. SITE DATUM
All vertical dimensions and levels on new walling and buildings are derived from one fixed point on the site. This is referred to as the ‘site datum’ and is generally established
on site prior to any construction work starting. Site datum may also be referred to as a ‘temporary bench mark’ (TBM).
Site datum level is most commonly established on site at the same level as the horizontal damp proof course (DPC) of the proposed building or wall. For a completely isolated structure, datum can simply be established at a convenient point near to the proposed structure by inserting a wooden peg with its top at 150mm above finished ground level; 150mm is the minimum DPC height above finished ground level for new buildings as required by the Building Regulations. For a boundary or garden wall, despite there being no statutory requirement, there is little reason to deviate from this minimum height dimension.
For new walls or buildings that are adjacent or near to an existing structure, it is common to derive datum for the new brickwork from DPC level of the existing building. A peg is simply inserted at DPC level adjacent to the existing
The ‘datum peg’ should be in a position on site where it can be seen and easily accessed, but where it will not be vulnerable to any damage from machinery or falling materials or passers-by. Maintaining the accuracy of the site datum is of fundamental importance since all vertical dimensions and levels for the new walling are derived from it and a site datum that alters during the course of the work can have disastrous consequences.
The site datum peg should be driven into the ground at its required location and then, ideally, surrounded by concrete (see Fig 56). The bigger the project, the longer the datum will be required, so consideration should be given to providing it with additional protection in the form of a triangular timber framework, as shown.
Fig. 56 Site datum peg with protective timber framework and concrete.
Once datum has been established it must be transferred to both ends of the wall being built, or all the corners for a new building, again, using either a spirit level and straight-edge, or a Cowley level. Concrete foundations cannot be relied upon to be flat or level, so datum is needed at every corner of a building or end-point of a wall, so that the bricklayer, when constructing the corners, can check the gauge of the brickwork from the top of the foundation concrete. In this way, any adjustments to gauge (‘picking up’ or ‘grinding down’) can be made below ground level, thus ensuring that all the brickwork will be the same level by the time DPC level is reached. For more on the method of checking gauge from the top of foundation concrete, see Chapter 9.
In order to avoid the need for making adjustment to gauge below ground, the datum peg may be used as a basis to calculate how deep to excavate the foundation trench so that, when the concrete has been placed, the vertical distance between the top of the foundation and datum level works to the gauge of the brickwork without the need to adjust the thickness of bed joints (in other words, in multiples of 75mm).
As an example, a simple strip foundation would require a minimum thickness of 150mm and assume a ground cover between the top of the concrete to finished ground level of 1000mm. Assuming that datum has been established at DPC
level of 150mm above finished ground level, the overall distance from datum to the bottom of the foundation trench would be 1300mm (calculated as 150mm + 1000mm + 150mm), with the top of the finished foundation being 150mm higher at a depth of 1150mm below datum. This total of 1150mm divided by a brickwork gauge of 75mm works out to 15.33 courses of brickwork from the top of the foundation to datum level. Clearly, this will not work to gauge – 15 courses would finish short and 16 courses would finish too high. Given that the shortfall is only 0.33 of a course (approximately 25mm), ordinarily the bricklayer would choose to adjust gauge by ‘picking up’ the bed joints as the brickwork is built out of the ground. (For more on basic bricklaying skills, see Chapter 9.)
There is an alternative to picking up the bed joints and that is to round up to the next full course. This would mean making 16 courses from foundation level to datum level but excavating the foundation trench a little deeper to maintain a gauge that requires no adjustment. Rounding down to 15 courses is also feasible, but this will mean raising the level of the foundation, with the possible risk of compromising any minimum foundation depth requirements. On the basis of rounding up to 16 courses, the depth of the foundation trench will need to be excavated to a depth below datum level of 1350mm, that is to say, the sum of 16 courses x 75mm + 150mm (for the concrete thickness).
Clearly there is a judgement to be made by the bricklayer, based on the following options: the bed joints may be picked up to gain 25mm, or a 75mm gauge may be maintained by going to the trouble of setting-out and excavating an additional depth of 50mm along the whole of the foundation trench, and laying an extra course of bricks. Obviously, the latter would take additional time, the extra excavated soil would need carting away and there would be an increase in the quantity and cost of bricks and mortar. In most cases then, the convenience of not having to adjust the thickness of bed joints comes at a high price that is never really likely to be viable. Rounding down to 15 courses avoids all the extra excavation and additional bricks and mortar, but still requires time to set-out the revised depth accurately. Most bricklayers would probably still opt to pick up bed joints to gain the 25mm required.