Setting-out a building or a wall with ranging lines attached to pegs driven into the ground at the corners is all well and good until it comes to the time to excavate for the foundations. Corner pegs will be in the way. The pegs will have to be removed to allow excavation to take place, which means that all the original setting-out will be lost.
To avoid having to go through the whole setting-out process again in order to establish the position of the walls on the new foundation, it is necessary to find a way of holding ranging lines in their correct position above the ground but away from the point of excavation. This is done by transferring the ranging lines to profile boards that are erected away from the confines of the outer wall lines of the building or structure, in a position where they will not be disturbed by the excavation. This will typically be around 500mmm away from the point where the foundation trench is to be excavated, but it will need to be further (sometimes up to several metres) in case of mechanical excavation, to allow for machinery access.
The profile itself comprises a horizontal board (approximately 75mm x 30mm x 850mm), face-fixed through two supporting pegs (see Fig 55). When profiles have been erected all the way round the building, the ranging lines can be detached from the nails in the top of the corner pegs and then extended back to the profile boards, making sure that they precisely retain their original linear position. This is achieved by keeping the ranging line tight and just in contact with the nail in the top of the corner peg (in other words, its original fixing point) as the line is extended back to the profile board. The line is held carefully in place and the top edge of the profile board is marked with a pencil. Where the profile has been marked, a clout nail is inserted or a shallow saw cut made, ready to secure the ranging line.
When all the profiles have been marked and made ready to receive the ranging lines, the original corner pegs can be removed and the ranging lines attached to the profile boards. At this stage, it is wise to re-check the setting-out for both square and accuracy of dimensions; this should be done at least every time the ranging lines are altered, or taken off and re-attached. Pegs and profile boards can remain in place for extended periods of time so it is good practice to check the accuracy of the setting-out at the start of every working day – there is always a possibility that pegs and/or profiles could have been disturbed at some point.
In the example (see Fig 55), the external wall lines of the garage are shown on plan extended to timber profiles.
Profiles can contain additional information, such as the width of the foundation, where extra ranging lines can be attached so that excavation lines can be marked out on the ground. This can be simply achieved by spreading a thin line of lime immediately below the ranging line, or by using spray paint specifically made for the purpose. Some excavator drivers prefer the centre line of the foundation trench to be marked in addition or instead, as it allows them accurately and easily to centre the excavator’s bucket.
Once excavation has been carried out and the concrete foundation placed, the ranging lines can be re-erected at the external wall-line positions, and then checked for accuracy. After this, the wall lines can be transferred down to foundation level and the brickwork may be set-out ready for construction.