Construction methods
A construction method (also called delivery system) is the way an owner arranges who designs and who builds. The three common ones are:
- Separate trades. The owner hires each trade (mason, carpenter, electrician) directly. Most control and often lower price, but the owner must coordinate everyone.
- General contractor. The owner signs one contract with a main builder, who hires and manages the trades. Simple for the owner. The designer is separate and can check the work.
- Design-build. One firm designs and builds. It is fast and has one point of responsibility, but there is less independent checking.
There is also a choice about how parts are made: on site (cast and built in place) or prefabricated (made in a factory, then lifted and joined). Prefab is quick and neat but needs good transport and cranes.
Construction contracts
A contract is a written agreement between owner and builder. It states the work (drawings and specifications), the price, the dates, how payment is made and what happens if something goes wrong.
- Fixed price (lump sum). One price for the whole job. If the real cost rises, the contractor bears the loss. Good when the drawings are complete.
- Cost-plus. The owner pays real cost plus a fee or percentage. Good when the work is unclear, but the owner carries the risk of a rise.
- Unit price. A price per unit (per m³ of concrete, per metre of pipe) multiplied by the real quantity.
A variation (change order) is a written change to the work, with its price and time.
Construction planning
A construction plan lists the tasks, how long each takes and the order (what must finish before the next can start). Drawn as bars on a time line it is a bar chart (Gantt chart). The chain of tasks that sets the end date is the critical path: a delay on it delays the whole job.
The plan also covers workers, materials, machines and safety.
Supervision
Supervision (also called site inspection) means checking that the work follows the drawings and specifications, in good quality, on time and safely. The supervisor:
- compares real progress with planned progress every week;
- checks materials and tests (for example concrete cubes);
- checks work before it is covered, like steel bars before concrete;
- records problems and orders corrections.
If the job is behind, the cause is found first, then more workers, shifts or machines are added.
Try it
Plan a small job: building a bookshelf. List 4 tasks, give each a time in hours and mark which must come first. Add the hours to find the finish time. Then imagine the wood arrives 2 hours late: which tasks move?
Key formulas and definitions
- Contract = work + price + time + payment terms
- Fixed price: owner pays the agreed sum. Cost-plus: owner pays real cost + fee
- Total time = sum of tasks on the critical path
- Progress gap = real progress − planned progress
Worked examples
1. A house is bid at a fixed price of 50 lakh. Steel prices rise and the real cost becomes 54 lakh. Who pays the extra 4 lakh?
The contractor, because the price is fixed. Extra work asked by the owner would be a separate variation.
2. Same job under cost-plus with a 10% fee and real cost 54 lakh. What does the owner pay?
54 + 10% of 54 = 54 + 5.4 = 59.4 lakh.
3. Tasks run one after another: foundation 3 weeks, frame 4, walls 3, finish 2. The foundation is delayed by 2 weeks. New finish time?
Normal total = 12 weeks. With the delay, 14 weeks, because each later task starts 2 weeks later.
4. At week 6 the plan says 50% should be done but the supervisor sees 40%. What does this mean?
The job is 10 percentage points behind. Find the cause and add workers or extra shifts.
Common mistakes
- Choosing a contract type before the drawings are clear. If the work is unclear, a fixed price causes arguments.
- Thinking the contractor and the supervisor are the same. The supervisor checks the contractor.
- Ignoring a small delay. If it is on the critical path, the whole job slips.
- Changing the work by speaking only. Changes must be written as a variation with price and time.