Section 2: Sizes of structural elements for certain residential buildings and other small buildings of traditional construction

Section 2: Sizes of structural elements for certain residential buildings and other small buildings of traditional construction

General
2.1 This section is presented as follows:

Section 2a
Basic requirements for stability.

Section 2b

Sizes of certain timber members in floors and roofs for dwellings.
Areas at risk from house longhorn beetle.

Section 2c

Thickness of masonry walls in certain residential buildings of not more than three storeys, small single-storey non-residential buildings and annexes.

Section 2d

Proportions for masonry chimneys.

Section 2e

Foundations of plain concrete.

Buttressing wall

A wall designed and constructed to afford lateral support to another wall perpendicular to it, support being provided from the base to the top of the wall.

Cavity width

The horizontal distance between the two leaves of a cavity wall.

Compartment wall

A wall constructed as a compartment wall to meet the requirements of regulation B3(2).

Dead load

The load due to the weight of all walls, permanent partitions, floors, roofs and finishes including services, and all other permanent construction.

Imposed load

The load assumed to be produced by the intended occupancy or use, including the weight of movable partitions, distributed, concentrated, impact, inertia and snow loads, but excluding wind loads.

Pier

A member which forms an integral part of a wall, in the form of a thickened section at intervals along the wall, so as to afford lateral support to the wall to which it is bonded or securely tied.

Separating wall

A wall or part of a wall which is common to adjoining buildings, and constructed to meet the requirements of regulation B3(2).

Spacing

The distance between the longitudinal centres of any two adjacent timber members of the same type, measured in the plane of floor, ceiling or roof structure.

Span

The distance measured along the centre line of a member between the centres of any two adjacent bearings or supports.

Supported wall

A wall to which lateral support is afforded by a combination of buttressing walls, piers or chimneys acting in conjunction with floor(s) or roof.

Wind load

The load due to the effect of wind pressure or suction.

Section 2A: Basic requirement for stability

2A1 This section must be used in conjunction with sections 2B and 2C and its principles relate to all forms of low-rise residential buildings.

2A2 Adequate provision shall be made to ensure that the building is stable under the likely imposed and wind loading conditions. This will commonly necessitate meeting the following requirements:

a. That the overall size and proportioning of the building are limited in accordance with the specific guidance for each form of construction.

b. That a suitable layout of walls (both internal and external) forming a robust 3dimensional box structure in plan is constructed with restriction on the maximum size of cells measured in accordance with the specific guidance for each form of construction.

c. That the internal and external walls are adequately connected either by masonry bonding or by using mechanical connections.

d. That the intermediate floors and roof are of such construction and interconnection with the walls that they provide local support to the walls and also act as horizontal diaphragms capable of transferring the wind forces to buttressing elements of the building.

Notes: A traditional cut timber roof (i.e. using rafters, purlins and ceiling joists) generally has sufficient built in resistance to instability and wind forces (e.g. from hipped ends, tiling battens, rigid sarking or the like).However, the need for diagonal rafter bracing equivalent to that recommended in BS EN 1995-1-1:2004 with its UK National Annex and additional guidance given in BSI Published Document PD 6693-1:2012 and BS 8103-3:2009 for trussed rafter roofs should be considered, especially for single-hipped and non-hipped roofs of greater than 40° pitch to detached houses.

Section 2B: Sizes of certain timber members in floors and roofs for dwellings. Areas at risk from house longhorn beetle

Sizing of members

2B1 Guidance on the sizing of certain members in floors and roofs is given in 'Span tables for solid timber members in floors, ceilings and roofs (excluding trussed rafter roofs) for dwellings', published by TRADA, available from Chiltern House, Stocking Lane, Hughenden Valley, High Wycombe, Bucks HP14 4ND.

Alternative guidance is available in BS EN 1995- 1-1:2004 Design of timber structures with its UK National Annex and additional guidance given in BSI Published Document PD 6693-1:2012 and also BS 8103-3:2009 Structural design of low- rise buildings, Code of practice for timber floors and roofs for housing.

House longhorn beetle

2B2 In the geographical areas specified in Table 1, softwood timber for roof construction or fixed in the roof space, including ceiling joists within the void spaces of the roof, should be adequately treated to prevent infestation by the house longhorn beetle (Hylotrupes bajulus L.).

Guidance on suitable preservative treatments is given within The Wood Protection Association’s manual ‘Industrial Wood Preservation: Specification and Practice’ (2012), available from 5C Flemming Court, Castleford,West Yorkshire, WF10 5HW

Table 1 Areas at risk from house longhorn beetle

Section 2C: Thickness of walls in certain small buildings

Application

2C1 This section applies to the following building types:

a. Residential buildings of not more than three storeys;

b. Small single-storey non-residential buildings;

c. Small buildings forming annexes to residential buildings (including garages and outbuildings).

Wall types

2C2 Only the types of wall given in Table 2, which must extend to the full storey height, and parapet walls are considered in this section.

The use of this section

2C3 When using this section it should be noted that:

a. This section must be used in conjunction with Section 2A;

b. If wall thickness is to be determined according to paragraphs 2C5 to 2C13, all appropriate design conditions given in this section must be satisfied;

c. Requirements of BS EN 1996-2:2006 with its UK National Annex and additional guidance given in BSI Published Document PD 6697:2010, except as regards the conditions given in paragraphs 2C4 and 2C14 to 2C38;

d. In formulating the guidance of this section the worst combination of circumstances likely to arise was taken into account. If a requirement of this part is considered too onerous in a particular case it may be appropriate to consider a minor departure on the basis of judgement and experience, or to show adequacy by calculation in respect of the aspect of the wall which is subject to the departure rather than for the entire wall;

e. The guidance given is based upon the compressive strengths of bricks and blocks being not less than indicated in Tables 6 and 7.

BS 5628-1:1992 gives design strengths for walls where the suitability for use of masonry units of other compressive strengths is being considered.
[Table 2]

Conditions relating to the building of which the wall forms part

2C4 This Section applies only to buildings having proportions within the following parameters (see Diagrams 1 and 2):

a. residential buildings of not more than three storeys:

i. the maximum height of the building measured from the lowest finished ground level adjoining the building to the highest point of any wall or roof should not be greater than 15m, subject to the limits of paragraph 2C16;

ii. the height of the building H should not exceed twice the least width of the building W1;

iii. the height of the wing H2 should not exceed twice the least width of the wing W2 where the projection P exceeds twice the width W2;

b. small single-storey non-residential buildings: height H should not exceed 3m and W (being the greatest length or width of the building) should not exceed 9m (see Diagram 2), subject to the limits of paragraph 2C16;

c. annexes: height H as variously indicated in Diagram 2 should not exceed 3m, subject to the limits of paragraph 2C16.

Table 2 Wall types considered in this section

Diagram 1 Size and proportion of residential buildings of not more than three storeys

Thickness of walls

2C5 General wall thickness may be determined according to this section provided:

a. Conditions relating to the building of which the wall forms part (see paragraphs 2C4, 2C14 to 2C16, 2C38); and

b. Conditions relating to the wall (see paragraphs 2C17 to 2C37) are met. (See Diagram 3.)

2C6 Solid external walls, compartment walls and separating walls in coursed brickwork or blockwork: solid walls constructed of coursed brickwork or blockwork should be at least as thick as 1/16 of the strorey height. Further requirements are given in table 3.

2C7 Solid external walls, compartment walls and separating walls in uncoursed stone, flints,etc.: the thickness of walls constructed in uncoursed stone, flint, clunches, bricks or other burnt or vitrified materials should not be less than 1.33 times the thickness determined by paragraph 2C6

2C8 Cavity walls in coursed brickwork or blockwork:
All cavity walls should have leaves at least 90mm thick and cavities at least 50mm wide. The wall ties should have a horizontal spacing of 900mm and a vertical spacing of 450mm, which is equivalent to 2.5 ties per square metre. Wall ties should also be provided, spaced not more than 300mm apart vertically, within a distance of 225mm from the vertical edges of all openings, movement joints and roof verges. For selection of wall ties for use in a range of cavity widths refer to Table 5. For specification of cavity wall ties refer to paragraph 2C19.

For external walls, compartment walls and separating walls in cavity construction, the combined thickness of the two leaves plus 10mm should not be less than the thickness determined by paragraph 2C6 and Table 3 for a solid wall of the same height and length.

2C9 Walls providing vertical support to other walls:

Irrespective of the material used in the construction, a wall should not be less in thickness than any part of the wall to which it gives vertical support.

2C10 Internal load-bearing walls in brickwork or blockwork

(except compartment walls or separating walls): All internal load-bearing walls should have a thickness not less than:

except for a wall in the lowest storey of a three storey building, carrying load from both upper storeys, which should have a thickness as determined by the equation or 140mm whichever is the greatest.

Diagram 2 Size and proportion of non-residential buildings and annexes

Diagram 3 Determination of wall thickness

Table 3 Minimum thickness of certain external walls, compartment walls and separating walls

Diagram 4 Parapet walls: height

2C11 Parapet walls:

The minimum thickness and maximum height of parapet walls should be as given in Diagram 4.

2C12 Single leaves of certain external walls:

The single leaf of external walls of small single- storey non-residential buildings and of annexes need be only 90mm thick, notwithstanding paragraphs 2C38.

2C13 Modular bricks and blocks

Where walls are constructed of bricks or blocks having modular dimensions, wall thicknesses prescribed in this section which derive from a dimension of brick or block may be reduced by an amount not exceeding the deviation from work size permitted by a British Standard relating to equivalent sized bricks or blocks made of the same material.

2C14 Maximum floor area:

The guidance of this section assumes that no floor enclosed by structural walls on all sides exceeds 70m2, and that no floor without a structural wall on one side exceeds 36m2. (See Diagram 5.)

2C15 Imposed loads on roofs, floors and ceilings:

The design considerations given in this section are intended to be adequate for the imposed loads given in Table 4.

2C16 Maximum height of buildings:

The design guidance in this section is based on BS EN 1991-1-4:2005 with its UK National Annex. The maximum heights of buildings given in Table c of Diagram 7 correlate to various site exposure conditions and wind speeds. A map showing wind speeds is given in Figure 1 of Diagram 6.

Conditions relating to the wall

2C17 Maximum allowable length and height of the wall: This section does not deal with walls longer than 12m, measured from centre to centre of buttressing walls, piers or chimneys providing restraint, or with walls exceeding 12m in height (see also Table 3).

Diagram 5 Maximum floor area enclosed by structural walls

Table 4 Imposed loads

2C18 Rules of Measurement for heights of walls and storeys:

The height of a wall or a storey should be measured in accordance with rules in Diagram 8.

Construction materials and workmanship

2C19 Wall ties:Wall ties should comply with BS EN 845-1 and should be material references 1 or 3 in BS EN 845-1 Table A1 austenitic stainless steel. Wall ties should be selected in accordance with Table 5 of this Approved Document

2C20 Masonry units:

Masonry units: Walls should be properly bonded and solidly put together with mortar and constructed of masonry units conforming to:

a. clay bricks or blocks to BS EN 771-1;

b. calcium silicate bricks or blocks to BS EN 771-2;

c. concrete bricks or blocks to BS EN 771-3 or BS EN 771-4;

d. manufactured stone to BS EN 771-5;

e. square dressed natural stone to the appropriate requirements described in BS EN 771-6.

2C21 Compressive strength of masonry units:

Minimum compressive strength requirements for masonry units according to BS EN Standards are given in Diagram 9, where the masonry units indicated for Conditions A, B and C should have declared compressive strengths of not less than the values given in Table 6. Normalised compressive strengths for block sized clay and calcium silicate masonry units not complying with brick dimensional format are given in Table 7.

Diagram 6 Map showing wind speeds in m/s for maximum height of buildings Figure 1 and 2

Diagram 6 Map showing wind speeds in m/s for maximum height of buildings Figures 3a and 3b

Diagram 7 Maximum height of buildings

Diagram 8 Measuring storey and wall heights

Table 5 Cavity wall ties

Table 6 Declared compressive strength of masonry units complying with BS EN 771-1 to -5 (N/mm2)

Diagram 9 Declared compressive strength of masonry units

Table 7 Normalised compressive strength of masonry units of clay and calcium silicate blocks complying with BS EN 771-1 and 2 (N/mm2)

2C22 Mortar:

Mortar should be:

a. one of the following:

(i). Mortar designation (iii) according to BS EN 1996-1-1:2005 with its UK National Annex;

(ii). Strength class M4 according to BS EN 998-2:2010;

(iii). 1:1:5 to 6 CEM I, lime, and fine aggregate measured by volume of dry materials, or

b. of equivalent or greater strength and durability to the specifications in a. above.

Loading on walls

2C23 Maximum span of floors:

The maximum span for any floor supported by a wall is 6m where the span is measured centre to centre of bearing (see Diagram 10).

2C24 Other loading conditions:

a. Vertical loading on walls should be distributed. This may be assumed for concrete floor slabs, precast concrete floors, and timber floors designed in accordance with section 2B, and where the bearing length for lintels is 150mm or greater. Where a lintel has a clear span of 1200mm or less the bearing length may be reduced to 100mm.

b. Differences in level of ground or other solid construction between one side of the wall and the other should be less than 4 times the thickness of the wall as shown in Diagram 11

c. The combined dead and imposed load should not exceed 70kN/m at base of wall (see Diagram 11).

d. Walls should not be subjected to lateral load other than from wind, and that covered by paragraph 2C24(b).

Diagram 10 Maximum span of floors

End restraint

2C25 Vertical Lateral Restraint to Walls

The ends of every wall should be bonded or otherwise securely tied throughout their full height to a buttressing wall, pier or chimney. Long walls may be provided with intermediate buttressing walls, piers or chimneys dividing the wall into distinct lengths within each storey; each distinct length is a supported wall for the purposes of this section. The intermediate buttressing walls, piers or chimneys should provide lateral restraint to the full height of the supported wall, but they may be staggered at each storey.

2C26 Buttressing Walls

If the buttressing wall is not itself a supported wall its thickness T2 should not be less than:

a. Half the thickness required by this section for an external or separating wall of similar height and length less 5mm; or

b. 75mm if the wall forms part of a dwelling house and does not exceed 6m in total height and 10m in length; and

c. 90mm in other cases.

Diagram 11 Differences in ground levels

Diagram 12 Openings in a buttressing wall

The length of the buttressing wall should be at least 1/6 of the overall height of the supported wall and be bonded or securely tied to the supporting wall and at the other end to a buttressing wall, pier or chimney.

The size of any opening in the buttressing wall should be restricted as shown in Diagram 12.

2C27 Design criteria for piers and chimneys providing restraint:

a: Piers should measure at least 3 times the thickness of the supported wall and chimneys twice the thickness, measured at right angles to the wall. Piers should have a minimum width of 190mm (see Diagram 13);

b: The sectional area on plan of chimneys (excluding openings for fireplaces and flues) should be not less than the area required for a pier in the same wall, and the overall thickness should not be less than twice the required thickness of the supported wall (see Diagram 13).

Openings, recesses, overhangs and chases

2C28 General:

The number, size and position of openings and recesses should not impair the stability of a wall or the lateral restraint afforded by a buttressing wall to a supported wall. Construction over openings and recesses should be adequately supported.

Diagram 13 Buttressing

2C29 Dimensional criteria for openings and recesses:
The dimensional criteria are given in Diagram 14 and Table 8.

No openings should be provided in walls below ground floor except for small holes for services and ventilation, etc. which should be limited to a maximum area of 0.1m2 at not less than 2m centres.

2C30 Chases:

a: Vertical chases should not be deeper than 1/3 of the wall thickness or, in cavity walls, 1/3 of the thickness of the leaf;

b: Horizontal chases should not be deeper than 1/6 of the thickness of the leaf of the wall;

c: Chases should not be so positioned as to impair the stability of the wall, particularly where hollow blocks are used.

2C31 Overhangs:

The amount of any projection should not impair the stability of the wall.

Lateral support by roofs and floors

2C32 A wall in each storey of a building should extend to the full height of that storey, and have horizontal lateral supports to restrict movement of the wall at right angles to its plane.

2C33 Floors and roofs should:

a: Act to transfer lateral forces from walls to buttressing walls, piers or chimneys; and

b: Be secured to the supported wall by connections specified in paragraphs 2C34 and 2C35.

2C34 The requirements for lateral restraint of walls at roof and floor levels are given in Table 9 and guidance on satisfying the requirements is given in paragraphs 2C35 and 2C36.

2C35 Walls should be strapped to floors above ground level, at intervals not exceeding 2m and as shown in Diagram 15 by tension straps conforming to BS EN 845-1. For corrosion resistance purposes, the tension straps should be material reference 14 or 16.1 or 16.2 (galvanised steel) or other more resistant specifications including material references 1 or 3 (austenitic stainless steel). The declared tensile strength of tension straps should not be less than 8kN.

Diagram 14 Sizes of openings and recesses

Table 8 Value of Factor ‘X’ (see Diagram 14)

Table 9 Lateral support for walls

Diagram 15 Lateral support by floors

Tension straps need not be provided:

a: In the longitudinal direction of joists in houses of not more than 2 storeys, if the joists are at not more than 1.2m centres and have at least 90mm bearing on the supported walls or 75mm bearing on a timber wall-plate at each end, and

b: In the longitudinal direction of joists in houses of not more than 2 storeys, if the joists are carried on the supported wall by joist hangers in accordance with BS EN 845-1 of the restraint type described by additional guidance given in BSI Published Document PD 6697:2010 and shown in Diagram 15(c), and are incorporated at not more than 2m centres, and

c: When a concrete floor has at least 90mm bearing on the supported wall (see Diagram 15(d)), and

d: Where floors are at or about the same level on each side of a supported wall, and contact between the floors and wall is either continuous or at intervals not exceeding 2m. Where contact is intermittent, the points of contact should be in line or nearly in line on plan (see Diagram 15(e)).

2C36 Gable walls should be strapped to roofs as shown in Diagram 16(a) and (b) by tension straps as described in 2C35.

Vertical strapping at least 1m in length should be provided at eaves level at intervals not exceeding 2m as shown in Diagram 16(c) and (d). Vertical strapping may be omitted if the roof:

a: Has a pitch of 15deg or more, and

b: Is tiled or slated, and

c: Is of a type known by local experience to be resistant to wind gusts, and

d: Has main timber members spanning onto the supported wall at not more than 1.2m centres.

Diagram 16 Lateral support at roof level

Interruption of lateral support

2C37. Where an opening in a floor or roof for a stairway or the like adjoins a supported wall and interrupts the continuity of lateral support, the following conditions should be satisfied for the purposes of Section 2C:

a: The maximum permitted length of the opening is to be 3m, measured parallel to the supported wall, and

b: Where a connection is provided by means other than by anchor, this should be provided throughout the length of each portion of the wall situated on each side of the opening, and

c: Where a connection is provided by mild steel anchors, these should be spaced closer than 2m on each side of the opening to provide the same number of anchors as if there were no opening, and

d: There should be no other interruption of lateral support.

Small single-storey non-residential buildings and annexes

2C38 Size and proportion

i. General

The guidance given applies in the following circumstances:

a: The floor area of the building or annexe does not exceed 36m

b: The walls are solidly constructed in brickwork or blockwork using materials which comply with paragraphs 2C19 to 2C22.

c: Where the floor area of the building or annexe exceeds 10m

Note: There is no surface mass limitation recommended for floor areas of 10sqm or less

d: Access to the roof is only for the purposes of maintenance and repair.

e: The only lateral loads are wind loads.

f: The maximum length or width of the building or annexe does not exceed 9m.

g: The height of the building or annexe does not exceed the lower value derived from Diagram 2.

h: The roof is braced at rafter level, horizontally at eaves level and at the base of any gable by roof decking, rigid sarking or diagonal timber bracing, as appropriate, in accordance with BS EN 1995-1-1:2004 with its UK National Annex and additional guidance given in BS Published Document PD 6693-1:2012 or BS 8103-3:2009.

i: Walls are tied to the roof structure vertically and horizontally in accordance with paragraphs 2C32 to 2C36 and with horizontal lateral restraint at roof level in accordance with paragraph (iv) below.

j: The roof structure of an annexe is secured to the structure of the main building at both rafter and eaves level.

Diagram 17 Size and Location of openings

Diagram 18 Wall thickness

ii. Size and location of openings

One or two major openings not more than 2.1m in height are permitted in one wall of the building or annexe only. The width of a single opening or the combined width of two openings should not exceed 5m.

The only other openings permitted in a building or annexe are for windows and a single leaf door. The size and location of these openings should be in accordance with Diagram 17.

Diagram 19 Lateral restraint at roof level

iii. Wall thickness and recommendations for piers

The walls should have a minimum thickness of 90mm

Walls which do not contain a major opening but exceed 2.5m in length or height should be bonded or tied to piers for their full height at not more than 3m centres as shown in Diagram 18a.

Walls which contain one or two major openings should in addition have piers as shown in Diagrams 18b and 18c. Where ties are used to connect piers to walls they should be flat, 20mm x 3mm in cross section, be in stainless steel in accordance with clause 2C19, be placed in pairs and be spaced at not more than 300mm centre vertically.

iv. Horizontal lateral restraint at roof level

Walls should be tied horizontally at no more than 2m centres to the roof structure at eaves level, base of gables and along roof slopes as shown in Diagram 19 with straps fixed in accordance with paragraphs 2C35 and 2C36. Where straps cannot pass through a wall they should be adequately secured to the masonry using suitable fixings. Isolated columns should also be tied to the roof structure (see Diagram 19).

Section 2D: Proportions for masonry chimneys above the roof surface

height to width relationship

2D1 where a chimney is not adequately supported by ties or securely restrained in any way, its height if measured from the highest point of intersection with the roof surface, gutter, etc. should not exceed 4.5W, provided the density of the masonry is greater than 1500kg/m3, where:

W is the least horizontal dimension of the chimney measured at the same point of intersection, and

H is measured to the top of any chimney pot or other flue terminal (see diagram 20)

Diagram 20 Proportions for masonry chimneys

Section 2E: Foundations of plain concrete

Conditions relating to the ground

2E1. There should not be:

a: Non-engineered fill (as described in BRE Digest 427) or wide variation in ground conditions within the loaded area; nor

b: Weaker or more compressible ground at such a depth below the foundation as could impair the stability of the structure.

Design provisions

2E2. The following design provisions relate to foundations:

A: The foundations should be situated centrally under the wall;

B: For foundations in chemically aggressive soil conditions guidance in BS 8500-1 and BRE Special Digest 1 should be followed. In non-aggressive soils, concrete should be composed of Portland cement to BS EN 197-1 and -2 and fine and coarse aggregate conforming to BS EN 12620 and the mix should comply with one of the following recommendations:

i: In proportion of 50kg of Portland cement to not more than 200kg (0.1 cubic metres) of fine aggregate and 400kg (0.2 cubic metres) of coarse aggregate; or

ii: Grade ST2 or grade GEN I concrete to BS 8500-2;

c: Minimum thickness T of concrete foundation should be 150mm or P, whichever is the greater where P is derived using Table 10 and Diagram 23. Trench fill foundations may be used as an acceptable alternative to strip foundations;

d: Foundations stepped on elevation should overlap by twice the height of the step, by the thickness of the foundation, or 300mm, whichever is greater (see Diagram 21).For trench fill foundations the overlap should be twice the height of the step or 1m, whichever is greater;

e: Steps in foundations should not be of greater height than the thickness of the foundation (see Diagram 21);

f: Foundations for piers, buttresses and chimneys should project as indicated in Diagram 22 and the projection X should never be less than the value of P where there is no local thickening of the wall.

Minimum width of strip foundations

2E3. The recommended minimum widths of foundations given in Table 10 may be used.

Diagram 21 Elevation of stepped foundation

Diagram 22 Piers and Chimneys

Diagram 23 Foundation dimensions

Table 10 Minimum width of strip footings

The table is applicable only within the strict terms of the criteria described within it.

Minimum depth of strip foundations

2E4. Except where strip foundations are founded on rock, the strip foundations should have a minimum depth of 0.45m to their underside to avoid the action of frost. This depth, however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground.

In clay soils subject to volume change on drying (‘shrinkable clays’, with Modified Plasticity Index greater than or equal to 10%), strip foundations should be taken to a depth where anticipated ground movements will not impairthe stability of any part of the building taking due consideration of the influence of vegetation and trees on the ground. The depth to the underside of foundations on clay soils should not be less than 0.75m on low shrinkage clay soils, 0.9m on medium shrinkage clay soils and 1.0m on high shrinkage clay soils, although these depths may need to be increased in order to transfer the loading onto satisfactory ground, or where there are trees nearby.