The world isn't flat... but your warehouse floor should be

A “flat” floor sounds like a fairly basic proposition. Aren’t all concrete floors flat, or at least flat-ish?

Actually, no. Currently, the Conslab team are building several world-class floors for Very Narrow Aisle (VNA) racking warehouses across New Zealand, and I can tell you right now that there’s a lot more to a “flat” floor than meets the eye.

What we’re seeing is that VNA systems are becoming a trend not just in the region, but worldwide. In the last few years in Asia, nearly 50% of the warehouse floors I constructed were designed to cater to VNA, which guarantees faster material handling and more storage room — effectively giving you more bang for your buck. What’s not to like about that?

The thing is, we still see a lot of confusion in the market when it comes to picking the right floor flatness specification for VNA projects, which typically require a very tight tolerance for surface regularity. With that in mind, I thought it’d be a good time to revisit the topic of floor flatness.

When I’m presenting a seminar or conference talk on this subject, I like to get people thinking about it in terms of a few key principles:

1. What "floor flatness" actually means.
2. Why it matters.
3. Why different logistics systems demand different approaches to floor flatness.
4. Why UK and USA specs are okay, but not German ones — and please leave your straight edge at home!
5. Why your floor flatness specification can have a radical impact on your construction method, time, and cost.

What do we mean when we talk about "floor flatness"?

I think it’d be a good idea to establish our terminology before we dig too much deeper. Generally, when people in the English-speaking world talk about “flat flooring” or floor flatness specs, they’re broadly referring to what we in the industry call floor surface regularity. If you wanted to impress someone at a party, you could mention that your warehouse floor has "excellent surface regularity in terms of the departures in elevation from a theoretically flat plane," but most of the time, just talking about a“flat floor” is fine.

In the specifications we use, "flatness" is determined using short wavelength measurements, which look at the way the floor profile changes over 300 mm. In practice, these measurements tell you three things: how bumpy your floor is, how good your finishing process was, and whether you’ll be able to run your forklift operation smoothly.

I think it’s a good idea to point out that “flatness” isn’t the same as “levelness” — which is the other key surface regularity criteria to think about when you’re building a warehouse floor. Levelness relates to measurements over a longer distance (typically 3 m), and is determined by how well the floor is "struck off" during the casting process. It’s just as important as flatness: if you get your levelling wrong, you’ll quickly run into problems with your racking system.

Why is floor flatness important?

When you really get down to nuts and bolts, a warehouse floor has one critical function: it’s a surface to store stuff on and move things around on.

Of course, different businesses and industries have different goals for their warehouse floors. E-commerce giants like Amazon are obsessed with fast order fulfilment, and they need nifty little robots running around on the floor to make this happen. An FMCG beverage giant like Coca Cola or PepsiCo might favour Automated Storage and Retrieval Systems for their products, while a paper manufacturer in South East Asia might be happy to store big rolls of paper directly on their warehouse floor.

Nevertheless, the floor is a fundamental enabler in each of these scenarios. After all, you can’t run robots efficiently, install an ASRS system properly, or move paper rolls in and out if your floor isn’t up to the task.

This is exactly what makes your floor’s surface profile so important. Imagine for a second how ridiculous it would be to build a shiny new warehouse or distribution centre and then decide to cut costs by leaving sub-base flooring (rocks) in place, instead of a concrete floor. I think we can all agree that there’d be no point in having built the warehouse in the first place.

The things is, you make the same mistake when you build a warehouse without thinking about floor flatness, which ultimately determines:

• The types and heights of logistics system you can operate, and how much you can store;
• How quickly your material handling equipment (e.g. forklifts) can move stuff in and out;
• How much you’ll spend on maintenance for your material handling equipment;
• How safe your warehouse is.

I like to compare floor flatness to the air in your car tyres. It doesn't matter whether you’re driving a new Ferrari or a Toyota Prius, you don't want to ride around on flat tyres. In my eyes, building a warehouse without getting the floor flatness right is like refusing to put air in your brand new car’s tyres.

At the tail end of the curve, bad floor flatness can lead to some pretty serious problems...

Why do different types of warehouses have different floor flatness requirements?

I’ve said it before and I’ll say it again: it’s horses for courses in construction. Everyone sets up their warehouse with different goals in mind, and they’ll use different systems and technology to make their goals happen.

When it comes to concrete flooring, your movement and storage needs will ultimately determine your flatness requirements. Traditionally, we make decisions about warehouse floor flatness based on the kinds of racking layouts and forklifts an operator needs to use. Regardless of whether you’re a mega billion dollar company like Amazon or a South-East Asian paper manufacturer, your warehouse floor will almost always fall into one of two categories: "free movement", or "defined movement.”

As you can probably guess, these terms tell us how vehicles will traffic on the floor. When forklifts and other material handling equipment are free to turn and run along or across an aisle, you get a “free movement” situation. Traditional "wide aisle" selective pallet racking warehouses, or warehouses where pallets are block-stacked on the floor are both classic “free movement” situations.

I like to use the analogy of driving to work in the morning to explain the concept of a free movement floor. Generally, I drive the same route to work every day. However, I might drive in several different lanes or stop off for a coffee somewhere, and if there are roadworks I might duck down a side street to avoid the holdup. This exactly how a forklift operates in a "free movement" environment.

This is a pretty typical free movement warehouse. Forklifts can move relatively freely throughout

On the other hand...

In Very Narrow Aisle (VNA) warehouses, you’ll find forklifts moving on fixed pathways between high, closely spaced racking. This is a classic “defined movement” situation, where forklifts have to be guided by automation to move stuff in and out. In this situation, the forklift runs the same wheel path over and over again.

It’s like catching the train to work in the morning instead of taking the car. The train will run from A to B on the same tracks every day, so you won’t stop for coffee or duck down a side street on your way in.

Understanding this distinction between free and defined movement warehouses is important, because it leads us to a couple of key realisations:

1. A Defined Movement (VNA) warehouse needs a higher standard of floor flatness, compared to a Free Movement environment. Because of the close spacing and height of the racking in VNA warehouses, even small issues with the floor profile can become a big problem at the top of the forklift mast. Similarly, forklifts running on a fixed wheel path can’t deal with uneven flooring. Think about it this way: a car could drive around a pothole in the road if it needed to, but a train definitely couldn’t. See the problem?
2. We have to take a different approach to measuring floor flatness in Free Movement vs. Defined Movement situations In a defined movement environment (VNA warehouse), we can measure the fixed wheel path the forklift will operate on and calibrate floor flatness accordingly. In a free movement environment, we don’t have that luxury — it’s just not practical to measure the infinite number of wheel paths a forklift might traffic. Instead, we take a sample of the overall floor flatness to satisfy ourselves that the surface regularity is good enough.

Small issues with flatness in a VNA aisle can have a big impact on how a forklift operates - a 3 mm difference at floor level can mean a 45 mm lean at the top of the forklift with high level racking

Why UK and USA specs are okay, but not German ones — and please leave your straight edge at home!

In general, the European, UK, and USA material handling and construction industries all have effective ways of specifying floor surface regularity. In Europe, this takes the form of EN15620, which mirrors the UK Concrete Society TR34 (4th Edition), while in the USA the preference is for ACI Fmin and F-number systems.

Fundamentally, all these specifications have the same goals. In free movement warehouses, they control for flatness and levelness using short and long wavelength measurements. In defined movement scenarios, they measure elevation across and along the aisle, as well as the rate of change — or "bumpiness" — of those metrics, effectively quantifying whether an MHE will lean or rock.

However, US specs aren’t quite as sharp as En15620 or TR34, because they don’t specify a control from datum (i.e. whether the surface of the floor overall is within +/-15 mm of the specified level). Also, the American F-number system doesn’t measure levelness as comprehensively as the the European and UK methods, which both sample from an orthogonal grid across the entire floor.

I’ll be frank about German standards — they’re not ideal. Although they’re occasionally used (erroneously) by MHE suppliers, German specs like DIN 15185 don’t control for rate of change in defined movement scenarios. They’ll give you a floor that looks flat, but they won’t stop dynamic lean or MHE sway. More recent VDMA standards aren’t much better. Although they do include a "waviness index," VDMA metrics seem poorly calibrated: a couple sheets of printer paper on the floor can take you from a pass to fail.

In New Zealand, we’re not much better. Kiwi specifications like NZS 3114 that control for floor flatness are often completely unsuited for warehouse construction. For example, they recommend taking a 3 m long “straight edge” measurement between two points to determine flatness. Needless to say, it’s wildly impractical to measure large areas of floor this way, and the construction industry tends to ignore the specs entirely — making them meaningless.

Why does your floor flatness specification have such a radical impact on your construction method, time, and cost?

Defined movement floors need tight tolerances to keep their forklift operations running, so they’re traditionally cast using a "long strip” method.

Long strip construction involves laying the floor in long narrow strips — typically two aisles wide. This allows formwork to be set very accurately to the correct height, and the strip is screeded “board to board,” giving you very precise flatness across the width of the aisle. Skilled technicians then use highway straight edges to cut high spots and fill low spots for accurate longitudinal flatness.

This meticulous method can make VNA flooring about 25% - 50% more expensive than free movement floors, which are levelled fairly easily and cost-effectively using semi-automated, laser-guided equipment (Laser Screed).

Long strip construction also tends to be slower, leaving a floor with a lot of joints and/or saw cuts. As weak points in the floor, these joints can require ongoing maintenance.

Fortunately, modern construction methods do make tight floor flatness tolerance less costly. While building New Zealand’s first independently certified super flat VNA floor for the James Pascoe Group, Conslab was able to develop a hybrid "cast & grind" approach which was recognised by Concrete NZ as a very cost efficient way of delivering “DM1,” the highest VNA flatness standard in the world. Our methodology also allows the floor to be constructed with the minimum number of joints, reducing floor maintenance and ownership costs over the long term.

Using "cast and grind" techniques has allowed Conslab to construct fast and cost effective VNA floors

At the end of the day, the increased storage density per m2 of floor and faster operating efficiency tend to make VNA buildings very cost effective over their asset life — even though their up-front cost can be higher than for a typical selective pallet racking warehouse. It’s this fact that’s been driving the big uptick in VNA system use over the last few years.