Use of a Brick Rod
Bricks are easy to cut, using a bolster or brick saw, and traditionally it has been common to make up any desired length of wall or opening by cutting a closer. Nevertheless, it is preferable to make all the lengths in multiples of bricks or half-bricks. If the brickwork is to be seen, avoiding closers always gives a neater appearance. If it is to be concealed, by plastering for example, using exact brick dimensions is less important, but it makes for slightly easier laying.
A brick rod is a set of measurements showing the height or length of brickwork of any number of whole or half bricks in length, and any number of courses in height. On the job, the bricklayers will use an actual rod (a piece of timber or metal) marked with the heights of each course, for setting out the courses as the work is laid. Lengths are set out with a tape, by laying a brick at each end of a wall, stretching a string line between them, and filling in between the end bricks. The last brick laid at one end (next to the end brick) closes the course, and is therefore called a closer. The term "closer" usually refers to a brick cut to fit in the available space, but can also be applied when the setout allows full bricks to be used throughout.
Designing to a vertical brick rod in exact course heights is even more important than in the horizontal dimensions. Cutting bricks for a part-height course is more difficult (it is possible to cut some dry-pressed bricks with a bolster, but in most cases a brick saw is necessary), and it is more unsightly. While closers within a course can be arranged so that they are never less than a half-brick in length, a cut course does not have that option.
One option for making up the height of a wall is to use a soldier course at the top. The soldiers (bricks laid vertically on end) can be cut to any suitable length.
Standard brick sizes and brick rods
The size of bricks varies from country to country, but at present there are Australian Standards which are met fairly consistently across the country. A standard metric brick is 230 x 110 x 76mm high, and a metric modular brick is 90 x 290 x 90 high. Although the dimensions of the standard brick are not obviously modular, when the joints are added each brick occupies 240mm in length, and therefore a module of 600 or 1200mm can be accommodated by full and half bricks. Seven courses of 86mm would make up a height of 602mm, but the bed joints are adjusted so that seven courses come to a module of 600mm. A modular metric brick and its joints occupies 300 x 100 mm on the face, so that modular dimensions of 100 and 300mm are easily met.
Charts are available showing the dimensions of a pier or opening using standard brick sizes and dimensions, as shown here. The difference is that a pier has one less perpend than it has bricks; the width of an opening has one joint more than the number of bricks.
To increase productivity, a range of bricks are available with a larger face size. Thicker bricks are also produced specifically to provide the fire and acoustic isolation needed for party walls. In most cases they retain the standard 230 length, but vary in width and height. Most of these are not intended to be used for facework, but can be used for both internal and external walls that are intended to be rendered and painted. Some larger units are made for facework in larger-scale and commercial buildings. Typical sizes are:
- 230 x 150 x 76 (thick for party wall)
- 230 x 110 x 119 (one-and-a-half course height)
- 230 x 150 x 119 (thick, one-and-a-half course height)
- 230 x 90 x 162 (thinner, two-course height)
- 290 x 90 x 162 (two-course height face blocks, made in Western Australia)
- 290 x 90 x 119 (one-and-a-half course height face blocks, made in Western Australia).
Roman brickwork, as well as some modern brickwork in Europe and the United States, uses bricks with a much longer, shallower proportion. The proportions can be very attractive, but a shallower standard-length brick would increase the cost of laying, and a longer brick would have to be also wider to maintain the 2:1 proportion on plan that allows corners to be turned neatly. One company is known to be producing bricks 230 x 110 x 50 (10 courses plus joints = 600 height) at the time of writing.
Because of the manufacturing process, bricks can be expected to vary in size a little. There will be a variation in size between individual bricks from the one batch, and the average size for a batch may vary from the standard. A small variation between individual bricks can easily be accommodated in the joint thickness. If the average size differs much from the standard, it might not be possible to maintain the standard rod dimensions. Darker coloured bricks are usually burned longer, and at higher temperatures, than the lighter coloured ones. This results in greater shrinkage during firing, and dark bricks are usually smaller than light ones. This is a particular problem if polychromatic brickwork (bricks of different colours used in a pattern in the same wall) is to be used.
Manufacturers generally test fire each new clay deposit coming into production, to ensure that the end product complies with standards. The standard defines the average size by measuring a set of 20 bricks together, and individual variations by sampling individual bricks. The requirements of AS/NZS 4455-1997, "Masonry Units and segmental pavers" are discussed in Size Tolerance in Brick Types.
At earlier times in the history of Australian brickmaking, considerable variation in brick size occurred, but there remains very real frustration among architects and contractors about current size variation, given the current sophisticated mechanised production techniques.
Typical dimensions of dry-pressed face brickwork in some older Sydney buildings are: (all in mm)
||Maximum length variation
Although length variation is smaller and perpends are tighter than today's brickwork, it should not be concluded that there was less size variability in the past, as there was more non-face work to absorb the out-of-size bricks. For high-quality work, bricklayers would cull the worst of the out-of-size or distorted bricks and use them as commons elsewhere in the job.
For face brickwork, it is important to ascertain whether the chosen bricks will be sufficiently uniform and accurate in size that standard horizontal and vertical rods can be used. A few millimetres of size tolerance can be accommodated in the joints, but horizontal joints larger than the assumed 10mm are not desirable.
In order to achieve a particular dimension, the joints can be adjusted a little to get the desired result -- provided this adjustment is not cumulative with an adjustment for the actual non-standard size of the bricks! A typical 3m storey height requires 35 courses. By making the bed joints 9 or 11mm instead of 10, the rod can be adjusted up or down by 35mm, almost half a course either way. A horizontal bay of 6m needs 25 bricks, so that a 1mm variation in the width of the perpends would give a variation of 25mm either way. As can be seen from the examples of measurements on historical buildings above, tight perpends have been common in the past, and are generally not objectionable. On the other hand, extra wide perpends tend to look unsightly.
Preference for standard dimensions
Using a standard brick rod becomes important when off-the-shelf windows are to be incorporated. These are made to suit a 300mm module, and any multiple of 600mm will fit without cutting bricks if the standard rod is used.
When details are being worked out, it should be remembered that a pier of, say, four bricks width is actually 4 bricks plus 3 joints = 950mm, while an opening of four bricks width is 4 bricks plus 5 joints = 970mm. With the height of walls, there is a bed joint under the lowest course, so that the height of the wall is a simple multiple of the course height. The height of an opening contains an extra joint thickness, but if there is a metal arch bar at the top, this will occupy the equivalent of one joint. If there is a precast or reinforced brickwork lintel, then the opening gains the extra bed joint in height.
Brickwork built off a shelf angle doesn't have a mortar joint under the first course, but the soft joint under the angle will be larger than a standard joint, and the vertical rod will not be continuous from one floor to the next. (See Control Joints.) In most cases, if the external walls are supported from the structure at a given level, all the brickwork will be discontinuous at the same level, so that the rod can start again at each level of support.
Set-downs in slabs and balconies
In full-brick construction, the set-down at the edge of a slab should be in multiples of brick courses, so that courses in the inner skin (built off the slab), and the outer skin (built off the set-down) will line up. This is necessary so that the cavity ties will be level, and it simplifies detailing around the heads and sills of openings. The same argument applies at the set-down at a balcony.
When the external skin is brickwork and the inner walls either studwork or blockwork, these arguments do not apply. However, in most cases a one-course step is a good minimum size for effective waterproofing, except perhaps in regions of very high wind, or in the upper stories of taller buildings, where a greater height might be needed.
Setting out the first course
If the joints are in a contrasting mortar colour, they will be quite prominent, and the setout of the joint pattern deserves some thought. It is generally considered preferable for the edges of the openings to align with the centre of a brick (left diagram) than with a perpend (centre diagram). There will probably be a sill under the opening, maybe a soldier course feature over the head, and maybe a frame of coloured, projecting, or recessed bricks all around (right diagram).
In any of these cases, to make sure that the setout is the way the designer intended, it is necessary to detail enough of the facade to show how many courses occur under the sill, and from sill to head, and to determine whether the first course begins with a full or half brick.