On Site or Off Site Manufacture?

Dave Mills
A succession of reports has promoted off site manufacture to help improve construction outcomes in terms of reduced cost and programme and increased quality delivered in a safer manner but are off site solutions the greatest things since sliced bread?

The Challenges

Off site solutions vary from panelised systems at one end of the spectrum to modules which bolt together with all the fit out included at the other.

Panelised systems, utilising a timber frame construction pose some of the greatest challenges in terms of risk management, particularly regarding the protection of the construction workforce and existing adjacent properties from an increased fire risk.

If, having undertaken a risk assessment for the use of timber frame, it’s use is precluded, options can now be explored for other systems e.g. steel framed stud work, panelised or volumetric solutions. Logistics and site access need to play an important part in the evaluation. Some panelised frame systems can be manufactured in sections small enough for two people to carry short distances from delivery vehicles if there is no crane access, whereas volumetric solutions need good vehicle and crane access.

To be successful and reap all the potential benefits offsite solutions have to offer, design, site access, Health and Safety and procurement all need to be considered at the project initiation phase.

Confined sites by their very nature have neighbours near so the use of offsite construction can reduce the disruption and inconvenience to stakeholders, however the installation period (all be it short) should not be overlooked as this can involve road closures and weekend working. Access to site, overhead lines, turning radii, one way streets will all present challenges.

However, we have found several Design and Build contractors actively seek to use off site solutions for the construction of schools, recognising the positive benefits of a reduced-on site construction period on the day to day operation.

Safety and Access Considered, Can Offsite Solutions Accommodate The Proposed Design?

Timber frame and modular work best in uniform designs with stacking load bearing walls. Flying brickwork can be accommodated but with secondary support creating a hybrid solution. This introduces complexity, additional trades and therefore cost. This increases risks and dilutes the benefit of a single trade providing a significant part of the structure and fabric.

Volumetric module sizes are limited by UK Highway Laws for road transfer and whilst units over 2.9m x 18.65m can be accommodated by some manufacturers, each unit is then classed as an abnormal load adding further cost and complexity especially around site logistics.

Designers need to plan for and coordinate the required breaks between modules with natural breaks in building layout e.g. wall lines. If layout and fenestration are critical a system will need to be selected early (RIBA stage 1/2) to inform the design process. This may result in the need to tender for a provider outside of the normal RIBA stages, bringing procurement into play early in the project life cycle. This, then in turn challenges some procurement processes, and requires early engagement with procurement teams to either prescribe a new route or agree appropriate waivers. Where programme critical, lead in times of the modular solution may need to be considered if expecting a contractor to purchase the structure. If the appointment date of the contractor doesn’t consider the lead time for the modular solution there could be a gap in the programme and it may result in delayed finish and ultimately a dilution in realisation of benefit, however accepting that this is the case and planning to adopt an offsite solution early in the project life cycle can enable the benefits, of reduced time on site, and increased quality to be realised.

Designers need to plan for and coordinate the required breaks between modules with natural breaks in building layout...

Prefabricated and fitted out bathroom pods have been around in the industry for over 10 years with some hotel chains using them routinely to achieve consistency, and quality of fit out in the construction of their new hotels. These types of product work well where repetition and quality of fit out are the order of the day and have also seen some success in student accommodation.

So How Far Can I Take The Modular or Off Site Solution?

It is great for schools as it reduces the time spent on site, minimising disruption, construction risk and safeguarding issues associated with having a construction site within a school.

Fast food chains and high street supermarket small town centre stores common use this technology. These units are typically repetitive in design and require a high quality fit out first time. It has the bonus of being a good neighbour through reduced time on site and can start generating income earlier. The Chinese have been adopting this approach for some time, an extreme example of this can be shown here. The latter is particularly true if the layout is sufficiently uniform to risk construction ahead of planning permission being granted with any locally applied external finishes being applied as the fit-out progresses.

Why Aren't we Building More Off Site?

Mortgage lending for non-traditional construction types has historically been a challenge with only selected lenders being prepared to support the purchase of these types of properties. This approach is slowly changing as they start to see the increasing quality attainable and the stigma of the post war non-traditional house types is diluted.

Modular/offsite has its place but it is important to recognise that it is not the right solution in every case. Traditional methods still have a place in the 21st century construction industry to create iconic and interesting buildings that inspire and excite. However, where speed of construction and repetition are key drivers it firmly has a place in the construction industry of today and tomorrow.

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