Building Regulations: Floor Construction

When you build an extension, its floor isn't just a surface; it needs to be strong enough to support itself, everything inside the room, and everyone who uses it. If you're constructing a ground floor, it also has crucial jobs like preventing moisture from the ground from getting in and stopping heat from escaping (this is called thermal insulation).

Understanding Ground Floor Construction Types

Solid Floor Construction

This is a common choice for ground floors. Here's what a typical solid floor involves:

• First, you'll lay a robust hardcore base.

• Next, a layer of sand blinding covers the hardcore.

• Then, a concrete slab is poured on top.

• Finally, a sand and cement screed creates a smooth, level surface for your finishes.

It's vital to incorporate both a proper damp proof membrane (DPM) to stop moisture and thermal insulation to prevent heat loss. You can install these either directly over the sand blinding or on top of the concrete slab.

Ensure your DPM connects correctly with the existing damp proof course (DPC) in both the outer walls and any relevant internal walls surrounding your floor. The specific thickness of each floor component will vary depending on the ground conditions where you're building and the exact order you layer them.

Important consideration: If your current house uses air bricks to ventilate voids under its existing floor, you'll need to install ducting right through your new solid floor. This allows air to continue flowing into the void beneath the original house. You will then place new air bricks into the extension wall itself.

Suspended Timber Floor Construction

If you opt for a suspended timber floor, the Building Regulations insist on specific measures:

• You must first lay a concrete slab over the ground to stop weeds and other plant growth from coming up.

• Crucially, maintain a ventilated gap of at least 150mm between the underside of your timber joists and this concrete layer. This air gap is essential to prevent moisture from damaging your timber.

When you're installing timber floor joists, make sure they are correctly sized. Their size depends on their 'span' – this is the distance they stretch between their supports. Generally, you'll lay them across the shortest distance from wall to wall. Sometimes, to shorten the span and reduce the required joist thickness, you might need to build an extra internal wall with a small foundation.

Remember to insert a damp proof course (DPC) between any timber and the wall. You will then fit insulation material between the joists; its thickness will depend on the product you choose. Finally, you must install air vents to create cross-ventilation throughout the void beneath the floor, ensuring air flows freely from one side of your extension to the other.

Suspended Concrete Floor Construction

A suspended concrete floor shares many principles with its timber counterpart, but it uses concrete components. You'll either use pre-cast concrete planks or small pre-cast concrete beams with concrete blocks placed between them.

One advantage is that these concrete systems can typically span longer distances compared to timber joists. Just like with suspended timber floors, you'll need to provide adequate ventilation. For accurate sizing of beams, manufacturers often supply the necessary calculations, or you can always get expert advice from a structural engineer.

Addressing Contaminated Ground and Radon Gas

You need to be aware that in some specific locations, the ground might be contaminated with harmful gases, for example, from old landfill sites. Additionally, naturally occurring radon gas can be a concern in certain regions. If your site falls into one of these areas, you must take precautions.

To protect your extension, you will need to implement specific ventilation strategies and install a special gas membrane. These measures are crucial to stop these hazardous gases from entering your new building.