Section 2C Thickness of walls in certain small buildings

Thickness of walls in certain small buildings

  1. Application
    1. Wall types
      1. The use of this section
        1. Conditions relating to the building of which the wall forms part
          1. Thickness of walls
            1. 2C6 Solid external walls, compartment walls and separating walls in coursed brickwork or blockwork:
              1. 2C7 Solid external walls, compartment walls and separating walls in uncoursed stone, flints, etc.:
                1. 2C8 Cavity walls in coursed brickwork or blockwork:
                  1. 2C9 Walls providing vertical support to other walls:
                    1. 2C10 Internal load-bearing walls in brickwork or blockwork
                      1. 2C11 Parapet walls:
                        1. 2C12 Single leaves of certain external walls:
                          1. 2C13 Modular bricks and blocks
                            1. 2C14 Maximum floor area:
                              1. 2C15 Imposed loads on roofs, floors and ceilings:
                                1. 2C16 Maximum height of buildings:
                                2. Conditions relating to the wall
                                  1. 2C18 Rules of measurement for heights of walls and storeys:
                                  2. Construction materials and workmanship
                                    1. 2C19 Wall ties:
                                      1. 2C20 Masonry units:
                                        1. 2C21 Compressive strength of masonry units:
                                          1. 2C22 Mortar:
                                          2. Loading on walls
                                            1. 2C23 Maximum span of floors:
                                              1. 2C24 Other loading conditions:
                                              2. End restraint
                                                1. 2C25 Vertical Lateral Restraint to Walls
                                                  1. 2C26 Buttressing Walls
                                                    1. 2C27 Design criteria for piers and chimneys providing restraint:
                                                    2. Openings, recesses, overhangs and chases
                                                      1. 2C28 General:
                                                        1. 2C29 Dimensional criteria for openings and recesses:
                                                          1. 2C30 Chases:
                                                            1. 2C31 Overhangs:
                                                            2. Lateral support by roofs and floors
                                                              1. Interruption of lateral support
                                                                1. Small single-storey non-residential buildings and annexes
                                                                  1. Size and proportion

                                                                  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

                                                                  Table 2

                                                                  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.

                                                                  ***** Image Table 2 Wall Types considered in this section

                                                                  Conditions relating to the building of which the wall forms part

                                                                  Diagram 1

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

                                                                  • residential buildings of not more than three storeys:
                                                                    • 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;
                                                                  • 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;
                                                                  • annexes: height H as variously indicated in Diagram 2 should not exceed 3m, subject to the limits of paragraph 2C16.

                                                                  **** Image 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 storey 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, flints, clunches, bricks or other burnt or vitrified material 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:

                                                                  (specified thickness from Table 3) divided by 2 minus 5mm

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

                                                                  ***** Image Diagram 3 Determination of wall thickness

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

                                                                  ***** Image Diagram 4 Parapet walls: height

                                                                  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.

                                                                  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).

                                                                  ***** Image Diagram 5 Maximum floor area enclosed by structural walls

                                                                  ***** ImageTable 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 the rules in Diagram 8.

                                                                  Construction materials and workmanship

                                                                  2C19 Wall ties:

                                                                  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:

                                                                  • clay bricks or blocks to BS EN 771-1;
                                                                  • calcium silicate bricks or blocks to BS EN 771-2;
                                                                  • concrete bricks or blocks to BS EN 771-3 or BS EN 771-4;
                                                                  • manufactured stone to BS EN 771-5;
                                                                  • 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.

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

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

                                                                  **** Image Diagram 7 Maximum height of buildings

                                                                  **** Image Diagram 8 Measuring storey and wall heights

                                                                  **** Image Table 5 Cavity wall ties

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

                                                                  **** Image Diagram 9 Declared compressive strength of masonry units 

                                                                  **** Image 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:

                                                                  • Mortar designation (iii) according to BS EN 1996-1-1:2005 with its UK National Annex;
                                                                  • Strength class M4 according to BS EN 998-2:2010;
                                                                  • 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).

                                                                  **** Image 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.

                                                                  ****Image Diagram 11 Differences in ground levels 

                                                                  ****Image 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.

                                                                  ****Image 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 0.1m

                                                                  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 resistantspecifications including material references 1 or 3 (austenitic stainless steel). The declared tensile strength of tension straps should not be less than 8kN.

                                                                  **** Image Diagram 14 Sizes of openings and recesses

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

                                                                  **** Image Table 9 Lateral support for walls

                                                                  **** Image 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.

                                                                  ****Image 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

                                                                  Size and proportion

                                                                  2C38 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.

                                                                   ***** Image Diagram 17 Size and Location of openings

                                                                   ***** Image 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.

                                                                   ***** Image 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).