Construction management of services work
Installing building services is a team job. The site manager watches four goals together: Quality (work as designed), Cost, time (delivery date) and Safety.
The main tools are:
- Schedule (Gantt chart): tasks as bars on a timeline.
- Critical path: the longest chain of linked tasks. If one task on it is late, the whole job is late. Tasks off the path have spare time (float).
- Quality checks: inspect materials when they arrive, test pipes before they are hidden in walls, and keep records.
- Safety: safe ladders, lifting, hot work and electrical isolation, with a short talk every morning.
- Coordination: trades meet often so that pipes, ducts and cables do not clash.
Making the schedule
Steps to schedule a job: (1) list the tasks, (2) estimate how long each takes, (3) decide which task must come before which, (4) draw bars, (5) find the longest chain. In the 3D, pipes (3 weeks), ducts and cables overlap, and the test (2 weeks) starts only after they finish. If the pipes are delayed, every following bar moves by the same amount.
Cost estimation of services work
An estimate is a careful guess of the cost before the work starts.
- Read the drawings and list every item (pipes, valves, fittings, ducts, cables).
- Quantity take-off: measure or count each item (metres, pieces).
- Multiply by the unit rate (price per metre or per piece, material + labour).
- Add the direct costs (material, labour, machines).
- Add overheads (site office, supervision, head office share) and profit.
- Add a small contingency for surprises.
Formula: Estimate = Σ(quantity x unit rate) + overheads + profit + contingency.
Keeping time and cost under control
During the job, compare progress with the plan every week. If a task on the critical path slips, add workers or work in shifts. Keep stores of material small but enough, so that work does not stop. Record every change, because extra work must be priced and agreed in writing.
Key formulas and definitions
- Cost of an item = quantity x unit rate
- Direct cost = material + labour + machines
- Estimate = direct cost + overheads + profit + contingency
- Series tasks: total time = sum of durations
- Critical path = longest chain of linked tasks
Worked examples
1. 40 m of pipe: material 12 and labour 8 per metre. Overheads 15%. Find the estimate.
Direct = 40 x (12 + 8) = 800. Overheads = 0.15 x 800 = 120. Estimate = 920.
2. Pipes 3 weeks (start week 0), ducts 3 weeks (start week 2), cables 3 weeks (start week 4), test 2 weeks after all work. Find the finish week.
Pipes end at 3, ducts at 5, cables at 7. The test starts at week 7 and ends at week 9.
3. In the last problem, the pipes are 2 weeks late and everything after moves. What is the new finish?
Every task moves by 2 weeks, so the finish is week 9 + 2 = week 11.
4. Direct cost 5000, overheads 12%, contingency 5% on the subtotal. Find the final estimate.
5000 x 1.12 = 5600. 5600 x 1.05 = 5880.
Common mistakes
- Forgetting overheads and contingency, so the estimate is too low.
- Counting tasks as in a line when some can run together.
- Starting the final test before all work is complete.
- Not recording changes, so extra work is never paid for.