Pictured above: office rooms built underneath a steel mezzanine deck, with the deck's mesh railing running along the top of the partition wall, photographed while the fit-out was still under way, from our project archive (V5-13). Here the office went below the deck. This article is about deciding which way round to do it.

It is a common request once a business has moved into an industrial unit. The stock has the floor, the admin team is squeezed into a corner, and nobody wants to give up pallet space for an office. A mezzanine looks like the obvious answer, and often it is.

There are two quite different ways to build it: the office on top of the deck with stock underneath, or a storage deck with the office built below. They look similar on a sketch, but they are not the same structure and they do not raise the same fire and submission questions.

Two ways round, and why they are different builds

Office on top. This is what we call an office mezzanine. Its load rating is lower than a storage deck — typically 200 kg/m² — but everything else is stricter: partitioning, acoustic decking and how the air-conditioning is integrated. It usually comes with an internal staircase, a glazed balustrade and partition walls for meeting rooms.

Office underneath. The deck above is a storage mezzanine, typically rated 500 kg/m² for pallet storage or 350 kg/m² for cartons. The office is a room built in the space below it, and the deck becomes that room's ceiling.

The difference that catches people out is the load rating. A deck is rated for what it was designed to carry, and that figure goes on its load notice. An office deck designed at 200 kg/m² is not a storage deck with desks on it. If there is any real chance that the upstairs office becomes stock space later, say so before the design is drawn. Designing for storage from the start is a decision; raising a finished deck's rating is an engineering job.

Headroom usually decides it for you

Before choosing, measure. For a mezzanine that is useful on both levels, you want a minimum of 4.5 m clear height between the slab and the ceiling structure. The deck itself takes around 300 mm, which leaves roughly 2.1 m of headroom above and 2.1 m below.

Where the clear height is between 3.5 m and 4.5 m, a mezzanine can still work, but only as a storage level with restricted headroom above. That settles the question in a lot of Singapore units: the level with restricted headroom can never be the office. If only one level can take people standing up all day, that is where the office goes, and the other level is storage. Below 3.5 m, a mezzanine usually does not make economic sense at all.

Two cautions on measuring. First, measure to the lowest thing hanging from the roof, not to the roof: sprinkler heads, beams, lights and ducts. One client measured 5.4 m of clear height; the sprinkler heads were at 4.8 m, and the BCA plan had to be redrawn. Second, an office underneath the deck gets its own ceiling layer — lights, air-conditioning, perhaps a false ceiling — and every bit of that comes out of the headroom below. More on reading clear height properly is in clear height: the warehouse number that decides how many racking levels you get.

If the office goes on top: people change the design

A storage deck is visited when stock goes up or comes down. An office deck has people on it all day, and the design follows the people.

• Stairs are sized to the occupant load. Mezzanine egress is calculated from how many people will be up there. A 400 m² mezzanine for 20 occupants may need only one stair; the same area for 80 occupants needs two stairs at opposite ends. Stairs are built to BCA and SCDF code — riser height, going depth, handrails and kickplates — which is covered in stairs, gates and handrails: the details that fail.
• Decide about a lift early. Some occupant loads or accessibility requirements mean a lift. We coordinate with a separate lift contractor, and the structural openings and the pit are part of our scope — which is exactly why it has to be known before the column grid is fixed, not added once the deck is up.
• Noise and air-conditioning. The office sits directly above a working floor. Of the common deck finishes, a composite deck with concrete topping gives the best acoustic performance, and air-conditioning has to be planned with the deck and partitions rather than hung off finished steel.

If the office goes underneath: the ceiling is a working deck

Putting the office below brings its own set of questions.

• Sprinklers. If your unit is sprinklered and the mezzanine obstructs the existing sprinklers, additional sprinkler heads must be installed below the deck — and the space below the deck is where your office would be. It adds cost and time, and it is either coordinated with your sprinkler contractor or included in the quotation — find out which before comparing prices.
• Smoke detection. SCDF generally requires smoke detection on the mezzanine level if the deck is above about 3 m.
• The deck finish and the sprinkler design go together. 22 mm particleboard is the cheapest option and suits non-sprinklered storage; chequer-plate steel takes heavier traffic and is sprinkler-friendly; open-grate steel is used where there are sprinklered floors below. Pick the deck with the office ceiling and the sprinkler layout on the same drawing.
• The deck moves when it is loaded. Pallets and cartons going on and off above a room with partitions and a ceiling is a stiffness question, not just a strength one. Why that matters is in mezzanine deflection limits.
• Columns land inside the rooms. Mezzanine column spacing has to suit whatever is underneath. If you also want pallet racking under the deck beside the office, the column grid has to work for the rooms and for the rack layout at the same time, which is why we design them together.

Does an office change the submission question?

It can, and this is the point to be careful with. The triggers for BCA submission we work to are: a mezzanine above approximately 300 m² gross floor area, a mezzanine carrying assembly use, a mezzanine used for any public-facing function, and any mezzanine that changes the building's GFA balance. SCDF submission comes in when a mezzanine affects the fire compartmentation of the unit, needs sprinkler tap-offs, changes the escape paths, or is above 300 m² gross.

You will often hear that a storage mezzanine below 300 m² in a single-occupant industrial unit does not need submission. That is often true — for storage — and even then we check the specific URA plot record first. An office is not storage. Enclosed rooms, more people and new walking routes are exactly the kind of change that can touch compartmentation and escape paths, and a room where you meet customers can start to look like a public-facing function. So do not carry the storage answer over to an office. Tell whoever surveys the unit exactly what the office is for, including whether clients will visit. The detail is in the conditions that trigger a mezzanine BCA submission and SCDF fire-safety endorsement for mezzanines.

Where submission is needed, we handle the PE-stamped structural calculation, BCA endorsement, SCDF fire-safety plan and Certificate of Occupancy coordination inside the same mezzanine quotation. Our billing also moves part of the fabrication milestone to a "submission accepted" trigger, so you are not paying for steel to be fabricated for a design that has not been accepted.

If the unit is leased, start with the landlord. The slab is assessed from the building's original structural drawings, which are typically obtainable from the landlord or BCA records, and you will want the landlord's agreement in writing before anyone draws a column grid.

Who builds the office itself

The mezzanine and the office fit-out are usually two trades, and the problems tend to happen where they meet. We often work alongside the client's interior designer or fit-out contractor on column placement, deck finishes and ceiling clearances. The rule we work to: the structural design holds, and the fit-out adapts to it. We do not sub-let any structural scope to fit-out contractors.

The practical advice is to get both parties looking at the same drawing before the column grid is fixed — where the doors go, which rooms a column lands in, and how much ceiling depth the fit-out needs under the deck. If you do not have a fit-out contractor, our sister company Larry Contractors does office fit-outs.

Plan for the day the office moves

The mezzanines we install are bolted construction, so they can be partially disassembled and extended; when a client asks, we design with that in mind, typically reserving one column line as the future expansion edge. Welded mezzanines are much harder to extend. Our smallest mezzanine is around 80 m², so if what you need is one small room, ask whether a mezzanine is the right answer at all.

Before you ask for a quote

• Office on top, underneath, or not sure yet.
• Clear height to the lowest obstruction, not to the roof.
• Floor area of the office and of the whole deck.
• How many people will work there, and whether clients will visit.
• Whether the unit is sprinklered, and whether you want racking under the deck.
• Who is doing the fit-out, and whether the landlord has agreed.
• Whether the office level might become storage later.

The site survey is free for serious enquiries.

The short version

• Office on top: an office mezzanine, typically 200 kg/m², stricter on partitions, acoustics and air-conditioning. Office underneath: below a storage deck rated 500 kg/m² for pallets or 350 kg/m² for cartons.
• If the office deck might ever carry stock, design it for storage from the start.
• 4.5 m clear height gives about 2.1 m on each level. Below that, the level with restricted headroom is storage, never the office.
• In a sprinklered unit, an office underneath may need its own sprinkler heads below the deck.
• Do not assume a storage mezzanine's "no submission" answer applies to an office.

If you are planning an office in your warehouse and want to know which way round works for your unit, WhatsApp us on 9107 2601 with the clear height, the floor area and what the office is for, and we will tell you honestly which layout the unit allows. Deck ratings and the regulatory triggers are on our mezzanine page.

Related reading: mezzanine platforms in Singapore · mezzanine BCA submission conditions · SCDF fire-safety endorsement for mezzanines