A slab roller flattens clay to an even, set thickness in one or two passes. It is a large, expensive piece of equipment that a rolling pin and two battens replicate slowly, and the question is whether the volume of slab work justifies the space and the money.
What it does
Clay goes between two cloths, the gap is set, and a roller passes over it to produce a slab of that exact thickness across its whole area. Evenness is the point: hand-rolled slabs vary, and variation shows up later as uneven drying, warping and cracked joins. For tiles and repeat slab work the consistency is worth a great deal.
When a rolling pin is enough
For occasional slab work, entirely. Two battens of equal thickness either side of the clay and a rolling pin across them gives the same result, more slowly and with more effort. Most potters making the occasional slab dish never need more, and buying a roller before the volume justifies it is a common way to fill a small studio.
Space and cost
Slab rollers are big, heavy and need a permanent home with room to work on both sides. Second-hand ones appear when studios close and are worth watching for, since nothing on them wears out except the cloths. That combination makes a used roller a much better proposition than a new one for most buyers.
Cloths and maintenance
Canvas cloths are consumable and are what keep the clay off the rollers. Keep them clean and dry between uses, because clay dried into canvas makes dust when it is next shaken out, which is the one dust point in an otherwise clean piece of equipment.
Questions people ask about slab roller
Do I need a slab roller?
Only if you make slab work in quantity. A rolling pin and two battens give the same even thickness more slowly.
Are used slab rollers worth buying?
Yes. Almost nothing on them wears out except the canvas cloths, and they appear regularly when studios close.
Why does slab thickness matter?
Uneven slabs dry unevenly, which warps the work and cracks the joins.