Prefab or built on site – when does it pay off?
Prefabricated piping is often sold as faster and cheaper. Sometimes it is. Here are the dividing lines – and the four questions that decide whether prefab pays off in your project.
Prefabricated piping solutions are often sold as faster and cheaper. Sometimes that is true. Other times you pay for flexibility you never use. Here are the real dividing lines.
What prefab actually is
Prefab means that pipe runs, modules or complete technical units are built in a workshop and shipped to site for installation. Instead of cutting, welding and testing in a cramped plant room, the work is done under controlled conditions, with fixed equipment and a fixed crew.
That sounds obviously better. It isn't always.
When prefab pays off
When work repeats. If you are building 40 identical bathrooms, 12 identical plant rooms or a series of identical cooling circuits, prefab is almost always right. Unit cost drops sharply after the first few, and quality is more consistent than when four different crews do the same job in four different places.
When site conditions are cramped or hazardous. Welding in a full plant room with other trades around you is both slow and risky. Move the work to the workshop and much of the HSE burden and trade coordination disappears.
When the schedule is tight. Prefab lets you produce in parallel with the building going up, instead of waiting for the shell to be ready. This is often where the real gain lies – not in the hourly rate, but in the calendar.
When quality must be documented. Workshop production gives traceability: who welded what, when, with which parameters and with what test result. In projects with strict documentation requirements, that alone justifies the choice.
When it doesn't pay off
When the design isn't finished. Prefab requires final dimensions. If a route changes after the modules are produced, you pay twice. Projects with many unresolved interfaces are poor prefab candidates.
When refurbishing older buildings. Old buildings rarely match their drawings. On-site flexibility is then worth more than workshop precision.
At low volumes. Without repetition, the setup cost doesn't pay for itself. A single plant room is rarely cheaper for being built in two places.
When logistics are difficult. Modules have to be transported, lifted and brought in through an opening. If access is poor, transport and crane hire eat the gain.
The questions that decide it
Before you decide, ask four questions:
- How much repeats?
- How locked is the design?
- What does a day of delay on site cost?
- Will the modules physically fit where they need to go?
If you answer "a lot", "locked", "a lot" and "yes", prefab is right. If you answer the opposite on two or more, build on site.
The most common mistake
It isn't choosing the wrong method. It's choosing too late.
Prefab has to be decided early enough for the design to be adapted to it. If the decision is made after the drawings are finished, you get the costs of prefab without the benefits. The assessment belongs in the early phase, together with the client and designers – not when the piping contract goes out to tender.
Rør System delivers both prefabricated piping solutions and design engineering, and is happy to make the assessment early. Read more about smart prefabrication and design engineering.