5.1 Substructure and ground bearing floors
SCOPE
DESIGN STANDARDS
Design that follows the guidance below will be acceptable for substructure and ground bearing floors.
STATUTORY REQUIREMENTS
Design should be in accordance with relevant Building Regulations and other statutory requirements.
TRANSFER OF LOADS
Studies of the site, carried out in accordance with Chapter 4.1 'Land quality - managing ground conditions' should be taken into account in the design of substructure.
Where the depth of infill exceeds 600mm, the floor must be designed as a suspended floor, as described in Chapter 5.2 'Suspended ground floors' (Design).
Loadbearing partitions should not be supported off ground bearing floors. They should have proper foundations (reference should be made to Chapter 4.4 'Strip and trench fill foundations' (each section)).
In Scotland, sleeper walls should not be built off ground bearing floors.
Movement joints should be aligned with those in the foundations. Details of movement joint design are given in Chapter 6.1 'External masonry walls' (Design).
GROUND CONDITIONS
Items to be taken into account include:
The main hazards likely to affect substructure and ground bearing floors are chemicals, particularly sulfates, contaminated material above or in the ground and waterlogged ground.
In certain parts of the country, special precautions may be necessary to reduce the entry of radon gas. Areas in England and Wales where special precautions are necessary are detailed in BRE Report 211.
When precautions are necessary they should be acceptable to NHBC.
Ground bearing floors may only be used where:
- trenches are backfilled with suitable material and the fill is properly compacted
- infill is less than 600mm in depth and properly compacted.
Where the bearing capacity and nature of the ground varies, a ground bearing floor may not be suitable, even if the depth of fill is less than 600mm. Special measures may be needed to restrict settlement, such as the use of suspended floor construction.
Shrinkable soil, expansive materials or other unstable soils may require suspended floor construction. Shrinkable soils are classified as those containing more than 35% fine particles (silt and clay) and have a modified Plasticity Index of 10% or greater (see Chapters 4.2 'Building near trees' (each section) and 5.2 'Suspended ground floors' (each section)).
A soil-testing laboratory should be consulted to verify the plasticity index of the soil.
Sloping ground may require steps in the substructure and possibly different floor levels. Where more than 600mm of infill is required at any point in a self-contained area, the floor over the whole of that area must be of suspended construction as described in Chapter 5.2 'Suspended ground floors' (Design).
Construction on steep slopes may involve walls below dpc level acting as retaining walls. These should be designed by an Engineer where the:
- height (H) of slab above ground level is greater than 4 times the total width of wall (T)
- difference between floor levels of adjacent structures (H) is greater than 4 times the total width of wall (T).
Special precautions may be needed to prevent damage to the substructure from site operations on adjoining ground, such as ground treatment or surcharging due to infill.
SERVICES AND DRAINAGE
Items to be taken into account include:
Surface and/or land drainage may be needed on sites where there is a risk of waterlogging. Ground or paths adjoining the dwelling should slope away at a slight fall. Ground or path level should be at least 150mm below dpc.
Walls which act as retaining walls may require land drains, hardcore fill and suitable outlets to dispose of any sub-soil water that collects behind the wall.
All existing services should be located and identified before work commences. Where existing services would be obstructed by the foundations and substructure, they should be:
- disconnected and grubbed up, or
- diverted and any remaining voids filled with concrete or grout, or
- protected where they are to remain active.
To avoid flooding around, in or under dwellings, existing active ground water drainage should be retained. Water from these drains may require diversion.
It is very difficult during a dry period to find out whether ground water drains are active, so where they are severed or disturbed, they should be re-connected to a suitable outfall.
It is important that the design drawings include all necessary details relating to the proposed underground services (see Clause D17).
Drain pipes passing through or under the building may require flexible connections or other means of accommodating differential movement. Further details are given in Sitework clause 5.1 - S2(e) and Chapter 5.3 'Drainage below ground' (Design and Sitework).
Services should be arranged so that future access, if required, can be obtained without affecting structural stability.
WALLS BELOW DPC
Generally, masonry walls below dpc should be designed and constructed as described in Chapter 6.1 'External masonry walls' (each section) and sleeper walls as described in Chapter 4.4 'Strip and trench fill foundations' (each section).
Recommendations for the design strength of bricks, masonry blocks and mortars are given in BS EN 1996-1-1.
Frost damage occurs on saturated masonry exposed to freezing conditions. Bricks, blocks and mortars which are located between 150mm above, and 150mm below ground level, are the most likely to be damaged by frost.
BRICKWORK
Fletton or common bricks are usually of durability designations F1,S2 or F1,S1. If in doubt as to their suitability, bricks of F2,S2 or F2,S1 designation should be specified or the manufacturer consulted.
Where bricks of grade F1,S2 or F1,S1 are to be used in the outer leaf below dpc, or where they could be frozen when saturated, it is most important to ensure that they are durable.
If there is any doubt about the suitability of a particular brick, written confirmation should be obtained from the brick manufacturer concerning its suitability for the:
- geographical location
- location in the structure.
Calcium silicate bricks for use below dpc should be at least compressive strength class 20.
Bricks used in walls which act as retaining walls should be of a type recommended by the manufacturer for the conditions of exposure.
BLOCKWORK
Concrete blocks for use below dpc should comply with BS EN 771 and have:
- a density exceeding 1500kg/m3, or
- a compressive strength not less than 7.3N/mm2.
Where necessary, to resist sulfate attack and ensure adequate durability, special blocks made with a higher than normal cement content and/or with sulfate-resisting cement should be used.
If there is any doubt about the suitability of a type of block, particularly where acids or sulfates occur, written confirmation should be obtained from the block manufacturer concerning its suitability for the:
- geographical location
- location in the structure.
MORTAR
The selection of mortar for use below dpc should follow the recommendations given in BS EN 1996-1-1. Alternatively, the mix may be 1 : 1 : 5½, cement : lime : sand, with plasticiser.
Sulfate-resisting cement should be used where recommended by the brick manufacturer and where sulfates are present in the ground or ground water.
Proprietary mortars and admixtures should only be used strictly in accordance with the manufacturer's recommendations, taking into account the type of masonry unit and its location.
For non-clay bricks or blocks, manufacturers' recommendations should be followed.
Walls which act as temporary retaining walls, due to the sequence of backfilling trenches and filling the wall cavity, should be designed:
- as retaining walls, or
- by an Engineer in accordance with Technical Requirement R5, or
- so that the thickness of the leaf acting as the temporary retaining wall is as given in Sitework clause 5.1 - S2(b).
The drawings and/or specification should be detailed accordingly.
GROUND BELOW FILL
Before fill is placed, all topsoil containing roots and vegetation should be removed and a suitable bearing surface prepared.
HAZARDOUS FILL
Details of materials, test requirements and sources of fill material are given in Appendix 5.1-A.
Precautions should be taken to avoid adverse effects by either:
- ensuring that made ground and fill materials are free from harmful or toxic substances, or
- designing the construction to contain, resist and prevent adverse effects of such materials, by means acceptable to NHBC.
Types of fill which require special precautions to be taken are given in Appendix 5.1-A.
Where sulfates or other harmful chemicals are present in the ground at levels likely to be harmful:
- concrete for the floor slab should be:
- of the appropriate mix to resist sulfate attack, and
- protected by an impervious layer of 1200 gauge (0.3mm) polyethylene sheet (or 1000 gauge (0.25mm) if assessed in accordance with Technical Requirement R3 ) which may also serve as a dpm
- mortar should be in accordance with the recommendations of BS EN 1996-1-1
- concrete blocks should have a sulfate resistance appropriate for the level of sulfate in the fill or ground.
Where expansive materials are present, a suspended floor system should be used.
FILL DEEPER THAN 600mm
Ground bearing slabs are not acceptable where fill exceeds 600mm in depth.
Where the depth of fill exceeds 600mm at any point within a self-contained area, the floor construction over the whole of that area is required to be independent of the fill and capable of supporting:
- self weight
- partitions
- other imposed loads.
For details, reference should be made to Chapter 5.2 'Suspended ground floors' (Design).
FLOOR SLAB DAMP-PROOFING
Items to be taken into account include:
Ground bearing concrete floor slabs should be protected against ground moisture by providing a continuous membrane, details of which are given in the Materials section. Care should be taken not to trap moisture when a combination of damp proofing and vapour control layers are used.
When the membrane is located below the slab, a blinding layer of sand should be provided to fill voids in the hardcore and so minimise the risk of puncturing the membrane.
The continuity of the membrane should be maintained as follows:
- laps in polyethylene should be at least 300mm and joints sealed, where necessary
- membranes beneath the slab should link with wall dpcs to form an impervious barrier to prevent moisture reaching the interior of the dwelling
- linking should take account of possible differential movement.
A clear cavity for at least 225mm below dpc should be maintained. When specialised foundations are used, including those for timber framed buildings, this depth may be reduced to 150mm below dpc if weep holes are provided and other necessary measures are taken to ensure that the cavity can drain freely.
Where ground water can exert pressure, land drainage may be necessary to prevent water entering the dwelling. At changes in floor level, eg stepped separating walls, special attention is required to ensure the continuity of the dpm.
Horizontal and vertical tanking should link with wall dpcs in a manner similar to a dpm.
DAMP-PROOF COURSE
A damp-proof course should be positioned at least 150mm above finished ground or paving level.
Horizontal dpcs should be impermeable. They should be either lapped (at least 100mm) or welted, where appropriate, and, in all cases, linked to the dpm.
Where dwellings are 'stepped' on a sloping site, care should be taken to link dpcs and dpms properly, so that all parts of each dwelling are protected.
THERMAL INSULATION
The BRE Report 'Thermal insulation: avoiding risks' discusses aspects of insulation relevant to ground bearing floors. In England and Wales account should be taken of Accredited Details.
Items to be taken into account include:
Thermal insulation materials for use below ground bearing slabs are given in the Materials section.
For details of thermal insulation above ground floor slabs, reference should be made to Chapter 8.3 'Floor finishes' (Design).
Where cavity insulation batts or slabs start below dpc level, the vertical and horizontal spacing of wall ties should be compatible with the spacing to be used above dpc level.
For details of insulating masonry walls, reference should be made to Chapter 6.1 'External masonry walls' (each section).
The design should ensure that any risk of cold bridging is minimised, giving particular attention to junctions between floor and external walls.
Precautions include:
- extending cavity insulation below floor slab level
- linking floor and wall insulation
- providing perimeter insulation to floors
- facing supporting substructure with insulation.
Where dwellings are stepped or staggered, the wall forming the step or stagger may require insulation.
GROUND FLOOR SLAB
Appropriate mixes for ground bearing concrete floor slabs are specified in Chapter 2.1 'Concrete and its reinforcement' (Design).
Ground bearing concrete floor slabs should be not less than 100mm thick, including monolithic screed where appropriate.
BASEMENTS
The design should take account of the BCA Approved Document "Basements for dwellings". Its principles should be followed in England, Wales, Scotland and Northern Ireland.
In this clause the term "basement" means construction which is wholly or partly below ground level and for which normal damp proofing arrangements are inappropriate.
Items to be taken into account include:
All basements should be designed by an Engineer in accordance with Technical Requirement R5.
Information from the site investigation, carried out in accordance with Chapter 4.1 'Land quality - managing ground conditions' should be taken into account in the design of basements.
Reference should be made to Chapter 4.2 'Building near trees' where trees, hedgerows or shrubs are present.
The design of the basement should take account of the current and future ground water conditions. Where it is uncertain what the future ground water conditions may be the waterproofing system should be designed to withstand the full hydrostatic head.
Any existing land drains which are disturbed by the basement excavation should be diverted to a suitable outfall. See Clause D5.
Walls and floors below external ground level should resist moisture from reaching the internal surfaces of walls or the upper surface of a floor.
The design should ensure that the level of protection against water and moisture reaching the internal surfaces is appropriate for the proposed use. Where there is any doubt about use, the level of protection required for habitable accommodation should be provided.
Basements to be used for habitable accommodation should be designed to allow no water penetration and provide a dry environment if maintained by adequate heating and ventilation. This is referred to as "Grade 3" in the "Basements for dwellings" Approved Document.
Walls and floors to basements to be used for parking cars, for storage or as plant rooms should be designed to allow no water penetration (unless a type C drained cavity) although moisture vapour is tolerated. This is referred to as "Grade 2" in the "Basements for dwellings" Approved Document.
The design should ensure that all materials and products used in the construction of a basement are compatible and used strictly in accordance with the manufacturer's recommendations.
Proprietary waterproofing materials should comply with Technical Requirement R3.
Appendix 5.1-B shows generic basement constructions that may be acceptable to NHBC subject to appropriate detailing.
They are:
- Type A tanked protection. The water resistance is achieved by waterproofing. This system is not suitable where the water table is either variable or high and the basement walls are masonry. See "Basements for dwellings" Approved Document.
Note
Internal tanking is generally not acceptable.
-
Type B structurally integral protection. The water resistance is achieved by the design of the concrete construction. An additional moisture barrier may be necessary.
This system is not suitable where the water table is either variable or high and the design is to BS EN 1992-1-1 unless there is additional waterproofing. -
Type C drained cavity. The water resistance is achieved by collecting any water in the internal cavity system. An additional moisture barrier may be necessary.
This system is reliant on collecting and disposing of any water within the cavity system to a suitable outfall. Any sumps and/or pumps will need to be accessible for maintenance.
The number of services passing through basement waterproofing should be kept to a minimum.
The design should detail how any penetrations for services prevent water or damp ingress.
Further details of services and drainage are given in Clause D5 and Appendix 5.1-C.
PROVISION OF INFORMATION
Clear and fully detailed drawings should be available on site to enable work to be carried out in accordance with the design.
Design drawings should include:
- all necessary plan dimensions and levels related to identified benchmarks
- information on all proposed underground services
- points of entry to the building for services
- penetration of services through the substructure, including support of the structure above
- details of trench backfill, infill and void formers
- the required sequence of trench backfill if this is relevant to the design of the walls below dpc
- work required to maintain the integrity of dpcs and dpms
- details of junctions between dpm, dpc and tanking
- details of underfloor and floor edge insulation and cavity insulation, where relevant.
Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.