Short note: Centre line method

The centre line method is a quick way of finding the quantities of earthwork, foundation and walls in a building whose walls have the same section.

Procedure

1. Find the total centre-line length of all the walls (main and partitions), measured along the middle of the walls.
2. Deduct for the junctions where walls meet: each T-junction takes away half of the width of the wall (the part counted twice); a cross-junction deducts the same on each side, i.e. it counts as two T-junctions.
3. Multiply the corrected length by the width and the depth of each section (trench, PCC, footing, plinth masonry, superstructure).

Formula

$$L_{corrected} = L_{total} - n \times \frac{b}{2}$$

where $n$ is the number of T-junctions (a cross-junction counts as two) and $b$ the width of the wall at that level.

Example

For a room 4 m x 3 m (clear) with a 0.23 m wall: $L = 2(4.23 + 3.23) = 14.92$ m. A partition with two T-junctions would deduct $2 \times 0.23/2 = 0.23$ m. The trench volume = $14.92 \times 0.9 \times 1.0 = 13.43$ m³.

Merits and limits

It is fast and simple, and suited to uniform rectangular buildings; it is not accurate where the wall sections differ, for which the long wall-short wall method is used.