Welcome to NHBC's Supplementary Resource area. Supplementary Resource is the result of an extensive review and consultation on the way NHBC. The edition of NHBC Standards applies to every new home registered with NHBC where foundations are begun on or after 1 January NHBC STANDARDS TOP LINE FACTS. WITH THE LATEST NHBC STANDARDS. RECENTLY ISSUED, MIDLAND LEAD. GIVES YOU THE LOWDOWN.
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saw the introduction of a brand new format for the NHBC Standards, such this edition of the NHBC Standards also contains new or. The NHBC Standards came into force for every new home registered with NHBC where foundations were begun on or after 1 January. The Standards will become effective for every NHBC Registered home whose foundations are begun on or after the 1st January and will.
Note Other structural elements may be designed by a Chartered Civil or Structural Engineer or others whose status including professional indemnity insurance is accepted by NHBC. This would not apply to matters for which NHBC sets standards. Account shall be taken of all parts of the following British Standards: Where hand mixing. Site-mixed concrete 3. Mix design should take account of strength and durability. Chapter 2. Except for very small quantities.
This applies to plain and reinforced concrete. Concrete should be mixed using an appropriate method to achieve the required strength and durability. Chapter 3. Where materials need to be stored. Storage of materials 3. Protective Measures. Design and specification information should be issued to site supervisors. Compliance 3. Provision of information 3. The information below applies to cement strength class Guidance for site mixed-concrete.
Table 3: Mix proportions by volume using a maximum 20mm aggregate size Cement strength class Standardised prescribed mix Consistence class slump in mm Number of 25 kg bags of cement Fine aggregate litres Coarse aggregate litres Where cement strength class Table 2: BRE Digest Higher grade concrete has greater resistance to chemical and mechanical damage and should be specified accordingly.
Concrete mixes should be suitable for particular end uses and specified in accordance with BS as either: In addition to the issues in this section. Concrete specification 3. Concrete in non-hazardous conditions Table 4: Standardised prescribed mixes should conform to Tables 2 and 3 in this chapter. BRE Special Digest 1.
When designated mixes are used. Other suppliers may be suitable if they operate to an equivalent quality standard acceptable to NHBC. BS Ready-mixed concrete is acceptable from suppliers who operate under a full quality control scheme such as: Ready-mixed concrete should be: Designated mixes should conform to Table 5 of BS Issues to be taken into account include: Mixes should also be designed for the expected conditions of the geographical location of the site and the location of the concrete element in the structure.
Delivery information should be checked to ensure that the concrete meets the requirements given in the design. In all other cases. This is equivalent to exposure class XC4 above. Table 5: Exposure classes and examples of where they may occur. ST4 mix may be used only for small quantities of concrete.
Many foundations. XC2 Wet. Concrete permanently submerged in water. External concrete sheltered from rain. XC3 Moderate humidity Concrete inside buildings with moderate or high air humidity. XF1 Moderate water saturation. Exposure to climatic and atmospheric conditions Any concrete mix should be designed for the conditions expected at the geographical location of the site and at the location of the element in the structure. XC4 Cyclic wet and dry Concrete surfaces subject to water contact.
Explanation of suffix symbols to ACEC class number: The information in Table 7 provides guidance on selecting mixes for concrete elements in aggressive ground.
Table 7: Design guide for concrete elements in the ground Concrete element Strip or trench fill foundation. C1s As Table 4 AC Table 6: Specialist advice should be sought for more aggressive conditions. For the full list of ACEC classes. Cured concrete can be damaged by chlorides in the ground.
Though they can increase the frost resistance of cured concrete and are recommended for paths. Admixtures 3. Table A. Accelerators produce early setting of the concrete.
Aggregates Aggregates should be of a grade which ensures adequate durability of the concrete. AC-2s and AC-2z. Where unfamiliar aggregate materials are used. Concrete admixtures. Admixtures containing chlorides can cause metal corrosion and should not be used in reinforced concrete. Where ground water is highly mobile. Air-entraining agents should not be used as an anti-freeze for fresh concrete. Issues that should be taken into account include: Effects of alkali-silica reaction Alkalis can cause expansion.
Certain types of aggregate are shrinkable and require special precautions in mixing. Proprietary and recovered aggregates should only be specified where they have been assessed in accordance with Technical Requirement R3. Certain types of aggregate may be susceptible to alkali attack or excessive moisture movement. Retarding agents can increase the risk of frost damage. Standardised prescribed mixes should conform to BS Damage can occur when all the following conditions are present: Effects of chlorides Chlorides.
Reinforced concrete should be designed by an engineer in accordance with Technical Requirement R5. Proprietary concrete. Issues to take into account include: Structural design should be in accordance with Technical Requirement R5 and the mix properly detailed. Reinforcement should therefore be provided in accordance with BS EN No ties or clips should protrude into the concrete cover.
BS EN Reinforced concrete shall be suitable for its intended use.
Concrete cover There should be adequate cover to the reinforcement. Where no-fines concrete is used. Reinforcement should comply with the standards listed below. Compliance with appropriate standards The steel specification should indicate the steel type.
Table 8: Minimum cover for reinforcement for concrete not designed by an engineer Position of the concrete Minimum cover mm In contact with the ground. Cover should be adequate for all reinforcement. Proprietary methods of reinforcement.
Drawings and bending schedules should be prepared in accordance with BS and include all necessary dimensions for completion of the sitework. BS can be used for the design of suspended ground floors in homes and garages. Lapping bars and mesh Reinforcing bars or mesh should be lapped according to type and size as indicated by the designer to ensure that loads are fully transferred across the lap.
Support for reinforcement Spacers should be either concrete blocks no more than 50 x 50mm or ready-made of steel or plastic. Supports should be placed no more than one metre apart. Supports for top steel should be chairs. Such corrosion can be reduced by providing as much concrete cover as possible.
Carbonation reduces the corrosion protection of the reinforcement by increasing porosity and decreasing alkalinity. Installation of reinforcement 3. Slab reinforcement Should be located near the bottom of the slab. Chapter 5. Spacers for parallel bars should be staggered to avoid creating a plane of weakness in the concrete. Beams span Should have the main reinforcing bars placed inside the links. Requirements for fire resistance are given in BS EN Cover required by BS EN will normally provide up to one hour of fire resistance for columns.
For concrete which is to be left untreated. Concrete cast in one operation i. Formwork should be dismantled without shock.
Accuracy is essential to ensure that the correct cover to the reinforcement is maintained. Any holes for bolts or spacers should be drilled with care to avoid disfiguring or splintering the formwork surface and giving a poor finish. Before work continues beyond the joint. Additional water should not be added to ready-mixed concrete unless under the supervision and approval of the supplier.
Before concreting starts: Concrete should not be placed in or under water. Casting 3. Concreting should.
Sufficient concrete should be mixed or ordered. Formwork should be capable of being struck without damage to the concrete. Concrete should be deposited as close as possible to its final location. Construction joints should be formed only where unavoidable and in consultation with the engineer. Fresh concrete is susceptible to frost damage. Site-mixed concrete should be placed within 30 minutes.
Transportation on site should be as fast and efficient as possible in order to avoid segregation and to ensure full compaction of the placed concrete. Blinding concrete should only be used: Joints between shutters should be constructed for easy stripping. Support for load-bearing elements should not be removed until the concrete has achieved sufficient strength.
Formwork should be accurately set out in relation to relevant reference lines and benchmarks. These should not be positioned next to a return in the foundation. Before concreting 3. Formwork and its supports should be rigid enough to maintain the correct position and to withstand extra loads and accidental knocks likely to occur during placement and compacting. Props under suspended floors or beams should be released from the centre.
Curing 3. It is recommended that plain unreinforced concrete made with ordinary Portland cement is left for at least four days to cure. Damp hessian. It takes into account the site natural or brownfield and the mobility and pH of ground water. It is possible to proceed with substructure masonry above strip or trench fill foundations on unreinforced ordinary Portland cement concrete at an early stage.
No load should be applied to the work until the concrete has cured sufficiently. Total potential sulfate TPS The total potential sulfate content is the result of the combination of sulfates already present in the ground and that which may be added due to the oxidation of pyrite in the ground. It is also dependent on the type of site. Design chemical class DC class This defines the qualities of concrete that are required to resist chemical attack. Freshly poured concrete should be kept moist by covering as soon as the surface is hard enough to resist damage.
Glossary 3. Curing agents should never be used on floors which are to receive either a topping or a screed.
Additional protective measures APM These are defined as the extra measures that could be taken to protect concrete where the basic concrete specification might not give adequate resistance to chemical attack.
Curing periods may be extended at low temperatures. Proof of testing. Vibrating beams or hand tamping may be used to consolidate slabs up to mm thick. Excessive use of vibration can cause segregation and prevent concrete reaching an adequate strength. Each increment in concrete quality is counted as an extra APM. Five levels of classification are given that are equivalent to those given in BRE Digest now superseded.
Reinforced concrete or concrete containing cement replacements. Ready-mixed concrete supplier should prepare test cubes in accordance with quality assurance procedures. The design should clearly indicate where there are any special requirements for curing concrete. These should be marked. Test cubes should be prepared as requested by the engineer.
Testing 3. This is particularly important in hot. Poker vibration should be carried out by experienced operators to ensure complete coverage and to avoid honeycombing. This will normally take seven days. Mobile ground water Sites where water is free to flow into an excavation to give a standing water level are affected by mobile ground water.
Concrete should: Where it is necessary to continue building during longer periods of colder weather. When concreting is undertaken during colder weather. Materials 3. Appropriate covers should be provided for bricks and blocks.
A thermometer should be sited in the shade and used to indicate if temperatures are rising or falling. Meteorological Office Allowance shall be made for cold weather conditions during construction. Concreting 3. External conditions 3. Where very severe frosts are expected. The following conditions should be considered when scheduling work: Materials should: Where air temperature is below. Where slight overnight frosts are expected. Sitework which complies with the guidance in this chapter will generally be acceptable.
Render should not be applied if: Where warm air heaters are used to warm the structure before screeding and plastering takes place. When laying masonry in cold weather: In prolonged or very severe cold weather: Masonry 3. Paint should not be applied: When using admixtures: Timber preservative treatments that comply with the guidance in this chapter will generally be acceptable. Codes of Practice. All preservatives should meet the requirements of the Control of Pesticides Regulations administered by the Health and Safety Executive.
Table 1: Timber component groups and preservative treatment Component group Examples Hazard Desired Preservative type required 1 Preservative treatment class service not required life Copper Organic Boron organic solvent or microemulsion Internal joinery.
It is important that treatment of timber and joinery is carried out to appropriate standards which are both suitable and safe. Roof timbers dry Pitched roofs: Treatments in accordance with procedures set out in British Standards.
Roof timbers risk of wetting Flat roofs joists. Timber and external joinery should either be: Timber component groups and preservative treatment required are shown in Table 1 below based on BS Table 2 provides information on the timber species and durability.
The safety instructions published by the manufacturers should be followed. Durability 3. The specification should state the specific treatment and standard required. Uncoated external timbers not in ground contact Timber in contact Decking timber in ground with the ground contact where the deck is up to mm from ground level 8 4 15 Yes No No Where timber used is heartwood only 2 and of durability class 1 — 2 3. Sole plates 4 2 60 Yes Yes Yes Where timber used is heartwood only 2 and of durability class 1 — 2 3.
Timber in contact Timber retaining walls with the ground greater than 1m high and within garden areas 7 4 30 Yes No No Where timber used is heartwood only 2 and of durability class 1 3. Durable 3. It should be assumed that timber is sapwood. Reference should be made to Chapter Treatment should be carried out in accordance with the WPA Manual. External joinery. Preservatives used should be resistant to leaching or.
Natural durability of building timbers heartwood only Durability class 1. Timber in contact Timber retaining walls up with the ground to 1m high and within garden areas 7 4 15 Yes No No Where timber used is heartwood only 2 and of durability class 1 — 2 3. Timber in contact Timber retaining walls up with the ground to mm high and in a boundary situation 7 4 30 Yes No No Where timber used is heartwood only 2 and of durability class 1 3.
Decking where the deck is 3 up to mm from ground level 8. Reference should be made to Chapters 7. Window frames. Timber component groups and preservative treatment. West Yorkshire. UK Douglas Engelmann. European whitewood.
Only in situations where colour tinting will affect the appearance of the timber fixed to the home will clear preservatives be acceptable. It is important when timber and joinery products are stored that they are: Compatibility with metal 3. WF10 5HW. None hybrid. Checks should ensure that. Where moisture is expected. Applied preservatives should be compatible with the original treatment. Further information 3. Treatment of cut surfaces 3.
This should be stated on the delivery note. Protection and storage 3. Timber should not be cut after treatment. Copper-containing treatments can cause corrosion between mild steel and aluminium. Effective communication within their organisation and with the client. They should: Risk management Ability to conduct risk assessments as required by the risk management process.
Suitable persons for the level of investigation The following skills and knowledge are required from the person responsible for the Initial Assessment. Basic Investigation and documentation and verification. Health and safety Awareness of occupational hygiene issues and Health and Safety legislation.
Project management Ability to manage a project team consisting of the appropriate disciplines. Reporting and communication Ability to prepare comprehensive and well presented reports. The following criteria should be used as guidance for the appointment of a consultant or specialist responsible for Detailed Investigation.
Items to be taken into account include: Quality assurance Use of a quality management system. For land contamination to occur. Appropriate discipline s Understanding of all relevant skills required on the project and access to other disciplines. Site investigation Ability to design site investigation programmes. Experience Similar types of site and development. Chapter 4. A written or diagrammatic representation of the land contamination known as a Conceptual Model.
Compliance 4. Professional indemnity insurance Appropriate cover for the work being carried out.
Geotechnical and contamination issues Assessment should be carried out by direct investigation and examination of the ground. Engineering design Understanding of effective risk reduction techniques. Legislation Understanding of legislation and liabilities associated with the site.
Where results are inconclusive. Documentation and verification NHBC requires documentation and verification to show that: Yes No 4. Initial Assessment: Failure to provide such information may delay the registration process. The results should be used to determine whether or not hazards are known or suspected. Hazards Where hazards are identified. NHBC must be notified in writing a minimum of eight weeks before work starts. If any unforeseen hazards are found during the course of construction.
Hazards known or suspected? Yes No Required where hazards are known or suspected. Further Investigation required? Initial Assessment — desk study all sites 4. Solution features in chalk and limestone. Key information sources include: A desk study is the collection and examination of existing information obtained from a wide variety of sources. Low bearing capacity ground Former buildings or structures Adjacent buildings Drains.
It should indicate potential hazards at an early stage and provide a basis for the investigation. Mining past. Potential problems should be assessed according to the current and historical uses of the site and surrounding area.
Infill and made ground. A photographic record of the site can help in the reporting of the walkover survey. Potential hazards. Initial results should be evaluated for suspected hazards and the results recorded. Initial Assessment — results 4. Indications of any potential hazards should provide a basis for the investigation. Water Lane. Brickfield Cottage. A walkover survey is a direct inspection of the site and the surrounding area carried out in conjunction with the desk study.
Where trial pits do not provide sufficient information. Further investigation should be conducted if the Detailed Investigation has not satisfactorily addressed all of the original objectives. The problems and liabilities which have to be managed in order to develop the site should be clearly communicated in the Detailed Investigation report.
BS EN Where hazards are not suspected. The distance from the edge of the foundation should not be less than the depth of the trial pit. In addition to the Basic Investigation. Detailed Investigation sites where hazards are identified or suspected 4. The number and depth of trial pits should be located so they are representative of the site and will depend upon the: Trial pits should be located outside the proposed foundation area.
The Basic Investigation aims to provide assurance for all sites. A Detailed Investigation should be carried out where hazards are identified or suspected: Basic geotechnical and contamination investigations should be conducted and include: During the excavation of the trial pits. Remediation techniques Solutions for dealing with contamination hazards include: The consultant or specialist should: The report should include the following information: Unforeseen hazards 4. Design precautions Solutions for dealing with geotechnical hazards include: Remediation method statement and report The remediation method statement should detail the strategy for the site and include the: Where additional or unforeseen hazards arise.
Geophysical methods rely on contrasts in the physical properties. Contrast may also be provided by faulting.
Indirect investigations use geophysical techniques. Yes No Yes No Contamination hazards present: Hollow stem methods are typically employed where sample retrieval is required. Trial pits and trenches should be positioned where they will not affect future foundations. Where the site is within an area susceptible to radon. Further Assessment and Basic Investigation Detailed Investigation Proposals to manage geotechnical risks Proposals to manage contamination risks 4.
Trenches are extended trial pits. Guidance for investigations 4. Direct investigation techniques involve intrusive activities to enable the retrieval and examination of the ground using trial pits. All relevant information. Trial pits allow the detailed inspection. Conducted from the surface. Continuous flight auger Exploratory boreholes may be drilled in soils by mechanical continuous flight augers of various sizes. Probing techniques Used to analyse the relative density of soils and for environmental sampling and monitoring such as chemical and physical testing of gases.
Boreholes are typically formed using the following techniques: Light cable percussion drilling A shell and auger rig — typically used in the UK to drill boreholes in soils and weak rocks.
Rotary drilling Either open-hole drilling or rotary coring. Investigation technique A site investigation normally comprises techniques which are classed as either indirect or direct. Identification and classification of soil: Part 1. These range from basic tests undertaken by geologists or engineers using simple hand-held devices or portable test kits to methods that require specialist personnel and equipment.
Sampling methods and groundwater measurements. Identification of the probable source and the measurement of gas flow are important for risk assessments. A wide variety of in-situ tests can be used to support the results of direct testing. Further information 4. Chemical tests on soils. Part 2. Ground water should be collected from appropriately designed monitoring wells which should be screened and sealed to ensure that the relevant stratum is being monitored.
Testing Testing may be undertaken in-situ. Physical tests on soil and rock materials are carried out to provide the following information on ground: Samples are used to enable soil and rock descriptions to be made and to provide material for physical and chemical testing. Identification and description. The services of a specialist arboriculturalist may be helpful for the identification of the type and condition of trees that may affect building work. Details should be provided with respect to: Where trees or hedgerows are either not shown or are in different positions and shrinkable soil is identified.
This includes trees both on and adjacent to the site. The relationship becomes less predictable as factors combine to produce extreme conditions. The interaction between trees. This chapter gives guidance for common foundation types to deal with the hazard and includes suitable foundation depths which have been established from field data. Provision of information 4. Foundations near trees. Depths greater than 2. The depths are not those at which root activity.
The following situations are beyond the scope of the guidance in this chapter and will require a site-specific assessment by an engineer see Technical Requirement R5: NHBC data and practical experience. All necessary dimensions and levels should be indicated and relate to at least one benchmark and reference points on the site.
In order to minimise this risk. This has the potential to affect foundations and damage the supported structure. The site plan should show the trees and hedgerows that affect the ground and works. These are signified by the need for deeper foundations. Removal of existing trees and hedgerows Statutory Requirements.
Damage to foundations resulting from the growth of trees and roots should be avoided by locating structures and services at a safe distance. Dead trees and hedgerows should be removed. If the location of previously removed vegetation is not known. The local planning authority should be consulted. All parts of the system are easily susceptible to damage which may not regenerate and which can affect the stability of the tree.
The shape of this area may change depending on specific factors such as local drainage. This can be caused by: An arboriculturist may be required to assess these factors is impractical. Before the site is cleared. If necessary. Where this cannot be achieved. Unstable trees should be made steady or felled. Where root growth is noted within shrinkable soil and where records are not available.
Where the species of a tree has been identified but is not listed. Water demand categories of common tree species are given in the table below.
These tests are carried out on the fine particles and any medium and fine sand particles. Water demand of broad-leaf trees by species Tree species Water demand All elms. The volume change potential should be established from site investigation and reliable local knowledge of the geology.
BRE Digest and local geological survey maps Foundations shall be designed to make allowance for the effect of trees. The following definitions are used to classify soil properties: Arboricultural Association.
Water demand. High volume change potential should be assumed if the volume change potential is unknown. This is a requirement of BS which specifies the test procedure. The resulting shrinkage or swelling can cause subsidence or heave damage to foundations.
This information: Where the species of a tree has not been identified. Sufficient samples should be taken to provide confidence that the results are representative. Arboricultural Advisory and Information Service. Soil classification. Where hedgerows contain trees. Information may be obtained from suitable alternative authoritative sources for trees not listed in this chapter.
When the species is known but the subspecies is not. Coniferous trees: A 50mm decrease can be made to the foundation depth determined in accordance with this chapter for every 50 miles distance north and west of London.
Thurso 0. Water demand Zone of influence High 1. Where it is unclear which zone applies. Table 3b: Zone of influence lateral extent of trees. Guidance for factors affecting the mature height and water demand of trees Influencing factor Guidance Heavy crown reduction or pollarding previously or planned The mature height should be used.
Removal of trees previously or planned The water demand of a semi-mature tree may be equal to that of a mature tree. Foundations in shrinkable soils 4. Landscape and foundation designs should be compatible. This is to avoid a situation where. Different foundation types should not be used to support the same structure unless the foundation and superstructure design are undertaken by an engineer.
Distance between tree and foundation The distance D between the centre of the trunk and the nearest face of the foundation should be used to derive the foundation depths.
Foundation type Foundations to all permanent structures. Foundation types that are acceptable in shrinkable soils include strip. Excavation of foundations 4.
Some root activity may be expected below the depths determined in accordance with this guidance. Root barriers are not an acceptable alternative to the guidance given.
Foundation depths should be measured on the centre line of the excavation and from ground level determined from Clause 4. Freestanding masonry walls should be constructed on foundations in accordance with this chapter or designed to accommodate potential ground movement. Method of assessment of foundation depths Foundation depths should be determined according to the guidance provided in this document.
If in doubt, assume the worst conditions or consult an engineer. Foundations deeper than 2. One of the following methods may be used to assess the foundation depth where foundations are in the zone of influence of existing or proposed trees. Design in accordance with this chapter to a depth derived from the charts in Clause 4. The most onerous conditions should be assumed in the absence of derived information. When this method is used and it results in foundation depths or other details less onerous than those derived from this chapter, the design should be submitted to NHBC for approval prior to work commencing on site.
Foundation depths related to the zone of influence of new tree planting Foundation depths relating to the zone of influence of proposed tree planting should be in accordance with any of the following:. Minimum foundation depths outside of the zone of influence of trees can be determined from Tables 4 and 5. Where foundation depths are in accordance with column A or column B in Table 4, tree planting should be restricted to: Foundation depths related to new shrub planting Shrubs have considerable potential to cause changes in soil moisture content.
The foundation design should consider shrub planting in accordance with Table 6. The foundation design should consider shrub planting as follows: Shrubs that have a maximum mature height of 1. Use foundation depth from column B and plant at least 1. Use foundation depth from column B and plant at least 0. Also see: Chapters 4. Design and construction of foundations in shrinkable soils 4. Reference should be made to Clause 4.
The following will only be acceptable if they are designed by an engineer and account for all potential movement of the soil on the foundations and substructure: Pier and beam foundations Pier depths not exceeding 2.
Pier depths greater than 2. Pile and beam foundations When selecting and designing pile and ground beam foundations, piles should be:. Sufficient anchorage should be provided below the depth of desiccated soil. Slip liners may be used to reduce uplift but the amount of reduction is small, as friction between materials cannot be eliminated. Bored, cast-in-place piles are well suited to counteracting heave. Most types have a straight-sided shaft, while some are produced with a contoured shaft to increase load capacity.
The design should allow for the enhanced tensile forces in these piles. Driven piles are less well suited to counteracting heave and are difficult to install in stiff desiccated clay without excessive noise and vibration. The joint design of these piles should be capable of transmitting tensile heave forces.
Climbing shrubs which require Use foundation depth from column B. Where required by NHBC, site inspections are to be undertaken by the engineer to verify suitable compaction of the fill. Foundations in shrinkable soils shall be designed to transmit loads to the ground safely and without excessive movement.
Strip and trench fill foundations in non-shrinkable soils overlying shrinkable soil Non shrinkable soils such as sands and gravels may overlie shrinkable soil. Foundations may be constructed on overlying non-shrinkable soil if all the following are satisfied:. Where any of the above are not met foundation depths should be determined as for shrinkable soil. Measurement of foundation depths Where ground levels are to remain unaltered, foundation depths should be measured from original ground level.
Figure 1: Measurement of foundation depths where ground levels are reduced or increased, either in the recent past or during construction, should be as shown in figures 1, 2 and 3.
Use the lower of: Figure 3: Granular infill should be placed beneath raft foundations on shrinkable soils as shown below. Steps in foundations On sloping ground, foundation trenches can be gradually stepped so that the required foundation depth is reasonably uniform below ground level. Where foundations are to be stepped to take account of the influence of trees, hedgerows and shrubs, they should be stepped gradually, with no step exceeding 0. Foundations, substructures and services shall be suitably designed and detailed to prevent excessive movement due to heave.
Heave precautions shall be incorporated into foundations and substructures in accordance with the design. Where foundations and substructure may be subject to heave, they should be protected by voids, void formers or compressible materials.
This section provides guidance on heave precautions for common building elements when located within the influence of trees which are to remain or be removed, including:. Potential for ground movement After the felling or removal of trees and hedgerows on shrinkable soils, heave can occur, as the absorbed moisture causes swelling.
Heave can also occur beneath a building where: Minimum void dimensions Voids should be provided to accommodate movement due to heave forces acting against foundations and suspended ground floors in accordance with Table 7. Volume change Void dimension against side of potential foundation and ground beam.
Void dimension under ground beams, and suspended in-situ concrete ground floor. Notes 1 Under suspended floors, the void dimension is measured from the underside of beam or joist to ground level and includes mm ventilation allowance. Void formers consist of materials that collapse to form a void into which the clay can swell. Proprietary materials to accommodate heave Compressible material compacts as clay expands; the void dimension is the amount the material should be able to compress to accommodate heave.
The correct placement of heave materials is essential to ensure the foundations and substructure are adequately protected from heave forces. Heave precautions for foundations Table 8 shows where heave precautions are required for trench fill, pier and beam, and pile and beam foundation types which are in the zone of influence of trees see Table 3b which are to remain or be removed.
External trench fill and pier foundations. Unless NHBC is satisfied that the soil is not desiccated compressible material should be provided to the:.
Inside faces of external wall foundations deeper than 1. External ground beams. External and internal ground beams. Compressible material, void former or void should be provided to the underside of:. It is essential that: It is essential that heave material is provided to the entire areas shown. Particular care should be taken to ensure that the full width of the ground beam is protected. Raft foundations constructed in accordance with Clause 4. Particular care should be taken to ensure that the full width of the ground beam and the areas around the piles are protected.
Other foundation types All foundations not covered in this chapter, but specifically designed to counteract heave, should be:. Suspended ground floors Suspended ground floors with voids in accordance with Table 7 should be used in situations where heave can occur within the area bounded by the foundations, including where:. Paths and driveways Paths and driveways should be designed and detailed to cater for the likely ground movement. Drainage shall be in accordance with the design and allow for ground movement.
To protect against the effects of heave, drainage should be designed:. Foundation depth charts 4. Where no value is given in the table, minimum foundation depths apply i. Chart 1: Chart 2: Site at Oxford. From laboratory tests: Plasticity Index. The process may be repeated to allow the foundation to be stepped as its distance from the tree increases. Step 1 Determine the volume change potential of the soil. Step 2 Establish the species. Services Part 9: Finishes Part External works. Part 2: Part 3: Part 4: Part 5: Part 6: Part 7: Part 8: Part 9: Part What's new in Standards ?
Chapter 3. Timber preservation natural solid timber Table 1 has been updated to recognise the use of water-based organic preservatives as an acceptable timber treatment. Chapter 5. Waterproofing of basements and other below ground structures The introduction of Chapter 5. Helping the industry to get this critical part of the build right, and reduce the significant rate of failure identified through our claims activities, remains one of our top priorities. To help provide further clarity on what is required to comply, a number of minor updates have been made to this chapter.
Chapter 6. External masonry walls Stone veneer cladding systems come in a number of forms. Typically they are either natural or reconstituted stone and can either be directly applied to masonry substrates with mechanical fixings or adhesive, or applied as a slip system on a backing board. Standards introduces guidance that will help to ensure that only suitable systems with appropriate verification of likely performance are used in the construction of homes with NHBC warranty.
Render and rendering systems Standards includes the launch of this new chapter which will set benchmark standards and provide helpful guidance for the application of render and rendering systems.
The chapter considers a range of commonly used types of render, including: Chapter 7. Flat roofs and balconies For many years NHBC Standards have led the way in ensuring that balconies are only constructed with suitably durable materials.
Further recognising the importance of getting this right, revised guidance for the use of timber in balcony constructions is introduced. Pitched roofs In addition to the current guidance in the NHBC Standards, supporting documents to building regulations and other authoritative resources, Standards includes an option for fixing lateral restraint straps to longitudinal binders.
A number of smaller amendments have been made to ensure that the chapter is consistent and in alignment with the latest version of BS Chapter 8. Internal services We have come across a number of designs where ventilation terminals have been concealed behind rainscreen cladding panels. This has led to questions about the performance of the ventilation system and durability of surrounding materials.
To address this, Standards contains guidance making it clear that extract terminals from ventilation systems should discharge to open air. Chapter