NHBC Standards 2006

6.3 Internal walls

SCOPE

This Chapter gives guidance on meeting the Technical Requirements and recommendations for internal walls, including separating walls and compartment walls.

DESIGN STANDARDS

6.3 - D1Design standards
6.3 - D2Statutory requirements
6.3 - D3Loadbearing masonry walls
6.3 - D4Loadbearing timber walls
6.3 - D5Beams and lintels
6.3 - D6Non-loadbearing internal partitions
6.3 - D7Fire resistance
6.3 - D8Movement joints
6.3 - D9Sound insulation
6.3 - D10Damp-proofing
6.3 - D11Fireplaces and flues
6.3 - D12-D13Provision of information

6.3 - D1
Design shall meet the Technical Requirements
 

Design that follows the guidance below will be acceptable for internal walls, including separating walls and compartment walls.

STATUTORY REQUIREMENTS

6.3 - D2
Design shall comply with all relevant statutory requirements
 

Design should be in accordance with relevant Building Regulations and other statutory requirements.

LOADBEARING MASONRY WALLS

6.3 - D3
Loadbearing masonry internal walls shall be designed to support and transfer loads to foundations safely and without undue movement
 

Structural design of masonry walls should be in accordance with BS 5628.

Items to be taken into account include:

(a)
provision of foundations
 

Any loadbearing wall should be provided with either:

  • a foundation, or
  • a means of support that transfers the load safely to a foundation.

Sleeper walls should be designed and constructed as described in Chapter 4.4 'Strip and trench fill foundations' (each section).

Where necessary, oversite concrete should be protected against sulfate attack by the use of a polyethylene sheet dpm, not less than 1200 gauge (0.3mm), (or 1000 gauge (0.25mm) if assessed in accordance with Technical Requirement R3) properly lapped.

Loadbearing walls built off a concrete groundbearing slab are acceptable only if the concrete forms part of a foundation which is designed by an Engineer in accordance with Technical Requirement R5.

Foundation design should take account of any site investigation or ground hazards.

(b)
structural elements
 

Bricks and blocks should be selected in accordance with their intended use. The table below gives recommended strengths of bricks and blocks to be used in buildings up to 3 storeys high:

Height of wallMinimum compressive strength of brick or block unit
1 or 2 storeysblocks- 2.9N/mm2

bricks- 9.0N/mm2
Lowest storey of a 3 storey wall or where individual storeys exceed 2.7m



blocks - 7.3N/mm2
bricks - 13.0N/mm2
(unless calculations show that lower strengths are suitable)
Upper storeys of 3 storey wall
blocks - 2.9N/mm2
bricks - 9.0N/mm2
(c)
lateral restraint
 

Loadbearing walls, including separating walls should be provided with lateral restraint:

  • at each floor level, and
  • at the ceiling level below a roof.

Concrete floors provide adequate restraint if:

  • they have a minimum 90mm bearing onto the wall, or
  • they abut each side of the wall (provided that both floors are at, or about, the same level).

Restraint straps should be provided at 2m centres to each floor when:

  • floors are not at, or near, the same level, and
  • the floor span is parallel to the wall, and
  • the floor is not built into the wall.
Restraint straps at 2m centres to each floor

Timber joists with a minimum of 90mm bearing normally provide adequate lateral restraint.

Where timber joists are not built into a masonry wall, restraint should be provided at 2m centres by means of either:

  • restraint type joist hangers to BS 5628 : Part 1, or
  • restraint straps with a cross section of at least 30mm x 5mm.

Where restraint is required and restraint-type hangers are not used, straps should be:

  • at least 300mm long on each joist
  • fixed with four fixings to each joist
  • not more than 2m apart.
Restraint type joist hanger

At least 300mm long on both joists

Where permitted, timber joists built into separating walls may provide lateral restraint but care should be taken to ensure that sound insulation and fire resistance requirements are also met. See Clauses D7 and D9.

(d)
bonding and tying
 

Where a separating wall abuts an external wall they may be tied or bonded together.

Tied joints should be formed using expanded metal strip, wall ties or equivalent fixings, at maximum 300mm vertical centres.

The external wall cavity should be closed at the junction with the separating wall with a flexible cavity stop, unless the cavity is fully filled with built-in insulation (where permitted).

(e)
movement joints
 

For guidance on the design of movement joints, reference should be made to Clause D8.

(f)
wall ties for cavity separating walls
 

Normally, the two leaves of a masonry cavity separating wall should be tied together to provide structural stability. The type of tie and spacing should limit the sound transmission across the cavity. Reference should be made to Clause D9 for details.

In England and Wales, separating walls may be built in accordance with Robust Details "Resistance to the passage of sound".

LOADBEARING TIMBER WALLS

6.3 - D4
Loadbearing timber internal walls shall be designed to support and transfer loads to foundations safely and without undue movement
 

Structural design of loadbearing timber walls should be in accordance with BS 5268.

Structural timber should be specified according to the strength classes in BS 5268 : Part 2, eg C16 or C24. When using the BS 4978 grading rules, the timber species should be included. The strength classes can then be determined from BS 5268.

Items to be taken into account include:

(a)
provision of foundations
 

Foundations should be provided as for structural masonry walls (see Clause D3(a)).

(b)
structural elements
 

Normally, individual studs, sills and headplates should be 38mm x 75mm. Larger timber section sizes may be required in order to achieve an adequate level of fire resistance. Studs should be spaced at maximum 600mm centres.

A lintel and cripple studs should be provided to any opening except when the stud spacing is unaffected. Normally, multiple studs should be provided to support multiple joists.

Where internal walls are made up of panels, structural continuity should be maintained, for example by the use of a continuous top binder.

Continuous top binder. Cripple studs support loadbearing lintels

Framing joints should be secured with not less than two nails per joint.

(c)
wall ties for cavity separating walls
&160;

For timber framed separating walls, ties should be specified in accordance with the system designer's recommendations. The type of tie and spacing should limit sound transmission across the cavity.

To limit sound transmission, metal tie straps should be:

  • not more than 3mm thick
  • fixed below ceiling level
  • spaced at least 1.2m apart horizontally.

Thicker ties, fixed at ceiling level or more closely spaced will increase sound transmission across the cavity.

BEAMS AND LINTELS

6.3 - D5
Beams and lintels shall be adequate for their purpose
 

Items to be taken into account include:

(a) loads and spans

Loads and spans should be either in accordance with manufacturers' recommendations or designed by an Engineer in accordance with Technical Requirement R5.

(b) materials

Concrete or steel lintels are suitable for use in masonry walls. Timber lintels should not be used to support masonry.

Lintels should extend beyond each end of openings in masonry as follows:

Span [m]Minimum length of bearing [mm]
Up to 1.2100
Over 1.2150

Where structurally necessary, padstones should be provided under the bearings of lintels and beams. Reference should be made to Chapter 6.5 'Steelwork support to upper floors and partitions'.

NON-LOADBEARING INTERNAL PARTITIONS

6.3 - D6
Non-loadbearing partitions shall have adequate strength and be adequately supported
 

Items to be taken into account include:

(a) partition construction

The following constructions are acceptable:

  • partitions of brick or block construction
  • timber stud partitions using studs, sills and headplates nominally 63mm x 38mm. Studs should be spaced to suit the thickness of plasterboard used, as follows:
    • not more than 450mm spacing for 9.5mm boards
    • not more than 600mm spacing for 10mm to 20mm boards
  • proprietary partitions of plasterboard, strawboard or other material, detailed and constructed in accordance with manufacturers' recommendations.

(b) movement joints

Reference should be made to Clause D8.

(c) method of supporting partition

Walls and partitions should be supported by the structural floor, not by a floating floor that incorporates a compressible layer, unless the material is specifically manufactured for that purpose.

Preferably, masonry partitions should be supported on:

  • other masonry partitions or walls (wherever possible the design of dwellings should be such that first floor masonry partitions are a continuation of those on the ground floor)
  • concrete floors
  • steel or concrete beams.

Where steel or concrete beams are to be used, it may be necessary to use padstones at bearings.

Masonry partitions should not be supported by timber joists or beams, unless they have been designed by an Engineer in accordance with Technical Requirement R5.

The design should make allowance for the relatively flexible nature of the timber and the relatively rigid nature of masonry.

Where stud partitions or proprietary plasterboard partitions are supported by a timber floor, extra noggings or joists should be specified, unless it can be shown that the deck can transfer the load without undue movement.

Allowance should be made for the possible deflection of floors at the head of partitions to prevent the partition becoming loadbearing.

Floor deflects when loaded. Gap for possible deflection

FIRE RESISTANCE

6.3 - D7
Internal walls shall, where necessary, have adequate resistance to the spread of fire
 

The guidance below applies in England, Wales, Northern Ireland and the Isle of Man. In Scotland, reference should be made to statutory requirements.

Items to be taken into account include:

(a) fire resistance of internal walls

In houses up to 2 storeys, the following walls should be fire-resisting:

  • loadbearing walls (half-hour minimum)
  • separating walls (one hour minimum)
  • walls between a house and an integral or attached garage (half-hour minimum).

In houses of 3 or more storeys, in addition to the above, the walls enclosing the staircase should have half-hour fire resistance.

For flats, the following walls should be fire-resisting:

  • loadbearing walls within the flat (half-hour for up to 2 storeys, one hour for 3 or more storeys)
  • separating walls, compartment walls and walls enclosing protected shafts, such as staircases (normally one hour)
  • walls enclosing hallways to second floor (and above) flats (half-hour for up to 2 storeys, one hour for 3 or more storeys).

(b) fire-stopping

Fire-resisting walls should be fire-stopped or constructed to resist fire spread:

  • at their perimeter
  • at junctions with other fire-resisting walls, floors and roofs
  • where openings are formed for doors, pipes, etc.

The following details should be designed carefully:

  • the junction between a separating or compartment wall and a pitched roof or a flat roof
  • where pipes and cables penetrate a fire-resisting wall (reference should also be made to Chapter 8.1 'Internal services' (Design and Sitework)).

The wall dividing an integral or attached garage and the floor above should act together to provide adequate resistance to fire spread. If there is either no ceiling or no floor to the space above the ceiling to the garage, vertical fire separation may be required.

When fire-stopping against timber construction, it is important to specify compressible material, such as mineral wool, to allow timber shrinkage to take place without affecting the fire-stopping.

(c) provision of cavity barriers

All internal walls of hollow or cavity construction, whether fire-resisting or not, should have cavity barriers installed at their perimeter and at junctions with fire-resisting floors and walls.

For cavity separating walls of masonry construction, the top of the wall needs to be closed for fire resistance.

Timber framed separating walls should be specified with cavity barriers of wire reinforced mineral wool at junctions with floors and ceilings. Reference should be made to Chapter 6.2 'External timber framed walls' for further information.

MOVEMENT JOINTS

6.3 - D8
Masonry walls shall, where necessary, be provided with movement joints
 

Movement joints should be provided in straight unbroken lengths of wall as shown in the following table:

Type of brick or blockJoint width [mm]Normal spacing
Clay brick1612m (15m max)
Calcium silicate brick107.5 to 9m
Concrete block and brick 106m

Movement joints in foundations should be continued through the superstructure.

SOUND INSULATION

6.3 - D9
Internal walls shall, where necessary, have adequate resistance to the transmission of sound
 

Items to be taken into account include:

(a)
all separating walls
 

In England and Wales, separating walls may be built in accordance with Robust Details Part E "Resistance to the passage of sound".

For details of separating walls in timber frame construction, reference should be made to Chapter 6.2 'External timber framed walls'.

To ensure an adequate level of sound insulation for masonry separating walls:

  • statutory requirements may permit the building in of joist ends into separating walls. Where permitted there should be a mortar joint around the joist perimeter and the face of the joint pointed with silicone sealant
  • all joints in the separating wall should be fully filled with mortar
  • flexible cavity stops should be provided at the junction of separating walls with external walls
  • avoid any reduction in the thickness of masonry, but where this is unavoidable, eg to accommodate electrical sockets, positions on opposite sides of the separating wall should be staggered.

Care should be taken when specifying dry lining to masonry separating walls. The thickness of plasterboard layers, the method of fixing and sealing may be critical. For fixing details, reference should be made to BS 8212.

(b)
cavity separating walls
 

Cavity separating walls should be constructed so that any external cavity wall insulation placed by blown or pumped methods, where permitted, cannot enter the separating wall cavity.

In masonry cavity separating walls, where the cavity is up to 75mm, flexible wall ties should be used, such as:

  • butterfly type ties, or
  • ties assessed for the purpose.

Where structural design permits, the omission of ties will enhance sound insulation.

Ties should be spaced 900mm horizontally and 450mm vertically. Closer spacing will increase sound transmission. Wider spacing may impair structural stability. Rigid ties transmit sound more readily than flexible ties.

Chases for services should be kept to a minimum and be well filled with mortar.

(c)
flanking transmission
 

The construction of the flanking wall, including the position of openings in it, will have an impact on sound transmission. The requirements of statutory regulations should be followed.

(d)
partitions to rooms containing a WC
 

The guidance below applies in Scotland, Northern Ireland and the Isle of Man. In England and Wales reference should be made to statutory requirements.

PARTITION PERFORMANCE

Any partition between a room containing a WC and a living room, dining room, study or bedroom should have a weighted sound reduction index of not less than 38dB over the frequency range 100-3150Hz, when tested in accordance with BS 2750. This does not apply to a partition between a WC and a bedroom where the WC forms an en-suite facility to the bedroom.

The following methods of construction will achieve the performance level stated above.

BLOCKWORK PARTITIONS

Blocks having a density of not less than 600kg/m3 finished on both sides with 13mm of plaster are acceptable.

Blockwork should be tied in every course to adjoining walls, with the joints filled solid.

For details of bonding and tying, reference should be made to Sitework clause 6.3 - S3(d).

TIMBER STUDWORK

Timber studwork is acceptable if used with one of the following combinations of materials:

One layer of 12.5mm plasterboard each side + no insulation. One layer of 12.5mm plasterboard each side + 25mm mineral wool quilt

5mm plaster coat on one layer of 9.5mm plasterboard each side + 25mm mineral wool quilt. One layer of 12mm panel board each side + 75mm mineral wool quilt

Other forms of timber studwork construction will be acceptable provided the criteria given under PARTITION PERFORMANCE is achieved and independent evidence of performance is available at the request of NHBC.

Where two thicknesses of board are used, the joints should be staggered and the joints in the outer layer properly filled.

Top plates, bottom plates and end studs should be secured and sealed to the adjoining structure so that sound paths cannot develop following shrinkage and/or deflection of the timber.

PROPRIETARY PARTITIONS

Certain proprietary partition systems meet the required standard. The partition manufacturer should have independent test evidence of the system's performance.

DAMP-PROOFING

6.3 - D10
Internal walls shall be designed to provide adequate resistance to moisture from the ground
 

Loadbearing walls built off foundation walls should have a dpc at their base at least the width of the wall. The dpc should link with any adjoining dpm.

Where steps are necessary in the ground floor slab, a dpc should be incorporated as a continuous link between the upper and lower dpcs. The vertical part of the dpc should be protected from damage during construction.

Dpc used as continuous link between upper and lower dpcs

Dpcs should be included below the following:

  • non-loadbearing partitions built off an in-situ ground slab with a dpm applied to the top of the slab

    Partition on dpc above polyethylene dpm. Screed on dpm above slab

    Dpc and dpm linked. Partition dpc. Applied dpm such as mastic asphalt above slab

  • partitions that may be adversely affected by residual damp (eg stud partitions) built off any type of concrete floor.

    Stud partition on dpc. Dpm below slab

FIREPLACES AND FLUES

6.3 - D11
Fireplaces and flues
 

See Chapter 6.8 'Fireplaces, chimneys and flues'.

PROVISION OF INFORMATION

6.3 - D12
Designs and specifications shall be produced in a clearly understandable format and include all relevant information
 

Drawings should give full information, including:

  • wall layout, with all dimensions shown
  • position and size of openings and lintels
  • details at junctions, indicating fire-stopping where applicable
  • details of wall constructions and materials, ties and restraints
  • details of foundations where required.
6.3 - D13
All relevant information shall be distributed to appropriate personnel
 

Ensure that design and specification information is issued to site supervisors and relevant specialist subcontractors and/or suppliers.

Where proprietary components are to be used, manufacturers usually have specific requirements for fixing and/or assembly of their products. This information should also be made available for reference on site so that work can be carried out satisfactorily in accordance with the design and specification.