Concrete floor systems in building construction

What are the best concrete floor systems?

Choosing the perfect material for your floor is a difficult one. You want something that’s strong, rigid, durable, soundproofed and suitable to accommodate heating.

These are all attributes that concrete floor systems can offer.

Originally popular within commercial construction, residential applications are becoming more common as builders explore ways to make it affordable for smaller projects.

Taking insulation and labour into account these are the average costs you may expect when looking into floor material options.

  • Concrete - £56/m²
  • Suspended timber - £58/m²
  • Block and beam - £81/m²

This is all very much dependent on the type of house build you’re looking at and whether it’s an extension to an existing structure or something being factored into a new build. Ground preparation is another factor to consider, as while on initial assessment block and beam may appear expensive, that can quickly be evened out if the installation can be done with less intensive preparation work.

In an ideal world, you’d be able to crane in a ready-made flooring system, complete with the necessary voids for ventilation and service pipes. However few sites will allow this luxury, so make your choice carefully before you begin and weigh up the pros, cons and associated costs of each.

Concrete floor mix options

The type of floor mix you choose depends very much on what you plan to put on top of it. Certain types of screed make tiling tricky, whereas others are less likely to give you a perfectly polished look if you’re leaving them exposed. Others are ideal for underfloor heating systems, helping to reduce energy bills by retaining heat for longer.

A screed mix is made up of cement, sand and fine aggregates. A smooth, free-flowing mix will be around one part cement to four parts sand, but that ratio can be adjusted depending on the specifics of each build.

  • Traditional screed - Cement and sand creating a surface suitable for tiles or vinyl. This usually has to be manually levelled.
  • Self-levelling screed - Most commonly seen in larger industrial units. The free-flowing nature is good for levelling, however, it’s not ideal for additives meaning less flexibility in application.
  • Fast drying screed - The industry benchmark is to allow a dry time of 1mm per day, Various screed mixes now contain additives to help reduce that so it doesn’t slow project progress.

As a rough guide, 1 tonne of 4mm washed sharp sand mixed at 4:1 with cement will cover approximately 15m² of 40mm thick floor. You can tell if you have the mix right by grabbing a handful screed and squeezing. It should stay as a firm lump with very little liquid coming out.

Once you’ve decided on the right concrete floor screed, you also have a decision of where and how to mix it.

Mixing by hand requires considerable effort so should only be used as a last resort, while readymade free-fall mixing has proven to be problematic for maintaining the required ratios to avoid cracking.

That leaves using a mobile volumetric concrete mixer which mixes sand, stone, cement and water on site in the exact ratios required, producing the exact amount of concrete needed.

Unless temperatures are reaching freezing point, it’s possible to pour screed, before leaving it to cure for 28 days. That time period is an ideal scenario and not one that all construction projects can accommodate. At the very least, screed should be firm enough to walk on before commencing the next stage which is likely to be around the 3-day mark. If you can hold out to 7 days, that should give you a solid enough base to build on.

As screed cures, a weak layer of fine particles — known as laitance — is deposited onto the surface. This is the most common failure point for flooring, and not removing this effectively before attempting to put any kind of adhesive on top will cause issues further down the line.

Concrete floor levelling tools and techniques

It goes without saying that the more attention you put into constructing your concrete floor up front, the less likely to are to encounter difficulties further down the line.

Levelling is one of those areas that requires considerable care.

It’s actually possible to use more than one type of screed within your construction project, especially where different types of flooring may need to be laid on top. A self, levelling screed may be an option for an area of underfloor heating where you don’t want to disturb the system with manual levelling techniques.

For manual levelling, the timber technique is by far the most common and practical.

This involves placing a spirit level on top of timber and systematically working your way across the floor, allowing you to see areas which need to be packed with extra mix.

A self-levelling compound does away with the manual effort, however, it does come with own complexities and disadvantages, the main one being its bonding capabilities with the subfloor. The subfloor needs to be spotless before you begin as any particles will affect how well it sticks, and even when it has bonded, it doesn’t provide additional structural integrity to the floor.

The fast drying time — as little as 20 minutes in some cases — means you need to have all your equipment ready to go as soon as the pouring process begins.

What’s the right screed thickness for floor insulation?

Every screed mix comes with its own advice when it comes to thickness. The optimum option for standard insulation is usually between 25–40mm but that may increase to 50mm if it’s unbonded and up to as much as 75mm if it’s expected to take very heavy loads.

If you’re laying underfloor heating, each system varies in terms of its depth requirement so read the guidelines carefully. The manufacturer may even provide a screed mix when supplying that’s specifically designed to work with their equipment.

How deep you pour obviously have a huge effect on the drying time, so factor this in when planning your project. If your project does require extra depth on the screed, it’s worth considering:

  • Whether you can wait for it to dry in layers before adding extra screed on top to make up the required depth
  • Whether screed is the right material to be used — once you go beyond 75mm there may be a more structurally sound alternative to consider before finishing with a screed layer on top

Building a concrete staircase

While stairs are technically part of the floor and many of the same rules apply, they do need some special consideration, especially when they’re going to be formed from concrete.

Taking into account the elevation, width and depth of your stairs, you should be able to estimate the amount of concrete that you’ll need.

Whether you’re using ready-mix concrete or pouring from a portable mixer, it makes sense to fill from the top of the form staircase to the bottom, smoothing as you go so you don't disturb steps as you work your way down and things can be left to settle and cure.

Depending on your design, a precast concrete staircase may be an easier and less time-consuming option. Available in standard or bespoke configurations, you can have integrated landings, a curved concrete staircase, or handrails set into the structure. All this is possible with poured concrete too, so long as you’re prepared to do the additional engineering on site.