Building with Bricks
Sean Wilkins advises on how to get the best from this ubiquitous building material
The versatility of the raw clay material and manufacturing process means that it is possible to fabricate bricks to a wide range of dimensions. The brick dimensions stated by the manufacturer are known as the work size and the most widely used work size for UK clay bricks is 215mm x 102.5mm x 65mm.
UK clay bricks tend to be a standard size but the manufacturer’s tolerance levels should be checked.
When specifying brick it is important to consider the tolerance and range values declared by the manufacturer. The tolerance is the difference between the stated work size and the average actual size.
The tolerance will be stated as T2 (generally the smallest deviation from the stated work size), T1 or Tm (deviation in mm from the stated work size declared by the manufacturer; it may be wider or closer than the other categories).
Tolerances (T2, T1 or Tm) in brick sizes
The range value covers the dimension difference within a sample, between the largest brick and the smallest. The range will be stated as R2 (generally the smallest range), R1 or Rm (range in mm declared by the manufacturer; it may be wider or closer than the other categories).
The range value (R2, R1 or Rm) gives the difference in mm between the largest and smallest brick in a sample
By ensuring that the setting out of the building horizontally and vertically is a multiple of the co-ordination size, you can reduce the need to cut bricks or adjust mortar joints.
The nature of the brick-making process means that the final colour and size can fluctuate between different batches. Bricks are made from natural materials, which can have different physical qualities. Whilst manufacturing systems minimise variations as much as possible, slight variations will remain. As variations may occur in clusters this can result in unacceptable patches, patterns or banding in completed brickwork unless good site practice is followed.
Although some manufacturing plants can shuffle bricks as they are being packed, there is no substitute for blending on site which will disperse any irregularities and achieve an overall harmonious appearance. Larger sites should agree a reference panel with the brick manufacturer prior to supply.
Bricks should be selected from a minimum of three packs and it is advisable to draw from the packs in a vertical/diagonal manner, rather than horizontal layers. If a manufacturer issues specific blending instructions, these should take precedence.
It should be noted that blending of bricks will be to little avail if mortar colour is not consistent and blending multicoloured bricks is just as important as blending bricks that are uniform in colour, to avoid one colour being grouped together.
Avoiding rainwater penetration
The diagram highlights the important design, specification, and workmanship requirements to ensure that water does not penetrate through the external brickwork cavity wall.
The key points for avoiding water penetration in to brick work are:
- Assess the severity of exposure to wind-driven rain. The UK west coast is generally wetter and windier.
- Assess the relative exposure of the project. Free-standing walls and parapets will be most severely exposed.
- The water absorption of a brick has relatively little impact on the rainwater penetration.
- Stronger mortars will generally reduce the amount of rainwater penetration.
- Well-tooled mortar joints will reduce the water penetration. Bucket handle and weather struck are the most effective (see mortar joints below).
- A wider cavity will reduce the amount of water penetration.
- A section of clear cavity, rather than fully filled insulation, will reduce the amount of water penetration.
- Keeping the cavity clear of mortar drops will prevent creating a bridge for moisture.
- Roof flashings should be installed in the mortar joint, underneath the cavity tray.
- Overhanging copings will reduce the extent to which brickwork becomes saturated.
- Fully fitted mortar joints will reduce the amount of water penetration.
- Cavity trays should include stop ends and weep vents.
- Continuous cavity trays should be installed immediately above openings.
- Larger areas of metal or glass can concentrate rainwater onto brickwork if there is no drip detail installed.
- Cavity trays should be included under all sill, copings and cappings.
- Half-brick thick single-skin walls, such as those used for garages, will let water through during prolonged bad weather.
About 18 percent of surface area for stretcher bond is mortar, so it plays an important part in the visual appearance of the brickwork. A mix of cement, lime and sand is recommended to get the best balance of workability and performance. The strength of the mortar should be suitable for the exposure condition, but not stronger than is necessary. How the joint is finished is also important.
Most low-rise residential construction does not require movement joints when constructed from clay brickwork. Unreinforced walls expand approximately 1mm per metre during the life of a building due to combined thermal and moisture movement changes. Not all clay types expand at the same rate and in some cases an allowance of greater than 1mm/m may be necessary. It is advisable to check this information with the brick manufacturer when specifying movement joints.
For clay bricks, the maximum recommended spacing between movement joints is 12m for a straight run of brickwork and reduced to 6m from a corner. For multiple corners, the 6m limit is effectively reset from each corner.
Short returns in brickwork that join two longer lengths have specific movement requirements.
Where the two long lengths of brickwork expand into a short return there can be a tendency to crack if the return is 675mm or less (which is equivalent to three bricks in stretcher bond). The solution is to de-bond the short return from one of the longer sections. If this is done at the internal angle, then the mastic can be colour matched to the mortar and the joint is not seen as different from a normal bonded corner.
Where the return is longer than this there will be sufficient length within the return to allow for expansion of the adjoining brickwork and there is no need to include a movement joint.
Sean Wilkins is Technical Manager at the Brick Development Association