‘Mortar joint’ is the term for the space of mortar between bricks. According to NZS4210:2001 Section 188.8.131.52 the thickness of a mortar bed, cross or perpend joint should be 10mm +/- 3mm. A joint thickness of up to 20mm may be accepted on the bottom course to accommodate any slab level issues.
Table 2.2 of NZS4210:2001 further stipulates that the maximum tolerance on mortar joint thickness shall be +/- 3mm on thickness specified
Good trade practice holds that Mortar Joint thickness should be as near to consistent throughout the brick veneer as possible. This can be achieved by spending time setting up a gauge (storey) rod prior to laying.
It is good practice to identify the average brick height by measuring a selection of 10 bricks (from at least 3 different pallets).
Before starting, a measurement should be taken to establish the distance between the top of the foundation to the soffit. It is good practice to check this measurement at several locations. This measurement is then divided by the brick height plus 10mm (to allow for mortar). This will help establish the number of courses required and establish a mortar joint thickness.
Substrate height = 3300mm
Brick height = 76mm Brick + Mortar height = 86mm
Substrate height (3300mm) divided by Brick + Mortar (86mm) height = 38.37
A decision needs to be made, whether to use 38 or 39 courses (this should be at the discretion of the bricklayer as a slightly larger joint may look better with a slightly larger brick).
For this example, 38 courses are being used.
Substrate (3300)/number of courses (38) = gauge height (86.8).
To make a gauge rod, a straight piece of batten with the substrate level marked on it should be used. A set of dividers should be set to the gauge height and the gauge rod marked (and courses numbered). This is then transferred to the profiles (at corners).
This method will enable the bricklayer to foresee any need to adjust the mortar thickness remembering that NZS4210:2001 Section 184.108.40.206 allows adjustments of +/- 3mm or increasing the bottom bed joint up to 20mm to make up the height.
You can read the next section here:
Types of Mortar Joints