Heather Campbell
A quick guide to subfloors and their role in a self build project.
In this article we cover:
- What a subfloor is and its function in construction.
- Materials commonly used in subfloor construction.
- Steps to build a subfloor, including insulation and heating.
- Differences between subfloor types, including concrete slabs and thermal screeds.
A subfloor is a critical structural layer in any building, placed directly on top of the foundations to provide a stable, flat base for the finished floor covering.
It acts as the foundation for everything above, ensuring the floor is level, strong and supports the building’s weight.
Often, this process often starts with hardcore (stone filling), followed by blinding (a layer of sand or weak concrete to create a level surface).
For homes in areas prone to radon gas, a radon barrier is installed to prevent harmful gases from entering the living space.
[adrotate banner="58"]Finally, a thick concrete slab (100-150mm) reinforced with steel mesh is poured to support the structure above.
Once the house is weather-tight, the next steps above the subfloor involve adding insulation boards, typically in two layers.
The first, thinner layer leaves space for running services — a task that can be DIY but is time-consuming.
Next come the underfloor heating pipes, usually installed by a plumber if required, followed by a screed for the final finish.
The screed can be liquid (hemihydrate, which needs sanding to remove laitance) or concrete.
Some subfloor systems combine the insulation within the screed for improved thermal performance.
Some self-builders argue that there’s no need for a concrete slab.
Instead, they place insulation directly above the radon barrier and cover the underfloor heating pipes with a thick concrete screed.
Always check your specific requirements and building regulations compliance with your engineer.
As a rule of thumb, the thicker the screed, the slower the response time for heating with underfloor systems.
English