Prefab vs. On-Site Construction: Making Informed Choices for Your Project

Construction methods have evolved significantly over the years, offering various approaches to building structures. Prefabricated (prefab) construction is gaining popularity for its numerous benefits, including less disturbance at the final property, reduction in waste and carbon footprint, and improved quality control. This article aims to compare a prefab approach with traditional, on-site building practices.

Definition and Process

Prefab Construction

Prefabricated construction involves assembling building components off-site in a controlled manufacturing environment. These components are then transported to the construction site for assembly. This method relies on standardized modules that are manufactured and assembled according to predetermined specifications. Larger projects combine multiple prefabricated ‘modules’, an approach known as Modular Construction. 

On-site Construction

On-site construction, sometimes referred to as ‘stick-built’ construction, entails assembling building components directly at the construction site using raw materials. This method relies on traditional construction techniques, where workers frame and erect the structure on-site, piece by piece.

Time On-site and Property Disturbance

Prefab construction significantly reduces the time spent on-site compared to traditional construction methods. With prefab, the majority of construction work is completed in a factory setting, minimizing disruption to the property and surrounding areas. This approach compresses the on-site construction window to as little as a few days, depending on utility connections and landscaping requirements.

On-site construction typically involves months or even years of construction activity, resulting in prolonged noise and pollution disturbance to the property and neighboring areas.  A larger footprint is required for delivery and storage of materials, supplies, and the preparation of building components on site.  Over longer timeframes, trash, debris, and chemicals are more likely to accumulate and make their way into the soil. Poor weather conditions can further extend construction timelines. 

Quality Control and Resiliency

Prefab construction offers high levels of quality control and consistency due to manufacturing in a controlled environment. On-site construction is more susceptible to defects in workmanship, which can extend timeframes to rework a section, or affect the quality and consistency of the final structure. 

Most importantly, excessive moisture levels in the framing materials because of exposure to rain in on-site construction can lead to mold growth and the accelerated breakdown of building materials. Many builders are unwilling to wait for the moisture content of their materials to drop down to the necessary 19% moisture content, even when building science specialists now believe the moisture content really shouldn’t be above 12% before insulating to avoid hazardous conditions. 

Cost Considerations

Prefab construction can lead to cost savings through reduced labor, shorter construction schedules, and minimized material waste. However, upfront manufacturing and transportation costs may offset some of these savings.

On-site construction may incur higher labor costs and longer construction timelines, especially in remote areas with extended daily transportation. Unplanned variables such as weather can contribute to unexpected delays and additional expenses, ultimately impacting overall project costs.

Material Waste Reduction

Prefab construction is known for its ability to reduce material waste through advanced planning and design. According to a study by Boston Consulting Group (BCG), construction waste accounts for 10% to 20% of the total materials in conventional construction projects, and that prefab construction could reduce a project’s waste by 50%. Even more optimistically, a report by the U.K. Waste & Resources Action Programme (WRAP) shows that prefab methods can achieve up to a 90% reduction. 

In conventional, site-built projects, materials are delivered to the site in bulk. There is generally less precise planning for quantities, as it is more efficient to have excess material on the jobsite than to run out.  The excess materials are then typically discarded at the end of the project. In contrast, for prefab facilities, excess materials are saved for use in the next build. 

Environmental Life-Cycle Performance

An Environmental Life-cycle Assessment (LCA) is the entire “cradle to grave” analysis of a building’s environmental impact. A study by researchers at McGraw-Hill Construction found that prefab buildings generally have better environmental life-cycle performance than conventional buildings. Factors such as reduced transportation emissions and optimized material use contribute to the overall sustainability of prefab construction.

A larger number of smaller vehicles will produce more carbon emissions than a smaller number of larger vehicles. Less on site construction time means less transportation movements to and from the project site.  According to the report “Waste Reduction Potential of Offsite Volumetric Construction,” prefab construction can reduce the total number of deliveries to sites by 90%, dramatically reducing the total number of trips by workers and for material deliveries.

Prefab construction generally involves a greater level of collaboration between project design teams, engineers, material and product suppliers, and manufacturers. This increased collaboration could lead to more integrated solutions that further enhanced sustainability.

Worker Wellbeing

Factory-based construction allows for steady, year-round work, and is much less affected by extreme weather that may cause work stoppages on conventional projects. It also counts as a significant improvement in working conditions to be able to work in a climate-controlled facility with the assistance of machinery to move, lift, and place components.

Design Flexibility

Site-built construction may be beneficial for projects with a lot of custom, site-specific design choices, allowing architects and builders to make real-time adjustments and modifications in response to the land throughout the entire construction process.  Prefabricated building systems may only offer design flexibility within predefined parameters.

Conclusion

Prefabrication is emerging as a viable and sustainable alternative to conventional construction methods, offering a multitude of benefits for the building industry and the environment.

Both prefab and on-site construction methods have their advantages and limitations, making them suitable for different project scenarios. Prefab construction offers speed, quality, and cost benefits, particularly for projects with standardized designs. On the other hand, on-site construction provides greater design flexibility and customization options, making it ideal for complex or unique structures. Ultimately, the choice between these methods depends on project-specific requirements, budget considerations, and desired outcomes.

References

Boston Consulting Group (BCG) (2019) The Offsite Revolution in Construction, USA: Boston Consulting Group.

Faludi, Jeremy & Lepech, Michael & Loisos, George. (2012). Using Life Cycle Assessment Methods To Guide Architectural Decision-Making For Sustainable Prefabricated Modular Buildings. Journal of Green Building. 7. 151-170. 10.3992/jgb.7.3.151. 

Jaillon,L., Poon, C.S., Chiang, Y.H. (2008) Quantifying the waste reduction potential of using prefabrication in building construction in Hong Kong. Elsevier 

Tam, C.M., Tam, V.W.Y., Chan, J.K.W., Ng, W.C.Y., (2005). Use of prefabrication to minimize construction waste – a case study approach. The Int. J. Construct. Manage. 5 (1), 91–101.

Table ‘HISTRATE’, ‘RIDIND 2010/11’ and ‘HISTINJ’. HSE RIDDOR - Reporting of Injuries, Diseases and Dangerous Occurrences Regulations. http://www.hse.gov.uk/statistics/tables/ [Accessed on 02/27/2024]

WRAP. ND. Waste Reduction Potential of Offsite Volumetric Construction. WAS003-003: Offsite Construction Case Study