A double-level office building is a great way to get more square metres without using any more land. It also creates a pleasant and elevated workspace with a view.
At XL Structural, we design and deliver new commercial buildings, often with a mezzanine floor. It's easy to incorporate a mezzanine floor in to your building plans, provided the structure is strong and well-engineered. Please note that we do not retrofit mezzanine floors into existing buildings.
So what should a mezzanine be constructed with — concrete, timber-framed, or cross-laminated timber (CLT)? While the answer ultimately depends on your requirements, we explore the pros and cons of each option in this article so you can make an informed decision.
A timber-framed mezzanine is constructed of timber joists that span between supporting beams or load-bearing walls. The joists typically have 400mm centres and can be standard SG timber (which is a milled timber such as 190x45, 240x45, or 290x45) or a laminated joist such as LVL, glulam, or hyjoist. Insulation is placed between the joists and plywood or strand board flooring, about 20mm thick, is laid over the joists.
If your floor requires a fire rating, the underside of the floor will need to be lined in fire-rated GIB. The ceiling can then be painted and left as is but, often in larger offices, a suspended ceiling will be installed to give space for services, air conditioning ducts, and similar features. Supporting walls will need to be lined in fire-rated GIB and supporting steel will need intumescent paint or fire GIB.
Below is a cross-section of a timber mezzanine floor:
This flooring system is used all over New Zealand in most houses and many commercial buildings, meaning all builders are familiar with it. Onsite changes are typically straightforward as the whole floor system is constructed onsite.
For smaller mezzanines under 150m², it tends to be a simple, cost-effective option, especially when the ground floor has walls that can be made load-bearing and are no more than 6m apart.
The main disadvantages arise for larger floors over 150 to 200m². In timber framed mezzanines, noise transfer is typically a lot higher than with concrete or CLT floors. This can be partially mitigated with insulation and larger joists but can be quite distracting when there is a large upper level with a lot of people working.
As the above images show, there are a lot of components to the floor, especially when a suspended ceiling will be installed below. This means that on larger floors the labour component can quickly add up, which will make it cost much the same as a concrete floor.
Additionally, on larger floors, a lot of steel or laminated beams and joists are required to avoid too many load-bearing walls. This also pushes the cost closer to concrete and CLT.
A concrete mezzanine consists of a concrete slab poured in situ and supported by steel beams and columns. The steel beams and columns are installed to provide a steel structure that supports the concrete. Then, a tray system such as ComFlor or HiBond is laid on top of the steelwork. Reinforcing is then laid on the floor, exterior boxing is placed and concrete is poured on top of the tray, or deck.
The concrete already has a fire rating so only the supporting steel needs to be fire-rated. This is usually done with intumescent paint or it can be lined with fire GIB. Often the underside of the floor is left exposed and services are also exposed, creating a modern industrial look. Alternatively, a ceiling can be suspended to conceal services. Below are some example details:
For a large floor over 300m², a concrete system becomes more cost comparable or even cheaper than a timber-framed option. While the steel and concrete in a concrete mezzanine floor add to the cost, there are fewer labour hours required, resulting in a shorter overall build time and lower labour costs.
A concrete floor has much better acoustic properties than a timber floor. If you have a large upper level with a lot of people working, a concrete floor will reduce the noise transfer between levels.
Leaving the concrete and supporting steel exposed downstairs gives a clean industrial look that is also very low maintenance.
Concrete and steel are a common system that commercial builders are well-versed in. There is also onsite flexibility before the pour as the formwork and piping can be adjusted.
A concrete floor is heavy, adding to the load on the ground. This is usually not an issue, unless the ground is poor. In this case, the Geotech investigations will advise the best course of action. As the floor is heavy, there are usually more bracing elements required in the office, in the form of steel diagonals or ‘K’ frames (pictured below).
These braces are not necessarily a large cost but do tend to impact the windows. Either the space where there is bracing lined over and ‘hidden’ or the brace is placed in front of the window as part of the look of the building.
In principle, a CLT panel floor is very similar to a concrete floor. However, instead of pouring a concrete floor on top of the steel structure, a 126mm thick timber panel is laid on top of the steel. The timber panel consists of solid layers of timber glued together to make an extremely thick, large panel. For further explanation, refer to this brochure.
Like a concrete slab, CLT panels have a fire rating so it is only the supporting steel that needs to be fire rated. This is achieved with intumescent paint or a fire GIB lining. Often the underside of the floor and services are left exposed or a ceiling can be suspended to conceal them.
CLT panels are a faster option than concrete because the timber only has to be laid onto the steel, whereas concrete requires trays and reinforcing to be installed and then the concrete poured and cured. Therefore, a CLT panel floor is likely cheaper than concrete and is more efficient. In a recent project, the builders allowed three days to install the CLT panel floor and had it completed in less than a day.
While concrete mezzanine floors often require specialist contractors for the Nelson studs and concrete pouring, CLT installation is easy to learn. With a bit of training, a builder can easily install CLT because it is a timber product, using typical fixing details.
CLT panels are a lot lighter than concrete, placing a lighter load on the footings, supporting steel, and bracing elements.
Red Stag says that every cubic metre of timber stores 250kg of carbon, while every cubic metre of concrete produced emits 110kg of carbon.
It can also look really smart underneath if it is unlined and left as timber, but this is a matter of opinion.
In the past, the supply of CLT has been unreliable, which is why XL Structural Steel and other commercial builders have not used it as much. It has typically been used in speciality projects that use a full timber system, or multi-level apartments. A lot of CLT comes from Australian-based XLAM but, recently, more manufacturing space has opened in NZ, giving more options in the industry.
Structurally, the timber does not span as far between intermediate beams as concrete. However, this does not typically have too much of a cost impact.
CLT requires additional protection, such as GIB, to get a fire rating above 60 minutes. Fortunately, this is not usually a problem as most floors typically only require a 60-minute rating. Additionally, the CLT surface is not as hard-wearing as concrete and may need coating to maximise durability. However, most offices have carpet or tiles anyway.
CLT panels are more difficult to adjust onsite because the penetration notches and joints are done in the factory with a CNC machine. This means it is treated similarly to pre-cast concrete, with its own set of shop drawings.
If your offices are small and the upstairs is less than 200m², a timber-framed option is more cost-effective because NZ3604 can be used to design the offices. This avoids extensive engineering and a lot of steel beams and, if your downstairs space has a lot of rooms that aren't going to change, the lower level walls can be made load-bearing, saving on bespoke beam designs.
If your office space is over 200m² per level, a concrete floor or CLT is likely better in terms of cost and functionality.
At XL Structural Steel we have an in-house team of commercial and industrial building specialists. If you're still unsure or would like advice more specific to your situation, talk to our in-house team of commercial and industrial building specialists who can work alongside you to understand your building project needs and collaborate with you to define your design and budget requirements.