Cavity Wall

This Calculator will estimate the quantities of bricks, blocks and mortar (sand & cement) insulation, ties & render required for a given area of cavity wall construction.

Dimensions

Length of Wall in Metres
Height of Wall in Metres Metres
Total m2

Materials

Block work outer leaf and block work inner leaf

Blocks Outer Blocks Inner m3 Mortar Cement in Kg Sand in Kg Insulation 450x900mm Wall Ties dpc (4")

Face brick work outer leaf, block work inner leaf

Metric Bricks Outer Blocks Inner m3 Mortar Cement in Kg Sand in Kg Insulation 450x900mm Wall Ties dpc (4")

The costs shown can only ever be used as a very rough guide, materials and labour costs vary greatly by geography, season, workload and many other factors.

Also the blockwork and brickwork prices shown are based on a certain amount of assumption and averaging on our part.

Good Practice

  • Brick Mixing - Natural materials such as clay bricks have characteristic variations in shade, colour and texture. However because patching or colour banding would not be desired in the finished work, it is important to blend the bricks. Although this is carried out at manufacturing there is still a need to further carry out this process on site, this is best achieved mixing from as many packs as possible and mixing from corners rather than layers to maximise colour consistency.
  • Mortar - The correct preparation is key to the overall aesthetics, durability, weather resistance and structural performance.
    • Mix Design - mortar is a combination of binder (cement or lime) with sand. Either with or without fillers or additives such as plasticiser. Good mortar design fills the voids in a well graded sand with the binder or filler.This is what produces the ratio.
      In general it is good practice to use the weakest possible mortar mix that is compatible with the brick or block being used and the exposure, unless otherwise specified. This mortar mix will exhibit less shrinkage and produce a superior brick/mortar bond, weaker mixes also tend to be more flexible.
    • Aesthetics - The colour and joint profile chosen will affect the overall appearance of the brickwork. Whether the aim is to contrast against the brickwork or blend subtly there is a wide choice.
    • Durability - Water and frost are the main elements that will affect brickwork. However, as the frost resistance of the mature mortar increases with the proportion of cement, designation (i) and (ii) mortars are considered more suitable for severely exposed applications such as in copings, cappings and cills and below DPC level. Designation (i) mortars should be used for paving.
      • If there are high levels of soluble salts and/or the brickwork is likely to be saturated for periods then the Ordinary Portland Cement should be replaced with sulphate-resisting Portland cement.
    • Rain - Resistance to this can be achieved or increased by good adhesion, well filled joints and an appropriate joint profile.
      Tooled bucket handle or weather struck joints are the most appropriate for exposed areas. Whilst recessed joints are only suitable for moderate exposure.
    • Structural - In design calculations, it is assumed that both the compressive strength value of the appropriate mortar designation is achieved in practice and that the joints are completely filled with mortar. This underlines the importance of good building practice.
    • Adhesion - Strength of bond is affected by the suction rate of the brick, the mortar constituents, water content and the weather. Where the suction rate of a brick exceeds the optimum value of 1.5kg/m2/min, it is recommended that the bricks are wetted prior to laying, particularly in hot & dry weather conditions.

In general terms the correct proportions of binders and sand is necessary for consistent mortar strength, avoidance of colour variation and a good bond between brick & mortar. This should be achieved by the use of batching boxes buckets, avoid the use of shovels for proportioning by volume.

  • Bricklaying
    • Mortar Consistency: In general, the correct mortar consistency is a matter of the bricklayers judgement. However, if mortars are too wet at the time of laying this can lead to staining of the brick edges and, in the worst cases, runs of mortar can form on the face of the brick. Ideally, mortar should 'hang' on the trowel without being sticky, should spread easily and should not ooze out of the joints when bricks are positioned.
    • Docking: It is accepted practice for bricks with a high absorbency to be wetted prior to laying. This procedure adjusts the suction rate of the brick to promote good adhesion between the brick and the mortar. However, bricks should not be soaked during the process as this could lead to subsequent staining problems. See above. Bricks with a low suction rate should never be wetted because they might 'float'; mortar that is mixed too wet will have the same effect.
    • Tooling: The bricklayer uses his skill and experience in determining when to begin tooling the joints after placing the bricks in mortar. Early tooling should be avoided as this can produce a dragged effect on the mortar surface and the cement fines may be drawn to the surface resulting in an unsightly patchy bloom on the joints in the mature work.
    • Winter Working: If the temperature of unhardened mortar is less than 40C, the performance of the joints produced will be inadequate. Consequently, bricklaying should stop when the air temperature reaches 30C and falling, unless the mortar temperature can be maintained at a minimum of 40C until it has hardened. Bricklaying can be resumed when the air temperature rises to 10C and is expected to continue rising to above 30C over the bricklaying period.

Special Precautions in Cavity Wall Construction

  1. Wherever possible, contact between 2 leaves should be avoided
  2. The cavity should be kept clear of droppings, and any on ties should be removed
  3. The main horizontal damp proof course must be in two separate widths, and the bottom of the cavity must be at least 150mm below this
  4. Heads of openings should be protected by a suitable cavity tray. Jambs must not be solid unless insulated damp proof courses are provided
  5. Projecting ends of stone or terra-cotta sills should be notched back from the inner leaf
  6. Cover flashings at intersections of lower buildings adjoining cavity walls must extend to at least 25mm beyond the inner face of the outer leaf
  7. Weep holes should be formed at the main horizontal damp proof course and above the damp proof courses over openings. No other ventilation to the cavity should be provided
  8. Built in floor timbers should be sound and their ends treated with preservative/capped
  9. The cavity should be continued up a parapet wall to the coping
  10. Ties should be rust proof, capable of preventing rain transmission and easily cleaned of droppings