Michael Hammett, Senior Architect, BDA
At the Brick Development Association we have noticed an increasing interest in the decorative nature of bonded brickwork and in particular more frequent enquiries about stack bonded work, especially in relation to stability and achieving regularity of joint alignment. This article highlights a number of the technical considerations to be taken into account when specifying it
In stack bonded brickwork the cross joints (vertical joints) between the bricks in each course are lined through with those in courses above and below. The bricks may be laid flat as stretchers or headers (ie as they are laid in a normal course), on-end as soldiers, or on-edge showing headers or bed faces. The choice is totally dependent on the decorative effect required from the resulting pattern.
Strictly speaking, such arrangements are not bonds, because there is no overlap of the bricks. Consequently the integrity of the assembly is impaired when compared with conventional brickwork in which units in each course overlap those in the courses immediately above and below. This reduction of integrity reduces the brickwork's resistance to lateral loads; a consideration relevant to structural masonry and also to brickwork as cladding, because both are subjected to lateral loading in the form of wind loads.
To compensate for lack of overlap in stack bonded brickwork, bed-joint reinforcement can be effective.
Welded, stainless steel, ladder-form, bed-joint reinforcement is typically used as, to be effective, the continuous tendons of reinforcement must be positioned towards the surfaces of the brickwork leaf, not at its centre. Although full consideration should be given to the details of any particular application proposed, bed-joint reinforcement is typically laid in joints at 225mm vertical spacing and wall ties are installed at the normal density for cavity walling. To assist the accommodation in mortar joints, oval or flat sectioned wire is advantageous, particularly when ties and reinforcement have to be installed in the same course.
For walling exposed to the weather, reinforcement and ties of stainless steel are recommended. As well as its enhanced resistance to corrosion the smaller cross-sectional area of steel necessary when using stainless steel, compared to galvanised mild steel, permits easier accommodation of the steel components within bed joints.
Other than requirements for protection against corrosion, no specification for the reinforcement itself or the frequency of its provision in the bed joints of stack bonded brickwork is included in the current British Standard Code of Practice for the Use of Masonry (BS 5628: Part 3), but BDA is aware that some specialist manufacturers of bed-joint reinforcement provide design advice for architects and structural engineers (Bekaert Building Products Ltd, 0114 2244488. BRC Building Products, 01785 222288).
Choice of brick
The colour and texture of the proposed brick should be considered in relation to the form of stack bonding proposed. As the bond depends on regularised mortar joints for it to read well, plain coloured bricks are preferable to multi-coloured types, which can camouflage the bond pattern. Bricks with a distinctive texture intended to be laid conventionally (flat), may not have a satisfactory appearance if laid on-end.
For good appearance of stack bonded brickwork the continuous horizontal and vertical mortar joints in the face of the work need to be straight and consistent in width. This requirement imposes greater stricture on consistency of the size and shape of the bricks used than does normally bonded work. Therefore, check on the practicalities of obtaining supplies of appropriate consistency when considering a particular brick. For clay bricks the limits of size permitted in BS 3921: 'Specification for clay bricks' will not provide an adequate method of regulating supplies and an alternative specification should be agreed with the manufacturer. There is no agreed industry standard for the maximum variation tolerable for such an application, but experience suggests that the difference in length between the longest and shortest bricks supplied should be no more than 3mm.
The straightness of a brick is also a consideration in regularity and bricks which are liable to be bowed may not give a very satisfactory appearance when stack bonded as soldiers. This aspect should also be discussed with the manufacturer, particularly when it is proposed to use bricks of long, thin proportions (7 and 8).
These requirements for bricks of more carefully controlled variations of size and form may well attract a slightly higher cost.
Choice of mortar
The colour, texture and profile of the mortar joint, as with all brickwork, strongly influences the overall appearance of the surface and should be chosen with due care. No special considerations apply to stack bonded work, except that the joints should be accentuated rather than disguised by using flush joints of mortar that matches the colour of the bricks.
The more rigorous geometry of stack bonds demands extra care and attention by the bricklayers, particularly if soldier courses are specified. Even when the bricks have been supplied as more consistent in size and form than is usual, constant checking of joint alignment will be required using string lines and a plumb level. When setting bricks as soldiers, a bricklayer should use a small boat level to check the verticality of individual bricks as they are laid (2, 7 and 8).
Care must also be taken with the alignment of the vertical surface of individual units so as to generate an overall flat plane of walling - 'hatching and grinning' describes the surface irregularity caused by the misalignment of face planes which is more noticeable when it occurs in stack bonding.
Prefabricated brickwork cladding
Stack bonding is often chosen by designers to signify that the brickwork is cladding and that it has no significant structural function, eg precast concrete cladding panels faced with brickwork. It may also deliberately exaggerate decorative forms to exceed the realistic capability of traditional brickwork (6).
Brickwork cladding on PowerGen's building in Coventry, by Bennetts Associates, is an interesting example of stack bonded brickwork (3). Here the reason for stack bonding was to ensure the alignment of perforations in the bricks to accommodate pre-stressed steel reinforcement tendons anchored to s/s angles at the top and bottom of each 3.6m x1.6m panel. The 102mm thick brick panels were prefabricated in a temporary workshop alongside the site and then craned into position when they were required.