1. Suspended Floor
A suspended floor is referred to “the floor that is raised and supported above the ground”. 1 Mostly suspended floors are used for upper storeys and are supported by columns, beams or walls. But also, they can be used to ground floors supported by piles. 2
In the 1700s the suspended floor began widely used on the ground floor to prevent the damp in the interface between wall and floor, yet the timber was laid directly onto the earth and this led to decay of the timber members.
During the Edwardian and Victorian era, suspended floors grew more popular and were supported off dwarf walls or sleeper walls. These walls were composed of little or no foundations and were most frequently constructed of brick to the underside of the joists. Air bricks or vents with metal cast iron covers was an external suggestion when there was suspended floor, but older houses usually had problems with dump because of the poor ventilation.
In the 1900s, the introduction of damp-proof courses signified the reducing of timber ground-floor joists and dwarf or sleeper walls were constructed using staggered brickwork, which added gaps and allowed air to flow across the floor, therefore the reducing of the damp. Although a typical Victorian terraced house had a solid concrete floor in the kitchen and hallway and the remaining ground floors were suspended. 3
During the first 20 years of the 20th century, several damp related improvements change the suspended timber floors. The entire floor was separated from the substructure by the DPCs and the bare earth was covered by a concrete slab, which was at, or above, external ground level to prevent the gradual increasing of water. Moreover, the slab prevented the growth of plants. The floor joists were supported by honeycombed sleeper walls, through which air could pass easily and the joists did not have contact with the external wall. Owing to the fact that most of these houses were detached or semi-detached, rather than terraced, the underfloor void was comparatively easy to ventilate (picture 1.1). 4
Despite this, by the 1950s the solid concrete floors were used extensively on the ground floor. 3
In nowadays, it is common to use suspended concrete floors in certain conditions and when the ground bearing slab is not proper. However, many developers would rather use suspended floors in all situations due to the noted risks of ground bearing floors. Until the 1970s, this type of floors were usually constructed from in situ concrete, but they were slow to construct and high-priced. These days, the floors most of the time are made from a series of beams with an inverted ‘T’ shape, 150-200mm thick and infill with a concrete block.
Before 2004, the ventilation of the underfloor space was necessary only in the situation when the ground was not well drained or if there was a risk of gas build-up. The Building regulations now require that the underfloor space should be vented, but DPMs are not required provided that the minimum recommended gaps between floor soffits and sub-soil are maintained. 4
1.3 Types of suspended floor
1.3.1 Suspended timber ground floor
The suspended timber floor’s construction usually consist of either floorboards or sheets, supported by timber joists, which are typically placed across the shortest span and they bear on wall plates which in rotation spread the load evenly along the sleeper walls. A DPS is installed between sleeper walls and the wall plate to block the moisture from the ground to soak the timberwork. 5, 6
1.3.2 Suspended concrete ground floor
Suspended concrete floor can be constructed with; beam and block, in situ reinforced slab and precast planks or slabs. The suspended concrete floor is usually used in slopes, in poor ground or when it has uncertain bearing capacity and where the ground is liable to volume change. 7
It is common to construct suspended concrete floors using the concrete block and the inverted T-beam method. The ends of the inverted T-beam are supported on the inner skin of the cavity wall or by separate sleeper walls. Furthermore, better mid span support can be provided by using intermediate sleeper walls. The beams are evenly spaced and receive the concrete infill blocks, which are laid edge to edge and are supported on the beam’s jutting bottom edge (picture 1.2). 5
1.3.3 Suspended timber upper floor
The timber upper floor consists of bridging joists supported at each of the ends by load bearing walls. The top of the joists is covered by floorboarding or sheeting to provide the floor surface and on their bottom are covered by plasterboard to form the ceiling. The end of the joists can be supported either by building them into the internal leaf of the cavity wall or by using metal joist hangers. If the floor requires openings for stairs, chimneybreasts etc., the joists which are around the opening must be trimmed or framed. 5
1.4 Advantages and Disadvantages
One of the advantages of the suspended floor is that it can usually be laid and loaded easily and in short duration. Also, it is more suitable for sloping sites as there is no need of extensive cutting and filling that would be required with a ground supported floor. Another advantage is that suspended floor is not affected by ground heave or clay shrinkage. Furthermore, the subfloor void is available for positioning services.
The suspended timber floor is lighter than the ground-supported floor, thus it helps reduce the dead load of the building. The suspended timber floor can give more comfort for carpeting the floor, as well as when the floor is well insulated, it can create a degree of sound proofing when it is used in upper floors. 8, 9
The suspended concrete floor has more benefits for the future as there will be little or no maintenance required. Cheaper construction on a sloping site rather than using a solid floor, which requires walls to support the ground underneath, is another advantage. 9
On the other hand, suspended timber floor condition can easily get worse and eventually the boards often come a little loose and they can start to get a few squeaks and creaks. Also, another problem associated with suspended floor is drafts.
Other disadvantages are that the suspended floor system requires a lot of handling and experienced professionals when being fitted as well as it is a lot more expensive than solid floor system. 10