How to level a sloping floor

#CraftedForLife

A common problem

One of the most frequent questions we are asked is, “I have a Victorian property with a sloping floor and would like it levelled. What are my options, and what would you recommend?” Our recommendation is to lift the existing floorboards and build up the subfloor to create a strong, stable, and truly level base. This approach addresses the root of the issue and provides a solid foundation for the new floor, ensuring longevity and performance.

Some companies suggest pouring a self levelling compound (SLC) directly over the existing floorboards. While this is undoubtedly a quicker and less involved solution, it simply leaves the original boards in place and levels over the top. Although this option can work in theory, there are several important factors that should be carefully considered before choosing to use SLC.

Improved self levelling compound

A few years ago, using SLC in this way would have been an impractical solution, as only basic SLCs were available and these required extensive preparation, typically over a sound concrete substrate. Construction technology has since moved on, and today there are highly advanced self levelling compounds that can be applied over a much wider range of subfloors.

While modern SLCs are far more adaptable and versatile than before, they are still not entirely universal. Each floor still needs to be carefully assessed to ensure the correct self levelling product and correct preparation method are used for a long lasting, high quality result.

Cracked Self Levelling Compound

The construction of a traditional suspended timber subfloor is relatively simple, consisting of timber joists with pine floorboards nailed across them. Pine is a softwood, which makes it naturally flexible. In older properties, these boards are often laid without a tongue and groove joint, meaning each board moves independently.

As a result, movement is especially noticeable when walking between the joists. This ongoing flexing and deflection is then transferred to the self levelling compound above, which is rigid by nature. Over time, this difference in movement almost inevitably leads to cracking within the SLC.

The need for a plywood subfloor

One effective way to overcome this issue is by introducing an intermediate layer over the existing floorboards. Our preferred solution is to install hardwood plywood, typically a minimum of 12mm thick, and often thicker where required. This creates a consistent, continuous surface that stabilises the original boards, ties them together, and bridges the gaps between them.

Before fitting the plywood subfloor, it is essential that the original floorboards are properly secured. This often involves adding extra fixings into the joists to eliminate movement, ensuring a solid and reliable base for the layers above.

Filling in the gaps

Self levelling compound is liquid, so any gaps between the floorboards would allow it to flow straight through. To prevent this, all perimeter edges around the plywood, particularly where it meets the walls, must be sealed with expansion foam to stop the SLC from dripping or running below.

Reducing the height of the room

The next stage is the application of the self levelling compound. At the highest points, the SLC should be laid at a minimum thickness of around 2 to 3mm to provide adequate structural strength. From there, it gradually builds up towards the lower areas, often reaching around 10mm. A high quality product such as Arditex NA can typically be laid up to this depth in its standard form. Anything beyond 10mm usually requires the addition of a coarse aggregate to maintain stability.

Additional weight

This build up of SLC and the supporting layers can add a significant amount of extra weight to the floor structure, often around 100 kg and in some cases considerably more, even approaching a tonne, concentrated over the joists. This places unnecessary load on both the subfloor and the joist structure. The key question is whether the building was ever designed to carry that amount of additional weight in one specific area.

Whilst increasing the weight of the floor is technically feasible, in practice we strongly advise against this approach, as it risks compromising the structural integrity of the property. The added weight can cause joists to sag or bow over time, resulting in an uneven, dipped surface, which ultimately defeats the original aim of levelling the floor.

This method is often chosen because it is cheaper and faster, and because it avoids the need to open up the subfloor, navigate pipework, or risk disturbing existing cables. However, the short term convenience can come at the expense of long term performance and safety.

Levelling the floor without Self Levelling Compound

We have extensive guidance on our website explaining how Victorian subfloors can be levelled correctly such as Floor Levelling Explained: An Expert Guide. Our approach typically involves removing the existing pine floorboards, then adding firrings to the joists or sistering the joists so each one can be individually adjusted as needed. Once the structure is properly levelled, we install new 18 mm hardwood plywood to form a strong and stable base.

Although this process requires more time and labour, it allows us to preserve as much of the room height as possible. By relying on skill and experience rather than excessive build up, we keep the floor level low and maintain the original proportions of the space. While advances in construction technology may continue to improve SLC based solutions, we remain committed to crafting for life and delivering the best long term outcomes.