Ground Floors

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Ground Floors – Part One

Solid FloorsToday most houses are built with concrete ground floors. This form of construction has been popular since the end of the Second World War when restrictions on timber imports led to the decline of timber ground floors. However, in the Victorian period and in the years before the War ground-bearing floors, usually in concrete, were not uncommon. Towards the end of the 19th century, publications of the time refer to concrete and asphalt as an alternative to timber for ground floors. This form of construction was often reserved for basements or sculleries. Most living rooms had timber floors.

By the late 1930s it was still uncommon to find concrete floors throughout at ground level, but not unknown. Developers usually provided some form of timber finish although tiles were still available (fitted carpets were rare so some form of finish was essential). Two common finishes were wood block floors or boarding fixed to battens.

Black, white and multi-coloured floor tiles were popular until the 1930s. They were normally laid in mortar on a concrete base. Cost meant they were usually reserved for hallways and entrance lobbies.

In poor quality housing clay tiles were sometimes found. These were typically laid on a bed of ashes/hardcore and pointed with a cement or hydraulic lime mortar.

If the DPM was below the slab it was often poured or trowelled over a thin concrete blinding on top of the hardcore

Timber block floors or timber boards nailed to battens fixed to slab or secured by dovetailing into screed. The DPM was sometimes ‘sandwiched’ between the concrete slab (ie the slab was poured in two halves).

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Until 1960 most ground floors were, in fact made from timber (see later section). Since the 1960s floors have usually been made from insitu concrete, ie, poured on site. Early floors would not have contained any insulation. A typical floor from the 1960s to 1980s includes a hardcore base, a polythene damp proof membrane, an insitu concrete slab, and a floor screed to provide a smooth even finish for tiles or carpets. The damp proof membrane can be located above or below the slab. This is partly due to preference on behalf of the designer but may also take into account the need to isolate the slab from aggressive chemicals in the ground. Under normal conditions a concrete slab will not be harmed if it remains damp. If the DPM is laid over the slab it can be formed from liquids (special bitumastic paints, hot bitumen etc) as well as sheet materials. Laid properly, liquids give the best protection.

The hardcore fills and levels out the sub floor void, reduces capillary action and provides a level stable base for the concrete. A thin layer of sand called blinding can then be spread over the hardcore to provide a smooth bed for the polythene damp proof membrane. The concrete is normlly 100mm thick and may be reinforced with mesh if the slab is for a garage floor or if the ground below has soft spots or trenches below it. A non-structural sand/cement screed about 50mm thick provides the finish.

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During the last 20 years there has been a slow, but steady, improvement in levels of thermal insulation. Ground floors now contain insulation as a matter of course. One option, using a floating floor, is shown on the right.

This type of floating floor finish can be laid over ground bearing concrete slabs or pre-cast suspended floors (explained larter in this section).

Where floors are finished with sand/cement screeds insulation can be positioned above or below the slab. Some insulation materials can even be positioned below the damp proof membrane. If the screed is laid directly on the slab it can be 38mm or so thick. Where a screed is laid over insulation boards it will need to be thicker (possibly up to 65mm) and may even need to be reinforced to prevent cracking. Where insulation is laid below the slab it should also run round its edge, not just below it, as it is the edge where most heat loss occurs.

In this example the floating floor comprises sheets of chipboard (with glued tongued and grooved edges) laid over a vapour check, itself laid over insulation boards. The floor is not fixed down – hence the term floating floor. These floors have a good ‘feel’ about them and are no more expensive than screeds. The vapour check below the chipboard is to prevent moisture in the slab affecting the chipboard. A disadvantage of these floors is that they can be ruined by, say, a washing machine leak.

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Ground Floors – Part Two

Suspended Floors

Pre-cast concrete floors

In many cases the use of a concrete ground bearing slab is not suitable. These include:

where there are aggressive chemicals in the ground where water tables are high on sloping ground where uneven depth of hardcore may lead to differential settlement on ground

likely to heave where the hardcore is likely to be more than 600mm thick

In these situations a pre-cast floor is more appropriate. They usually comprise a series of pre-stressed beams with a block infill. The floors are supported by the external and internal load bearing walls and are not in contact with the ground. DPMs are not required as long as minimum recommended gaps between floor soffits and sub-soil are maintained. Screed or chipboard floor finishes can be specified as outlined elsewhere in this section.

Suspended Timber Floors

In the Victorian period most houses had raised timber floors. They varied in quality, some of the better examples are still functioning perfectly well today. However, may of the poorer examaples have long since been replace with new timber or concrete floors. A typical example of a floor with a number of potential problems is shown below. The floor comprises a series of timber joists built into the external walls and supported mid span by sleeper walls to help keep the joists depth to a minimum. Air vents in the external walls allow air to circulate in the sub floor void to help combat damp and condensation, both of which can ultimately lead to dry rot in the timbers. The photo on the right shows a typical floor void – often very shallow and very close to the earth.

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By the 1930s raised timber floor construction had improved. The sleeper walls were honey-combed to aid the flow of air. The joist ends sat on a sleeper wall and did not touch the external load bearing wall. All the walls include DPCs, and a concrete oversite (no lower than external ground leve)l protects the floor from damp, vermin etc. Nowadays these floors are rare but not unknown. Modern raised timber floors can be built in a number of ways. The oversite can be a 100mm concrete slab or a 50mm layer of concrete blinding on a DPM. The joists can be supported on sleeper walls or on joist hangers. Insulation can be in between the joists or above them. Boarding is usually in the form of chipboard or strand board.

A typical floor from the 1930s to 1950s

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In modern construction timber floors are becoming more and more popular. The construction is similar to that of 70 years ago although there are five main differences:

joists are likely to span across an whole room – no sleeper walls are usually required nowadays. joists will be supported on hangers rather than built into the walls. timbers are nowadays usually treated against rot and insect attack. rules for ventilation are more onerous than in the past. most floors will be finished with chipboard and floors will be insulated with a quilt or boards,

usually positioned in between the joists

Questions – the answers are all in the above text.Solid floors

1. What is the function of a dpm and where can it be positioned? How can it be protected?

2. What is the function of a floor screed and what determines its thickness?

3. Describe typical construction of a floating chipboard floor (including slab etc)

4. Where can insulation be positioned in a ground bearing floor with a screed finish?

Suspended floors

1. When is a ground bearing slab not suitable?

2. Briefly describe the construction of a pre-cast concrete floor.

3. What improvements were made to raised timber floors in the 1920s and 1930s.

4. How do modern timber floors differ from those built 70 years or so ago?