Dunnage, racked and palletised warehouses

Three designs, one job

Every warehouse has the same task: hold casks safely for years, let them breathe and let staff count them. How it does that shapes maturation more than most new distillers expect. A cask on the top tier of a hot warehouse and one on the floor of a cool one can taste years apart after the same time in wood.

The three standard designs are dunnage, racked and palletised. Our guide to how whisky is made sets maturation in context.

Dunnage: low, cool and slow

A dunnage warehouse is the traditional Scottish building: thick stone walls, a low roof and an earth or cinder floor. Casks lie on their sides on wooden runners, stacked no more than three high. The thermal mass of the walls and the damp floor keep the temperature steady and the humidity high.

Maturation is gentle and even. The cost is space and labour. Casks are rolled in and out by hand. A dunnage warehouse also holds far fewer casks per square metre than the alternatives. Getting a cask from the back row means moving the ones in front of it.

Racked: tall, dense and layered

A racked warehouse holds casks on their sides in steel or timber racks, often many tiers high. Capacity per square metre is much higher and casks are easier to reach by forklift or by hand on each level.

The catch is stratification. Warm air rises, so the upper tiers run warmer and see bigger daily swings than the floor. In a tall warehouse the difference can be several degrees Celsius on a hot day. Upper casks extract faster and lose more spirit. Distillers manage this by rotating casks between levels or by blending across levels so that no batch depends on one position.

Palletised: casks stand on end

In a palletised warehouse, casks stand upright on pallets, usually four or six to a pallet, with pallets stacked several high. A forklift moves a whole pallet at once. It is the cheapest design to run per cask and the quickest to fill and empty.

Standing a cask on end changes things. The spirit no longer wets the bung stave in the same way. The head in contact with the spirit does more of the work. Sampling and regauging need the pallet brought down or a sampling hole in the head. Inspection is harder because you cannot see every stave. Our guide to sampling casks covers how access affects routine checks.

What changes in Indian heat

Indian maturation runs faster and loses more spirit than Scottish maturation. That alters the warehouse choice:

• Height costs more here. The top of a tall racked warehouse in a north Indian summer can sit well above 40 °C. Losses there climb sharply.
• Thermal mass is worth paying for. Thick walls, insulated roofs and shaded sites flatten the daily swing.
• Ventilation needs control. Night air can cool a warehouse. Hot afternoon air should be kept out.
• Fire rules apply. A warehouse of spirit is a flammable store. State fire and excise rules on layout, spacing and bonded storage vary and change, so check the current notification with your licensing adviser.

Our guide to temperature swings in Indian warehouses looks at the physics in more detail.

Floors, drainage and access

The floor matters more than it looks. Earth and cinder floors in dunnage stores hold moisture and keep humidity up. Concrete slabs are easier to clean and suit forklifts, but they dry the air unless the building is damp anyway. A slight fall towards a drain helps when a cask leaks, because spirit pooling on a floor is a fire risk as well as a loss.

Plan aisles for the equipment you will actually use. A cask rolled by hand needs a clear run and a gentle slope at doorways. A forklift needs turning space and racks rated for full casks. A 200 litre barrel of spirit weighs well over 150 kilograms. A 500 litre butt weighs several times that, so rack ratings and floor loads deserve an engineer's sign-off rather than a guess.

Matching the design to the distillery

A craft distillery filling a few hundred casks a year rarely needs a tall racked store. A low, well insulated building with casks two or three high, dunnage style, gives even maturation and is easy to inspect. Palletising makes sense once volume grows and labour becomes the main cost. Racking suits a mid-sized operation that needs density but wants each cask reachable.

Whatever you build, the location of every cask should be on record. A cask's warehouse, bay, row and tier is as much a part of its history as its fill date. It is the only way to explain later why two casks filled on the same day taste so different.

Put two loggers in before the first cask

Before deciding anything, measure. Hang one temperature and humidity logger near the roof and one near the floor of the building you plan to use. A month of readings across a hot spell tells you more about stratification than any drawing. Pick the cooler, steadier zone for your longest maturing stock.

Common questions

What is a dunnage warehouse?

A low, traditional warehouse with thick walls and an earth or cinder floor, where casks lie on wooden runners no more than three high. It gives cool, steady and humid conditions.

Why do casks on high racks mature faster?

Warm air rises, so upper tiers run hotter with bigger daily swings. The spirit moves in and out of the wood more and evaporation is higher.

Do palletised casks mature differently?

Yes, somewhat. Standing on end changes which wood the spirit contacts most and makes inspection and sampling harder, though the main driver of maturation is still temperature.