or overlay wooden floors—to fully expose the original pine floorboards.
Step 2 of floor levelling: cutting inspection pockets into floorboards to inspect subfloor joists and map floor height variations
Step 2: Cut inspection pockets & map floor heights
Before removing any floorboards, we cut small inspection sections into the boards above the joists. This allows us to access the subfloor structure to map the height variations across the entire surface, and locate the highest point, which will serve as the level benchmark..
Step 3: assessing architectural impacts on fireplace hearths, door heights, skirting, and radiators before raising subfloor levels.
Step 3: Assess the architectural implications and effects
Bringing the lowest areas up to the highest point alters the surface geometry. Before proceeding, we assess the impact on fireplace hearths, door heights, skirting boards, radiator clearance, electrical sockets and other specific elements..
Step 4: removing pine floorboards in controlled, sequential sections to protect fragile ceilings and clear debris between joists.
Step 4: Floorboards removal
Although it may seem intuitive to remove all the floorboards at once and expose all the joists we rather work in sections. This becomes all the more important when working on the upper floor. To protect the ceiling directly below and prevent structural accidents, we remove the original floorboards in controlled, sequential sections. Once a section is open, we vacuum and clear the cavities of decades of debris—especially if you are choosing to add acoustic or thermal insulation between the joists.
Step 5: installing custom taper-cut timber firrings onto exposed joists to level the subfloor without adding excessive weight.
Step 5: Install custom firrings (or sistering)
For each exposed joist, we custom-cut a tapering, wedge-shaped strip of structural timber (a firring) to bridge the gap between the joist's dip and our new level line. If a joist is structurally weakened or undersized, joist sistering will be the solution, fixing a new timber joist to its side to level and reinforce it simultaneously.
Step 6: laying structural WBP plywood sheets over firrings to create a rigid, unified subfloor deck ready for wooden flooring.
Step 6: Subfloor reconstruction with structural plywood
Once the opened subfloor section is corrected, WBP hardwood plywood will be screwed down, using full size sheets as much as possible. Unlike old softwood pine floorboards that act independently, the large plywood sheets tie entire joist sections together into a rigid, unified deck.
The Steps 4 through 6 will be repeated section by section until the entire subfloor is transformed into a sound, flat foundation ready to receive the new wooden floor.
Cracked historic ceiling in a period London property caused by structural movement and moisture damage before floor levelling.
Protecting historic lath & plaster ceilings
Working on the upper levels of period properties also requires an understanding of historic building techniques.
In most older London properties, the ceiling beneath the joists is made of original lath and plaster. Over a century or more, these ceilings become fragile. When the property has suffered from roof leaks, plumbing floods, or structural movement, the plaster keys holding the ceiling to the timber laths become incredibly brittle. Sometimes, they are so weak that minor, unavoidable structural vibrations can trigger failures.
Diagram showing a damaged historic lath and plaster ceiling with broken plaster keys caused by age, moisture, and structural vibration.
Transparency from Experience: Our years in the field have taught us just how unpredictable historic plaster can be. On one project in a first-floor flat, a week after we successfully completed a level floor installation, the original lath-and-plaster ceiling in the window bay directly beneath collapsed. Our structural levelling work was likely 'the straw that broke the camel's back' for a ceiling already failing from decades of hidden moisture. On another occasion, the flat below developed hairline cracks in the ceiling that extended into the upper wall plaster.
Because of these real-world structural realities, we take several precautions: The sectional approach: As outlined in Step 4, we only open the floor in small, controlled zones to minimize structural flexing across the joists.
Pre-project inspection & communication: We highly advise assessing the condition of the ceilings below before work begins, particularly if you live in a converted flat.
Careful mechanical fastening: When securing firrings, plywood, and structural elements, the vibrations are to be minimized by adapting the work sequence and using a variety of tools and equipment.
Technical comparison diagram showing joist firring using lightweight custom timber vs joist sistering using heavy full-size timber beams.
Why Firrings Over Sistering?
Sistering involves attaching full-size, heavy timber beams along the entire length of your existing floor structure. Firrings use only the exact amount of wood needed to compensate for the low areas, minimizing dead weight on your historic joists. Reduced long-term shrinkage: All new construction timber contains an amount of moisture and will experience shrinkage as it acclimates to a heated home, particularly in the winter. Because firrings utilize a much smaller cross-sectional area of new timber compared to sistering, potential long-term wood shrinkage is drastically reduced. This ensures your floor stays flat and sound
