How to dry slabs

Air-dry slabs and planks flat, straight and without big cracks.

Slabs dry mostly through their faces, but water leaves the end grain 10–15 times faster. The ends shrink before the rest of the board can follow, and that's where checks and splits start. Good slab drying is mostly about slowing the ends down, keeping air moving and keeping the boards flat while they shrink.

1. Seal the ends right away

Seal both ends within hours of cutting, before checks can start. Use end-grain sealer or wax emulsion, or in a pinch thick paint or PVA glue.
Put it on generously and go a few centimetres around onto the faces and edges.
Mill a little long: plan on losing 5–10 cm (2–4″) at each end anyway.
Slabs with the pith in them will usually crack along the pith. Many sawyers cut it out or accept a crack down the middle.

2. Stack on stickers

Build a flat, level base at least 30 cm (12″) off the ground, on blocks or beams. A twisted base gives twisted boards.
Separate every layer with stickers: dry, straight strips of the same thickness, about 20–25 mm (¾–1″).
Space the stickers about 30–40 cm (12–16″) apart, closer for thin or soft boards, and put one right at each end.
Keep the stickers lined up vertically from the base to the top, so the weight goes straight down and nothing bends.
Leave a small gap between boards in the same layer so air can move up through the stack.
Put weight on top (concrete blocks, heavy beams, or ratchet straps) to keep the top layers from cupping.

3. Choose the place

Good airflow, but out of direct sun and rain. Cover the top with a roof sheet or boards that overhang the stack, but don't wrap the sides.
Don't put green wood straight into a heated room or next to a stove. It dries too fast on the outside and cracks or case-hardens.
In hot, dry, windy weather, shade-cloth on the sides slows things down. Light-coloured woods such as maple can stain, so the first weeks of airflow matter most for them.

4. Be patient, and measure

The old rule of thumb is about a year per 25 mm (1″) of thickness for air drying. Thick slabs of dense hardwood like oak can take longer.
Air drying gets the wood down to the moisture of the outside air: often 12–20 %, depending on climate and season. Furniture for a heated home needs about 6–10 %, so finish drying indoors for a few weeks to months, or in a kiln.
Use a moisture meter on a freshly cut end or in the middle of the board, not just the surface.
Or weigh: put a small sample board (a short offcut from the same slab, ends sealed) in the stack and weigh it regularly. When its weight stops falling, the slab is at the same moisture as its surroundings.

Register the slab or the sample board in WoodDrying.net and add a weighing or moisture reading whenever you check it. The chart shows how fast it's drying and when it levels off.

Common problems

End checks: the ends weren't sealed, or not soon enough. Cut back to sound wood later.
Cupping, bowing and twist: stickers too far apart or not aligned, an uneven base, or no weight on top.
Sticker stain (dark stripes where the stickers were): wet stickers, or too little airflow early on. Use dry stickers.
Mould: not enough airflow. Open the stack up and keep it off the ground.
Surface checks: drying too fast in sun or wind. Shade and slow down the stack.