The following post includes excerpts from our book Understanding Architectural Details – Residential.
Introduction to Floor Details
The most common materials used for the construction of ground and upper floors tend to be concrete or timber. Required span, resistance to passage of sound, and fire resistance will often be factors to consider when deciding which material to use in floor design.
The Building Regulations in the UK provide guidance on roof design. You can find the Building Regulations here.
Types of Ground Floors
There are two types of concrete floor system commonly used in residential construction; ground bearing and suspended. Ground bearing floors, where the concrete slab is supported by the ground below. Suspended concrete floors do not rely on the ground below, but instead are supported by the external walls and therefore the foundations below.
Ground Bearing Concrete Floor
The ground bearing concrete floor slab is supported by the load bearing strata below. The slab abuts but is not tied to the external walls. The build up of this type of system can typically be:
- Hardcore fill to compacted to create suitable level (approx 200mm thick)
- Damp proof membrane (DPM), often laid over sand binding
- Rigid floor insulation (can be position above or below the concrete slab)
- Concrete floor cast in situ
- Floor screed if required, followed by floor finishes
There are many different options for positioning of damp proof membrane and insulation which can be seen in the following details.
Suspended Concrete Floor
Suspended concrete floors have become popular in UK house construction. They are typically used in situations such as sloping sites, the ground has poor bearing capacity, or there is likelihood of ground volume change. Suspended floors are also used when the water table is high, or when aggressive chemicals are present in the soil that may attack the concrete slab.
The ground floor is made up of suspended concrete slab or beam and block floor, supported by the external and internal load bearing walls, which transfer the loads to the foundations below. Both pre-cast concrete planks or slabs can be used, as well as cast in situ reinforced slabs along with beam and block systems.
Beam and block floor
These systems use concrete T beams that span between the walls, concrete infill blocks are then inserted between the beams to provide a solid base for the concrete topping or screed that is spread and levelled over the concrete units. Depth of the beams is between 130mm and 250mm and is determined by the span required, and the loads to be supported.
Damp Proof Membrane (DPM)
Concrete is not impervious to water and it is therefore necessary to protect the floor with a water resistant barrier to prevent moisture penetrating the building from the ground. This barrier is a damp proof membrane usually in the form of a thin sheet of polythene or polyethylene, although bitumen in various forms can also be used.
The membrane position varies according to the type of floor construction selected. If the membrane is positioned below the concrete slab, it is placed over the hardcore on a sand blinding to protect it from damage. The edges of the membrane are turned up the sides of the external walls so that it overlaps the damp proof course. When using rigid, impermeable insulation, it is prudent to use a vapour control layer on the warm side of the insulation (or position DPM on warm side of insulation) to minimise the possibility of interstitial condensation. While concrete does offer a good level of natural resistance, poor workmanship can sometimes result in the passing of vapour through the structure. Likewise, if a timber floor is being used, it is important to install a vapour control layer under the timber floor.
Damp Proof Course (DPC)
A DPC acts as a barrier to the passage of moisture into the structure, be it upwards in wall foundations, downwards in a parapet or horizontally in a window jamb. Generally made from dense polythene, the DPC should not penetrate the cavity, unless weep holes are provided that can direct water away from the cavity. In a traditional masonry cavity wall the DPC is laid in two separate strips, one internal and one on the external leaf, 150mm minimum above external ground level. The DPC should be lapped with the DPM to provide a good joint and prevent moisture tracking between them.
Although timber ground floor construction used to be a popular method, today it is not as common as the concrete alternatives. A suspended timber floor is constructed as a timber platform of boards nailed across timber joists supported on sleeper walls, and the external and internal load bearing walls surrounding them.
Ventilation must be provided throughout the underfloor void and through any sleeper walls with cross ventilation. Due to airtightness, acoustic and sometimes structural requirements of the Building Regulations, joist ends tend to be supported by joist hangers rather than being built into external or sleeper walls.
The floor is insulated in the form of rigid insulation between joists, or quilt between joists supported by netting. Floorboards are fixed over the floor joists and insulation, usually in the form of plywood, chipboard or OSB to required structural performance. These boards are then applied with a floor finish, such as carpet, vinyl and so on.
Underfloor heating floor details
Here are a couple of details options for underfloor heating.
Notes from the Building Regulations on Roofs
General Requirements – Floors
The following information is a partial list of requirements from the Building Regulations Approved Documents – for full and detailed explanations and requirements please consult the full publications.
- without causing such deflection or deformation of any part of the building as will impair the stability of any part of another building.
- swelling, shrinkage or freezing of the subsoil
- landslip or subsidence
- will not impair the stability of any part of the building
- all load bearing elements of structure of the building shall be capable of withstanding the effects of fire for an appropriate period without loss of stability;
- ideally the building should be subdivided by elements of fire-resisting construction into compartments;
- all openings in fire-separating elements shall be suitably protected in order to maintain the integrity of the continuity of the fire separation;
- any hidden voids in the construction shall be sealed and subdivided to inhibit the unseens spread of fire and products of combustion, in order to reduce the risk of structural failure, and the spread of fire.
- ground moisture
- interstitial and surface condensation
- spillage of water from or associated with sanitary fittings or fixed appliances.
- does not affect the health of the occupants of the dwelling
- will allow them to sleep, rest and engage in their normal activities in satisfactory conditions
- limiting heat gains and losses through thermal elements and other parts of the building fabric,
- from pipes ducts and vessels used for space heating, space cooling and hot water services.
- providing fixed building services which are energy efficient and have effective controls and are commissioned by testing and adjusting as necessary to ensure they use no more fuel and power than is reasonable in the circumstances.
- Ensure the ground is covered so as to resist moisture and prevent plant growth.
- Have a ventilated air space between he ground covering and the timber.
- Have a damp proof course between the timber and any material which can carry moisture from the ground
- the transmission of impact sound (such as speech, music, loudspeakers and impact sources such as footsteps or furniture moving)
- the flow of sound energy through walls and floors
- the level of airborne sound
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