Unit load design, or why the pallet is the least of it
The pallet, the product and the packaging form one structure, and most of the strength comes from the part people think of as wrapping. Design them separately and you will overbuild one and break the other.
June 14, 2022 · The grading bench · 9 min read · Craft
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- Stack strength carried by corners
- 60–70%
- Deck gap that matters
- over 3"
- Bridging
- Helps and hides problems
- Design unit
- All three together
Nobody designs a unit load. They specify a pallet, buy a carton, set a wrap programme, and hope the three get along.
How load actually reaches the ground
Put a layer of cartons on a deck. The weight does not spread evenly: each carton bears on whatever is underneath it, and underneath it is alternating board and gap. A carton sitting squarely over a deck board is supported along its whole base. A carton sitting over a gap is supported only at its edges, and the base panel has to bridge the span — which corrugated board is not good at, and which is why the bottom layer of a load is where crushing starts.
The thing that saves most loads is bridging: cartons are stiff enough and interlocked enough that the layer behaves partly like a plate rather than a row of independent boxes. Bridging is genuinely helpful and also conceals a badly matched pallet, because the load looks fine right up to the point where a slightly weaker carton or a slightly wider gap arrives.
The four interactions nobody specifies
Deck gap against carton footprint
If your carton's short dimension is smaller than the deck gap plus a board, a carton can sit almost entirely over air. Specify a maximum deck gap in inches — not a grade, which says nothing about it.
Carton compression against stack height
Corrugated loses roughly half its compression strength over a few months of storage and more again at high humidity. The published box compression test figure is a day-one, dry-conditions number.
Overhang against everything
A carton overhanging the pallet edge loses its corner support, which is where most of its strength lives. An inch of overhang can cost a fifth of stack strength.
Wrap tension against column alignment
Stretch wrap pulls the top layers inward. Too much tension at the top and the corner columns stop being columns; too little and the load shears. Both show up as crushing at the bottom.
A carton's strength is in its corners. Everything a pallet or a wrap does to the corners, it does to the whole load.
Corner columns
In a stacked unit load, 60 to 70 percent of the vertical strength is carried by the corner columns of the cartons. That is why column stacking — corners directly over corners, all the way up — is dramatically stronger than interlocked brick stacking, even though interlocking is more stable against shear. The trade-off is real: column stacking is strong and unstable, interlock is stable and weak, and the right answer depends on whether your risk is crushing or toppling.
Where the pallet enters this is at the bottom corner. A deck board missing at a corner, a lead board split along its length, or a cut-down unit with product hanging past the last board all remove support from exactly the place that carries most of the load. We condemn split lead boards for this reason rather than for appearance.
A practical sequence
- Start from the carton. Its footprint decides the pallet pattern, which decides whether a 48×40 is right or whether a cut-down saves you from overhang.
- Fit the pattern with no overhang. Flush or very slightly underhung. Underhang costs a little cube; overhang costs product.
- Then specify the deck. Maximum deck gap, minimum board thickness, no split lead boards, squareness tolerance. These are pallet facts, and they are not implied by a grade.
- Then set the wrap. Containment force at the bottom, less at the top, and never a wrap programme that pulls the top layers in off their corners.
- Then test the real thing. Build it, stack it as it will actually be stacked, leave it a week in the actual humidity, and look at the bottom layer.
Reference tables
The numbers referred to above, in full, so you do not have to take a sentence's word for them.
Footprints
| Footprint | Inches | Millimetres | Construction | Where it lives | Our stock | Per 53' van |
|---|---|---|---|---|---|---|
| GMA 48×40 | 48" × 40" | 1219 × 1016 | Stringer & block | Grocery, general distribution, retail | Deep | 480–560 |
| 48×48 | 48" × 48" | 1219 × 1219 | Stringer | Drums, reels, wire, square loads | Good | 392–420 |
| 48×45 | 48" × 45" | 1219 × 1143 | Stringer | Automotive, tyre | Good | 420–450 |
| 48×42 | 48" × 42" | 1219 × 1067 | Stringer | Telecom, paint, coatings | Good | 440–480 |
| 42×42 | 42" × 42" | 1067 × 1067 | Stringer | Chemicals, drums, paint | Good | 480–520 |
| 40×48 | 40" × 48" | 1016 × 1219 | Stringer | Beverage, rotated GMA | Good | 480–560 |
| 40×40 | 40" × 40" | 1016 × 1016 | Stringer | Dairy, beverage, bakery | Good | 520–580 |
| 36×36 | 36" × 36" | 914 × 914 | Stringer | Compact, display, light goods | Good | 640–700 |
| 32×30 | 32" × 30" | 813 × 762 | Stringer | Retail display, in-store | Limited | 900+ |
| 30×30 | 30" × 30" | 762 × 762 | Stringer | Light goods, display | Limited | 960+ |
| 48×20 half | 48" × 20" | 1219 × 508 | Stringer | Retail half pallet, promotions | Limited | 1,000+ |
| EPAL 1 | 47.24" × 31.50" | 1200 × 800 | Block, 9 blocks | European trade, retail | To order | 540–600 |
| EPAL 2 / ISO | 47.24" × 39.37" | 1200 × 1000 | Block | Europe, Asia, best container fit | To order | 480–520 |
| Asia square | 43.30" × 43.30" | 1100 × 1100 | Block | Japan, east Asia | To order | 460–500 |
| AU square | 44.88" × 44.88" | 1140 × 1140 | Block | Australia, chemical CP series | To order | 440–480 |
Trailer counts are for stacked, unstrapped loads of sound recycled stock and vary with repair thickness. We confirm the actual count on the bill of lading rather than estimating it. Anything not on this list we cut from reclaim — see custom milling.
Grades
| Grade A | Grade B | #1 premium | #2 economy | Custom cut | |
|---|---|---|---|---|---|
| Original boards | All | Some replaced | All | Several replaced | Mixed |
| Companion boards | None | Permitted | None | Permitted | Permitted |
| Stringer plugs | None | Permitted | None | Permitted | Permitted |
| Metal plates | None | Permitted, disclosed | None | Permitted | Permitted |
| Squareness | Within ¼" | Within ½" | Within ¼" | Within ½" | Within ½" |
| Colour uniformity | High | Mixed | High | Mixed | Mixed, fresh saw ends |
| Protruding fasteners | Zero | Zero | Zero | Zero | Zero |
| Moisture ceiling | 19% (15% on request) | 19% | 19% (15% on request) | 19% | 19% |
| Typical service | Retail-facing, export | Internal, bulk | Automation, racking | One-way, yard storage | Odd footprints |
| Relative price | Highest recycled | −18 to −30% | High | Lowest | ≈⅓ of a new build |
| Remaining life | Longest | Shorter — already repaired once | Longest | Shortest | Varies |
Grade A and #1 overlap: we use #1 for units that needed no repair at all and A for units that are cosmetically uniform. Where a customer holds their own written standard, that document governs and supersedes these names.
Load ratings and deflection
| Construction | Static | Dynamic | Racked (unsupported) | Unit weight |
|---|---|---|---|---|
| Recycled GMA 48×40 stringer, sound | 4,600 lb | 2,800 lb | 1,500 lb | 33–45 lb |
| Recycled 48×40 block, sound | 5,200 lb | 3,000 lb | 1,900 lb | 40–55 lb |
| Recycled 48×40 stringer, repaired (B) | 4,400 lb | 2,650 lb | 1,400 lb | 35–48 lb |
| New southern yellow pine, 5 top boards | 4,200 lb | 2,500 lb | 1,400 lb | 33–38 lb |
| New mixed hardwood block, 7 top boards | 6,000 lb | 3,500 lb | 2,400 lb | 48–58 lb |
| Heavy-duty new build, doubled stringers | 9,000 lb | 5,000 lb | 3,800 lb | 62–78 lb |
| Skid (no bottom deck) | 3,800 lb | 2,200 lb | Not for racking | 24–32 lb |
Representative planning figures for sound stock, not engineering certifications. Racked ratings assume the load spans two rails with nothing under the middle. Derate a further 20–30% for dwell times beyond a few weeks, because wood creeps under sustained load.
- Design ceiling: deflection of span ÷ 100 — about 0.4 inches across a 40-inch unsupported span.
- Orientation: rack a stringer pallet across the wrong axis and the deck boards carry the span instead of the stringers. Capacity can halve.
- Creep: wood deflects progressively under sustained load. A unit measured inside tolerance on day one can exceed it after six months in a rack.
- Stiffness, not strength, is the limit. Mixed hardwood outperforms southern yellow pine in racking by more than their strength figures suggest, because it is substantially stiffer.
- Drive-in racking gives a long unsupported span with no support at the leading edge. Derate hard, use block construction, consider full-perimeter bottom boards.
Moisture and dew point
| Moisture content | What happens | Typical source | Freight weight |
|---|---|---|---|
| 6–12% | Indoor equilibrium. No mould risk. Dimensionally stable. | Kiln dried, stored inside | Lightest |
| 12–19% | Safe working range — our standard ceiling. | Covered dry stack | Normal |
| 19–25% | Mould within days to weeks in warm conditions. | Rain-exposed stacks, green lumber | +5–10% |
| 25–40% | Mould rapid, blue stain likely. | Pallets left in the open through a wet week | +10–20% |
| Above 40% | Saturated. You are paying freight on water. | Standing water, flooded yard | +20%+ |
Mould needs free water. Below roughly 19% the water in wood is chemically bound in the cell walls and spores cannot germinate; above it, they can. Heat treatment kills insects and confers no moisture guarantee; kiln drying reduces moisture and is not a phytosanitary treatment. They are separate claims and conflating them causes both mould claims and border holds.
| Air temperature | Relative humidity | Equilibrium moisture content | Dew point — a pallet colder than this will sweat |
|---|---|---|---|
| 68 °F | 30% | ≈6% | ≈36 °F |
| 68 °F | 40% | ≈7.7% | ≈43 °F |
| 68 °F | 55% | ≈10.2% | ≈51 °F |
| 68 °F | 70% | ≈13.1% | ≈58 °F |
| 68 °F | 85% | ≈18.5% | ≈64 °F |
| 55 °F | 60% | ≈11.3% | ≈41 °F |
| 75 °F | 65% | ≈12.4% | ≈62 °F |
Approximate, for planning. Two separate lessons: wood will eventually equilibrate to whatever air it sits in, and any pallet colder than the dew point of your building's air will condense water on itself and then on your cartons. Acclimatise cold stock indoors for four to twelve hours before loading.
Fasteners
| Fastener | How it holds | Relative withdrawal | Correct for | Wrong for |
|---|---|---|---|---|
| Ring (annular) shank | Rings bite into the fibre; withdrawal must tear wood | 1.8–2.2 | Deck boards, repairs, anywhere a fork strikes | — |
| Helical (screw) shank | Threads itself into the fibre as it drives | 1.7–2.1 | Stringers, blocks, hardwood | — |
| Smooth shank | Friction only; wood shrinks away from it | 1.0 (baseline) | Nothing in a pallet | Every pallet joint |
| Staples | Two thin legs, wide crown, large bearing area | 0.5–0.8 | Collar panels, crate sheathing, lids | Deck and stringer joints |
| Galvanised (any shank) | As above, corrosion resistant | As above | Outdoor and wash-down service | — |
Doubling withdrawal resistance roughly doubles the number of trips a unit survives, for a fastener cost measured in cents. We use ring-shank and helical only, and every unit that has been apart is fully re-nailed. The MIBANT bend-angle test is the industry's rough proxy for nail ductility: too brittle and it snaps on impact, too soft and the joint loosens.
Conversions
| Inches | mm | Millimetres | Inches | Other |
|---|---|---|---|---|
| 20" | 508 | 600 mm | 23.62" | 1 board foot = 144 in³ |
| 24" | 610 | 800 mm | 31.50" | 1 cu yd = 27 cu ft |
| 30" | 762 | 1,000 mm | 39.37" | 1 short ton = 2,000 lb |
| 32" | 813 | 1,100 mm | 43.30" | 1 kg = 2.205 lb |
| 36" | 914 | 1,140 mm | 44.88" | 1" = 25.4 mm exactly |
| 40" | 1,016 | 1,200 mm | 47.24" | 56 °C = 132.8 °F |
| 42" | 1,067 | 1,219 mm | 48.00" | 19% MC = mould threshold |
| 44" | 1,118 | 1,016 mm | 40.00" | 32 bf ≈ one GMA unit |
| 45" | 1,143 | 914 mm | 36.00" | L/100 = deflection ceiling |
| 48" | 1,219 | 762 mm | 30.00" | ≈500 units per 53' van |
Decision trees
Recycled, new, or cut down?
- Does a standard footprint fit your carton pattern with no overhang? If yes → recycled.
- If no: does an oversized footprint exist in reclaim that can be sawn to your size? If yes → cut down, at roughly a third of a new build.
- Does anything automated read the pallet's geometry, or do you need ±⅛"? If yes → new build.
- Is the racked load above about 2,500 lb? If yes → new build, mixed hardwood, block construction.
- Is a documented chain of custody for the wood itself part of an audit? If yes → new build.
- Otherwise → recycled, at the lowest grade that survives the journey.
Buy, own a fleet, or rent from a pool?
- Measure your return rate first. Everything below depends on it and nothing else matters as much.
- Below 50% return → your pallet is packaging. Buy recycled one-way and price it as consumable.
- 50–85% return → the messy middle. Buy recycled on a managed programme, with collection on the same movements.
- Above 85% return, with controlled geography → own a fleet and use toll repair to keep your asset numbers.
- Mandated by a retailer's pool, or shipping to hundreds of points needing identical quality → pool rental, and budget for loss charges.
- Wash-down hygiene is a process requirement → plastic, and we will point you to somebody who sells it.
Stringer or block?
- Does it go into racking where you cannot guarantee orientation? Block.
- Do powered jacks enter from the 48" side? Block.
- Automated storage and retrieval? Block, or new.
- Heavy load sitting for months? Block, mixed hardwood.
- European or ISO-metric footprint? Block by specification, not preference.
- Otherwise → stringer: lighter, cheaper, deeper in recycled stock and easier to repair.
Repair or replace?
- Crack crossing a notch, or more than a third of a stringer's depth? Plug, or condemn.
- Three or more stringer repairs already present? Condemn — that is a pallet held together by optimism.
- Four or more deck boards gone on a low-value footprint? Harvest the boards.
- Above 24% moisture? Dry stack first, then decide.
- Contaminated with oil, chemicals or paint? Out of every reuse stream, permanently.
- Otherwise → repair and re-nail fully, at roughly a third of replacement cost.