# Rockwool slab

> A rockwool slab is a large, plastic-wrapped block of stone wool on which several rockwool blocks sit and finish their roots, the standard format for commercial drain-to-waste.

- Canonical: https://genetixbay.com/knowledge/glossary/rockwool-slab.html
- Also written: slab, slabs, rockwool block, rockwool blocks, stonewool slab, grow slab
- Glossary category: Systems and media · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

[Rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md) comes in a standard set of formats. Plugs and small cubes start seeds and cuttings; blocks, often about 10 × 10 cm (4 × 4 in) or larger, hold the young plant; and slabs, long plastic-wrapped bags of stone wool typically around 90-100 cm long, are what the blocks are set on to finish. Roots grow down from the block into the slab.

## In practice

Slabs are soaked in nutrient solution before planting, then [drainage](https://genetixbay.com/knowledge/glossary/runoff.md) slits are cut in the wrap. [Dripper](https://genetixbay.com/knowledge/glossary/drip-irrigation.md) stakes feed each block. The grower tracks slab weight or water-content sensors to control [dryback](https://genetixbay.com/knowledge/glossary/dryback.md), and slab [EC](https://genetixbay.com/knowledge/glossary/ec.md) to steer growth. Slabs differ in density and fibre orientation, which changes how they hold and release water; pick one designed for the crop and keep the type consistent run to run.

Used slabs can be recycled through specialist schemes in some areas.

## Don't confuse with

- **Rockwool cube or plug**: smaller starting formats.
- **Coco slab (grow bag)**: similar shape and use, but [coco coir](https://genetixbay.com/knowledge/glossary/coco-coir.md).

## Related terms

[Rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md), [Crop steering](https://genetixbay.com/knowledge/glossary/crop-steering.md), [Dryback](https://genetixbay.com/knowledge/glossary/dryback.md), [Drain-to-waste](https://genetixbay.com/knowledge/glossary/drain-to-waste.md), [Field capacity](https://genetixbay.com/knowledge/glossary/field-capacity.md)

## Read more

- [Growing in rockwool (stonewool cubes and slabs)](https://genetixbay.com/knowledge/systems/rockwool.md)
- [Drain-to-waste drip and crop steering (generative vs vegetative dryback)](https://genetixbay.com/knowledge/systems/drain-to-waste-crop-steering.md)

## Sources

1. Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier. https://doi.org/10.1016/C2015-0-01807-5