Container Loading Guide: Ten Decisions the Forklift Never Makes

Part of the Roofing Slate Guide's Buying & Importing section — the loading half of the story whose crate half lives in Roofing Slate Packaging and Crate Standards, and whose m² arithmetic lives in How Many Square Metres Fit in a Container?

People think loading a container of slate is straightforward: a forklift goes in, a few tonnes of crates go in, the doors close, done. I've been the man at the forklift for eleven years, and the honest truth is the opposite: the forklift is the last thing that loads the box. The box is loaded first on a sheet of paper — a grid, a column of weights, a sketch of where each crate will sit. The forklift is just the execution.

This page is that paper. I'm going to walk you through the loading decisions in the order we make them at the bay, with the numbers that matter and the mistakes I've personally paid for. If you're buying slate from China, you never have to make these decisions — but knowing what they are tells you exactly what "well loaded" means, what to ask for, and what to look at when the container opens at your end.

The Loading Sheet: Six Numbers That Must Be on Paper Before the Forklift Starts

Every container in our bay starts with a sheet. Not a blank sheet — a sheet with six lines already filled in, because the forklift only knows how to pick up a crate; it never knows how many to pick. The sheet knows.

You'll notice there is no line for "slate size." That's because the slate size was already locked into the crate size at packing. By the time a crate reaches the bay, its dimensions are the crate's dimensions — and the crate's dimensions were chosen in the packing standards so they tile the container floor without waste. A standard 60 m² crate sits about 1.05 × 0.7 m on the floor; four across is a snug fit in a 2.35 m-wide 20ft box. The m² math that says "1,130–1,450 m² for 7–9 mm slate" — the container capacity page runs it in full — is really saying "19 to 24 of these crates." The sheet simply adds the last step.

The Loading Order: Heavy Down, Far First, Door Last

Once the sheet is signed, the forklift earns its keep. There are three rules in the bay, and they don't come from a manual — they come from what a container does to its cargo at sea.

Rule one: heavy down. The heaviest crates go on the bottom row, the lightest on top — never the other way. A 1.3-tonne crate on top of a 0.9-tonne crate is asking the lower one to do the seaway's work twice: it must hold its own weight through every roll, plus the crate above it. Slate is strong against pressure, but it's a roof tile, not a pillar; it's the ground.

Rule two: far first, door last. The crates that go deepest into the box are loaded first, so the last crates in are the first ones out. That sounds trivial until you've seen the reverse: a receiver opens the door and pulls the closest crate first — and if the closest crate was the heaviest, the whole stack above it settles on nothing. The door-facing crates are the lightest crates and the last ones loaded. They're the first ones out. Always.

Rule three: the door row is an airlock, not a wall. This is the rule most buyers never see, because it's the rule that's visible only inside the box. The last row of crates sits 20–25 cm short of the door. Why? Because the door is where the container gets pushed, lifted, braked and strapped — the handle, the crane, the truck. A crate that touches the door directly gets the first shock of every port move. Leaving a shallow airlock and locking the last crate to it with wedges means the door takes the hits and the slate doesn't. It's also a moisture buffer: a slab of air that lets the temperature inside change without the crates feeling it.

Rule two and rule three together are the reason the loading order isn't a choice — it's a procedure. The door row has to be the last row in, and the last row in has to be light, and the light rows can't be the first rows down. The sheet fixes all three before the first crate moves.

The Two Small Spaces That Decide Everything: Under the Floor and Between the Crates

Two gaps in a container get almost no attention — the one under the crates and the one between them. Both are where the cargo actually moves at sea.

Under the floor. A container floor is not a flat sheet; it's a set of long joists — the deck bearers — running the length of the box, and the floor panels sit on them. The loading rule of the bay is: every crate should bear on at least two or three bearers, not sit like a seesaw on the gap between them. A crate sits square on the floor, and we lay the wedges and bearers under it along its full width, never just under a corner. A corner-weighted crate is the one that arrives with a warped frame and split slats — and it's the one that makes the receiver's photo look like the crate was dropped, when it was really just sat wrong.

Between the crates. Slate crates are not bricks; they're dry-stacked timber frames that settle, shift and breathe. Leave a true gap between two rows and the box will widen it with every roll — the rows clap against each other like hands, and the slates inside feel the clapping. The bay rule is: no gap bigger than a fist left unfilled. Small gaps get wedges and shims, the wide gaps get an air bag (dunnage bag) that we inflate in place until the two rows meet it snugly. When the receiver opens the door and finds a deflated bag lying flat, that's the bag doing its job — it took the motion so the crates didn't.

The Centre of Gravity: Why a Half-Tonne Difference Can Tip a Container

Here's the number most loading sheets never include — and the one that has cost me the most sleep. A 20ft container with 21 crates of slate holds roughly 21 tonnes, distributed over 6 meters of floor. If that weight is balanced evenly along the length, the container sits true on the trailer, the crane and the ship's hold. If it isn't, the container develops a habit: it tips toward the heavy end.

Two balance rules from the bay:

• Longitudinal balance — keep the centre of gravity within about ±2% of the container's length from its centre. For a 6 m box, that's roughly 12 cm of tolerance. A heavy cluster of crates at the front end makes the container nose-heavy; a heavy cluster at the door end makes it tail-heavy, and a tail-heavy 20ft box is the one that sits with the doors up and scrapes the ground on every ramp. The heavy crates go mid-length, slightly forward, and the light rows taper to the door.
• Lateral balance — keep the left–right difference under about 5% of the total weight. The crane doesn't care if a container is 100 kg heavier on one side; the sea does. On every roll the unbalanced side loads the other side's structure, and the container's whole weight swings on one corner. A 21-tonne load swinging on one corner is how container floors crack — not from a hit, from a repeat.

The simplest version is on the sheet: heaviest on the bottom, the total within a few hundred kg of the box's working limit, and the heavy rows as close to the middle of the box as the crate count allows. If the buyer reads nothing else from this page, read that line — it's the one that keeps a container off its tail.

The Loading Record: The Sheet Becomes the Contract's Numbers

The loading sheet has a second life. When the doors close and the seal goes on, the sheet is no longer a plan — it's a record, and it becomes the numbers on the packing list, the invoice and the bill of lading. This is the moment the documents guide means when it says the weight must match in three places: the weight on the sheet is the weight the invoice says, because the sheet said it first.

So a good bay signs the sheet in ink and files it. The position sketch — crate 1 at the far front, crate 21 at the door, airlock row marked "wedged" — is the answer to almost every claim that comes later. When a receiver opens the container and finds a damaged crate, the first question isn't "who dropped it," it's "where was it." The position record tells you that in one line. I've seen claims settled in a day on the strength of the loading record, and claims dragged for weeks because nobody had written down where the crate sat.

At the Receiver's End: Reading the Loading Plan Backwards

You'll never watch your slate being loaded. But when the container arrives, you can read the plan backwards, and it will tell you the whole story:

• The first crates you pull are the last crates loaded. They should be the lighter, door-row crates. If the first crate you pull is obviously heavy, the loading order was wrong.
• Look at the airlock. If the last row is pinned against the door with no gap — no airlock — that's a container that wasn't loaded to plan. If there's a wedge or a deflated air bag against the door, the plan was followed.
• The batch numbers on the crates should match the packing list. One batch, three places — the sheet's third line is the one that shows up here as a marking on wood.

That's the arrival inspection from a different angle: the receiving checklist looks for damage, but the loading plan tells you whether the damage was planned around. A crate that sat where it was supposed to sit, on dunnage, with an airlock — that crate's damage story is almost always a story about the sea or the port, not the loading. The breakage allowance guide tells you what damage is normal; the loading record tells you who owns it.

Five Loading Failures I've Seen at the Bay (and the One-Line Fix for Each)

Every bay has its war stories. These five are the ones I've seen repeat across different factories, and each has a one-line fix that would have prevented it:

• The missing loading sheet. Forklift starts before the plan is drawn; crates go in "as they come." The fix: a signed sheet before the first fork, with a position sketch. Non-negotiable.
• The corner-heavy crate. A crate set on one bearer corner, or on a floor patch that's cracked. The fix: the floor inspected before loading, dunnage laid along the crate's full length.
• The wall-squeeze. A crate pushed hard against the container wall, the slates touching metal. The fix: a shim or wedges so no slate touches the wall — slate against wood is a snug partner; slate against steel is a different story.
• The door row against the door. The last row flush against the door, so every port bump is transmitted straight into the cargo. The fix: the 20–25 cm airlock, wedged.
• The unrecorded load. The sheet thrown away after loading, so the batch numbers and positions exist nowhere. The fix: the signed sheet filed with the packing list. It costs one minute and it's the only record of where the damage happened.

None of these is exotic. All of them are visible in the box after loading — and all of them are avoidable with the sheet, the wedges, the airlock and the record that this page is about.

Before You Buy: What to Ask the Supplier About Loading

Most buyers never think to ask how the container will be loaded, because it sounds like the supplier's business. It is — and that's exactly why you should ask. The question is free, and it tells you more than the price does:

"Do you have a loading plan? Can I see it after loading?"

A factory that loads by plan will send you the sheet with the container — positions, weights, batch numbers. A factory that doesn't will tell you "we load carefully" and then there's nothing on paper to prove it. The difference is the whole difference between a cargo and a guess. The day the container arrives, the plan is your copy of the truth, and it also gives your pre-shipment inspector something to check against at the bay.

That's the container loading guide. The plan first, the weight in the middle, the light at the door, the airlock, the record. It's not a mysterious craft — it's a checklist, a sketch and a signature, repeated well. And if you want to see the craft on a real container, our roofing slate is loaded this way every time — ask us for a loading sheet sample with your next quote.

FAQ: Roofing Slate Container Loading