Roofing Slate Packaging and Crate Standards

I've spent twelve years on the export side of slate, which is a polite way of saying I've signed more packing lists than I've seen roofs. Somewhere in that time I learned a rule that the test reports never mention: a slate roof fails in three ways — bad stone, bad fixing, bad arrival. The first two get all the columns in the standards documents. The third one just gets blamed on the shipping line.

Most buyers I talk to treat packing the way they treat the label inside a shirt — something that happens to be there, vaguely useful, rarely read. Then the container arrives with a corner missing on one crate and eleven tiles that ring like cracked plates, and suddenly the phrase "shipping damage" appears on everyone's lips, though nobody can point to the line in the contract that said how it would be packed.

This guide is the thing I wish someone had handed me the first time I signed off a container: the packing spec, line by line, in the order the crate actually gets built. What every line does, what it costs when skipped, and the numbers that separate a crate that arrives whole from one that arrives "mostly."

Line 1 — Bundling: The Small Pack Within the Pack

Before the crate, there is the bundle. Serious slating factories don't throw tiles loose into a box; they tie them in bundles of ten to twenty pieces — a cardboard sheet between every second or third tile, a strap around each bundle, and the bundles laid in the crate like bricks in a wall, staggered so the corners of one bundle don't sit on the corners of the next.

Why does the middle of a bundle make any difference to a tile? Because slate breaks at the corners first. A loose pile of tiles in a crate is a pile of weapons pointed at each other; the top face of every tile is bearing the entire weight of the tiles above through four tiny contact points. Bundle the tiles and the cardboard spreads the load across the face. Stagger the bundles and no two corner stacks meet in the same column. The whole trick of packing slate is spreading pressure — pressure is the silent killer, and every bundling layer is a pressure distribution device.

Ask your supplier for the bundle count: "bundled in 10s" or "bundled in 15s" — and, more importantly, ask to see the packing plan on paper. A factory that bundles in 10s and crates in 100s has a total count that works out clean. A factory that bundles in 10s and says "around 1,200 pieces per crate" is asking you to do the arithmetic that they didn't.

Line 2 — The Interleaving: Cardboard, Foam, or Nothing at All

Between the bundle and the next bundle goes an interlayer. The trade name is interleaving, and the material decides the entire moisture and friction story of your shipment:

My honest rule for slate: corrugated paper is fine and nearly everyone uses it; foam is a premium you buy when the voyage is long or the climate is violent (think South Africa to UK in winter). What is never fine is "nothing" — on that one, I've opened crates where the tiles looked polished by hand. That is vibration damage, and it lives under the surface, invisible until the roof is on and the first frost arrives.

Line 3 — Corners: Where Most Slate Dies

Ask any port inspector what breaks first and the answer is always the same: corners. The corner of the tile is the thinnest edge, the least supported, the point that hits the crate wall when the whole crate shifts. Packing protects corners in three places:

• On the tile: corner protectors at the four corners of each bundle or crate load — cardboard or foam angles that stand the stack away from the timber.
• On the crate: the timber itself is the protector; the crate corners are where the wall frames take the impact of the neighbouring crate or the container wall. Crates with full-height corner posts survive a container shift better than edge-only frames.
• In the container: the space between the crates and the container wall is your last line of defence. Packed tight, or padded with the cardboard off the bundles, the walls of the container are actually load-bearing in the plan. (More in Line 10.)

Read the corner part of a packing spec like a lawyer: "corner protection" is a noun with no dimension. Corner protection by what — cardboard 3 mm? 10 mm foam? Half the crates I see arrive damaged arrived with cardboard corners in the brochure and bare timber in the corner of the container. Ask for the thickness, write it into the order, photograph it at loading.

Line 4 — Straps and the 24-Hour Re-tension

Steel or polyester strapping holds the crate together, and every export packing has some. The detail nobody puts on the brochure: strapping stretches. Polyester strap in particular — the same product that holds the crate — relaxes a measurable amount in the first 24 hours, and by the time the container is rolling across the sea the strap is holding the top of the pile against nothing.

A good packing line, the kind that has been doing this for years, comes back to re-tension the straps the next day — or uses a strap tensioner with a hydraulic or ratchet head that holds better. A great packing line straps twice: once at the packing station, once after the crate has sat overnight and the load has settled. When a supplier tells you "it's strapped," ask the second question: "strapped once, or re-tensioned in the morning?" — and treat the answer as a measure of whether the small labour, the one that nobody sees, is done at all.

And do not accept flimsy 12 mm strap on a crate that weighs 300 kg. The strap is the only thing holding the crate's face on, and the crate's face is what the forklift grabs. You want poly straps on light loads and steel on anything over 250 kg — that is not a luxury, that is the mathematics of a break.

Line 5 — The Crate Itself: Timber, Walls, and What They Actually Do

A close wooden crate — sides, lid, base, timber bracing inside — is the export standard for roofing slate, and for ocean freight there is no second option worth discussing. The crate is the armour. It costs timber, labour, and cubic metres in the container, and every one of those costs is cheaper than the tiles it protects.

Not all crates are built the same, and the differences are in the walls. A proper export crate has four structural elements: full-height corner posts that carry the stacking weight down through timber, not tile; a timber frame that keeps the crate walls off the bundle edges; internal bracing — cross-pieces that lock the bundles in position so nothing shifts when the ship rolls; and a lid that sits on the frame, not on the tiles. A crate that skips the internal bracing and lets the tiles hold the walls apart is a crate that arrives with a bow in the side and a crack in the tile that was doing the wall's job.

The timber choice is the other half: solid timber for the frame and the corner posts, plywood or oriented strand board for the wall panels. Solid timber takes the load; the panels keep the neighbouring crate off your bundles. A crate built entirely from thin plywood — no structural corner posts, just screwed panels — is lighter and cheaper and fine for a local truck; it is not fine for a container that lists 10° in a seaway. Ask for the crate spec the way you ask for the tile spec: timber dimensions, panel thickness, and whether the corner posts run the full height of the crate or stop at the lid line.

In practice, our roofing slate goes out in close wooden crates with full-height corner posts and internal timber bracing — never bare pallets, never wrap-and-go. If a supplier offers you "pallet and shrink wrap" as their export standard, they are describing a packing spec for a different trade, one where the insurance claim is someone else's problem. See our quotation comparison guide for how the packing line shows up in the price.

Line 6 — Sizes, Weight, and the 850 mm Stack

The crate must fit the 20ft/40ft box both in footprint and in loading height, and the height of the loaded crate is the number that decides everything. A crate that is 10 cm too tall does not go in the container; a crate that is 5 cm too wide costs you a row.

Take our 500×250 mm slates, 7–9 mm thick. One tile is roughly 2.4–3 kg, one crate holds about 100 tiles in bundles of 10, and that crate stacks up to about 850 mm tall and lands at 280–320 kg gross. Now run the container arithmetic: a 20ft box carries roughly 900–1,100 m² of roofing slate depending on thickness — the heavy, thicker material (9–11 mm) runs out of weight first at around 900–950 m²; the thinner 5–7 mm runs out of volume, pushes the box toward 1,100 m². The MOQ guide has the full container math, but the packing-side summary is: the crate count and the m² must agree to within one line on the packing list — that line is the only protection you have if the count is wrong.

Stack height has a mechanical ceiling you don't think about until it lands: the bottom crate carries the weight of every crate above it. Two crates is fine for our normal stacking; three is where you stop stacking without steel bracing and start worrying about the bottom tiles — the 100 tiles at the bottom of the bottom crate take the weight of two full crates pressing through the wood. The good packing plans either keep two high, or put a full timber frame under the base so the weight goes through the frame, not the tiles. Write "stacking height: 2 crates" into the packing list; you may need it to prove your breakage claim came from their load plan, not the sea.

Line 7 — Moisture, Part One: Load It Dry or Pay for It Later

Here is the shipping fact that confuses first-time slate buyers: the container is not dry. A steel container builds a micro-climate; a sunny day outside and a night at sea inside means the deck of the container sweats, the air inside cools against the cold steel, and condensation runs down the walls and onto the floor. Deck-face containers under the sun hit 60°C during the day and drop below 10°C at night. A container is a solar-powered dehumidifier in reverse.

So "wet" is the actual enemy of your crate, not "salt." The packing side of the battle starts before the crate exists:

• Load dry. The export moisture spec we work to is under 0.5% by weight; tiles packed within a day of washing carry water in the micro-cracks, and that water comes out during the temperature swings and never goes back. The factory quality control guide has the drying bed, the pin probe, and the moisture sheet in Gate 6 detail.
• Wrap the base. A PE film between the crate base and the stack keeps deck condensation off the first course of tiles. Most moisture damage is a first-course problem.
• Desiccant where it counts. A pouch or two per crate, and a second against the door face — the coldest spot. Cheap, and the shelf life of a pouch is finite, so ask for the date.

And do not mislead yourself on the outside either: the moisture layer on the exterior is an insurance claim you don't want. We keep every crate's exterior clean and the timbers dry so the crane straps and the forklift can see the wood. A crate arriving with rot-slimed timber already smells like a claim.

Line 8 — Moisture, Part Two: Vent or Seal

Here is the part of the moisture story that gets the least attention and causes the most confusion: some crates should breathe, and some should be sealed. A sealed crate traps its own condensation — the night's cooling and the day's warming form water inside the box, and a sealed crate has nowhere to let it out. A crate with a vented floor lets the air move, the water run out, and the dry air come in; the tiles breathe instead of stewing.

The good packing plans choose which one they are building: the vented crate for long ocean voyages where condensation builds, the sealed crate for short hauls or climates where the risk is outside rain. Ask how their crate breathes; a supplier who answers "we vent the floor, and the top course sits on a raised rail so air moves under it" has thought about the sea. A supplier who says "it's sealed tight, water can't get in" has thought about the brochure — the night's condensation is already inside, and they haven't counted it.

The tell is the interior. When the crate is opened at your port, look at the underside of the lid and the top course of tiles: water beading on the lid and damp staining on the top course is the signature of a sealed crate that should have been vented. The inspection checklist covers the lid and the top course on arrival.

Line 9 — Labels, Batch Numbers, and the Packing List Number

Then the paper: the crate's labels are a claim form. Every crate should show, stencilled or stickered: crate number, size and thickness of tile, bundle count, batch number, gross and net weight, and the export packing list number that ties the crate to the bill of lading. When you open a container and a crate's batch number doesn't match the packing list's batch line, the count line is your first conversation — the inspection checklist has the receiving-side version of that check.

Line 10 — ISPM 15: The Stamp That Customs Asks About

The second stamp is the one customs asks about, and it's a surprise to most first-timers: ISPM 15 — the international phytosanitary rule for solid wood packaging. A solid timber crate that crosses a border needs the IPPC stamp showing the wood was heat-treated or fumigated. Skip it, and the container can be held at the destination port, de-wooded, or refused — the wood is treated as a pest vehicle. A plywood or engineered timber crate — the layers already kiln-dried — doesn't need the stamp, which is why you see plywood-sided crates on roofing slate from China as often as you see thick "breathing" solid timber. That choice is not cosmetics; it's the documentation that travels with the wood.

Line 11 — Loading: The Two-Row Theory and the Free Space

The crate arrives at the container door; from here on the loading plan is the packing. Two rows of crates in a 20ft box, one row against each side wall, is the normal handshake — the middle aisle forms, and that aisle is where the bracing goes. Every wall of the stack that doesn't touch the container wall gets dunnage — foam, boards, the crushed cardboard from the bundles — jammed into the gap so nothing moves when the ship lists 10°. "Shake" is the word of the industry, and it's literal: at sea, everything settles and works itself loose.

Three things to ask for in the loading plan:

• Heaviest at the bottom. If the shipment mixes slate and lighter cargo, the load plan should be slate first, light cargo on top, bracing between.
• Bracing against the rear. The last few crates against the door go in with bracing against the door frame — that's the zone the lashing doesn't touch and the zone where the container breathes cold.
• The pre-loading photo and the loaded photo. A reliable exporter photographs the empty container, the first row, and the finished load — the photo set is the memory of the packing that goes with the packing list. If a supplier can show you all three from last week's container, they pack to a repeatable standard; if they can't, they pack to a mood.

Line 12 — The Weight List and the Claim Window

Finally, the list itself: the packing list is a legal document that talks numbers — gross weight, net weight, piece count, m² — and it must match the bill of lading and the manifest exactly. The gaps between these three papers are the most common reason a port hold goes on a slate container: the ship's manifest says 24,000 kg, the packing list says 23,600, and the port has a scale and a question. Weigh the crates on the weighbridge at the factory and print the list from the scale; a packing list written in the office and never weighed is a packing list that knows too much.

And when the container lands, the clock starts: the claim window for breakage is usually a matter of days, not months. You need the container survey, the photo set, and the count — all of it inside the carrier's deadline. The sourcing guide's final checkpoint covers the arrival sequence; here I'll just say: the day the container opens is the day you start the paperwork, not the day you decide whether to. The paper is the packing; the packing is the paper.

So What Actually Breaks, and What "Normal" Means

Every buyer asks the same two questions: how much breakage is normal, and what do I do when it's more. On a well-packed crate, 1–3% of tiles at the destination is the honest band — this is the number the freight insurance quote was built around, and it is the same 1–3% band the quotation guides discuss. Above 3%, the packing plan deserves your eyes before the insurance gets involved. And before you blame the sea, run the three-question litmus:

1. Was the crate actually built to the spec (bundle count, interleaving, corner boards, straps)?
2. Was the stack height more than two crates deep — or was the bottom of the container's door stacked with heavy on light?
3. Was the wood dry and stamped, and did the crate breathe? — because wet tiles don't break at the port; they break on the first frost a year later.

That last question is the sneaky one. Most "breakage" in the roofing slate trade isn't impact — it's moisture and thermal. The tile that was dry and stacked right is not going to crack in the container, but the tile that was damp in the box and locked in the dark is a tile that carries its own time bomb to the roof. When I hear a claim that the stone is bad, my first trip is to the moisture line, not the geology line.

And that is why the 12 lines are worth one pass before the container is sealed. The roofing slate range pages carry the specs of the stone; the contact form gets the packing plan written into the contract before the deposit — the crate's construction is cheap to change in the office and expensive to change after the seal. Our test results describe the stone; this article is about the part that no report describes: what the sea does to it, and what the packing does about it.

FAQ

Related Reading

• Roofing Slate Inspection Checklist — the receiving sequence that turns the packing spec into checks at the dock
• Factory Quality Control from Quarry to Packing — the eight gates, including the drying bed and the moisture sheet before the crate
• Roofing Slate MOQ Explained — the container arithmetic of m², weight and piece count that the packing list depends on
• How to Compare Roofing Slate Quotations — where packing specs show up in the price, and the 1–3% breakage band in practice
• How Roofing Slate Is Graded and Sorted — the sorting floor where the grades are decided before the crate is closed