The Short Answer
Baking
A loaf that spreads sideways instead of rising is the single most common sourdough complaint there is, and the usual response is to buy something. Before you do: two of the three causes cost nothing to fix, and no purchase will help until they are. Here they are in the order worth checking.
This is the most common cause by a wide margin. Fermentation builds gas, but it also slowly degrades the gluten network that has to hold that gas. Go too long and the structure gives out — the dough is full of air and has nothing left to trap it with. It will feel slack and soupy, it will not hold a shape on the bench, and it spreads the moment it is turned out.
Time is a bad guide here, because it depends entirely on how warm your kitchen is. A dough that needs four hours in a cold January kitchen can be finished in two on a warm August afternoon. Judge by the dough: roughly 50–75% larger, domed rather than flat on top, and jiggly when you shift the bowl. A gently floured fingertip pressed in should spring back slowly and leave a faint dent. If it collapses and stays collapsed, it has gone too far.
Shaping is not folding the dough into a parcel. It is dragging the outer skin taut so the loaf holds itself in a ball, the way a balloon holds its shape because the rubber is under tension. If that skin is slack, the dough has nothing resisting gravity and it flows outward.
The test is immediate and free: after shaping, the ball should sit on an unfloured patch of counter and hold a domed shape for a minute or two without visibly slumping. If it puddles straight away, pre-shape it, rest it 20 minutes under a cloth, and shape it again. Most people are too gentle here, and a slack dough almost always needs one more round.
This is the one that actually needs equipment. After shaping, the dough sits for its last rise — often an hour at room temperature, or overnight in the fridge. During that window it is soft, it is still fermenting, and gravity has hours to work on it. On a flat tray, a high-hydration dough spends that whole time slowly flowing outward. It goes into the oven as a disc, and no amount of oven spring recovers a disc.
What it needs is something holding it in from the sides, so the expansion has nowhere to go but up.
Nine inches round is the size that suits the loaf most sourdough recipes actually make — roughly a 750g to 1kg dough. This matters more than it sounds: a basket much wider than the dough lets it spread inside the basket, which is the exact problem you bought it to solve. Undersized is the safer error.
Buy them in twos. Sourdough runs on a schedule — one loaf retarding in the fridge overnight while you shape the next — and a single basket makes you wait on it. The Superbaking 9″ round banneton set is the straightforward version of this: cane baskets with linen liners, which give you both surfaces. Bare cane for the spiral pattern and maximum wicking, liner in for a smooth crust and much less sticking while you are learning.
Three honest limits. A basket will not rescue an over-proofed or badly shaped dough — it is the fix for the third cause, not the first two, and it is worth being clear about that before you spend anything. New cane sticks badly. Dust it heavily with rice flour, not wheat, for the first several bakes; rice flour does not absorb water and form glue the way wheat does. And you do not wash these. Knock the flour out, let it air dry completely, and it seasons over time. Washed and stored damp, cane grows mould.
And the free version, because it exists: a colander lined with a well-floured tea towel does the supporting job. It will not wick like cane and it will not give you the spiral, but if the aim is simply to find out whether lateral support is what your loaf has been missing, try that first. It costs nothing to test.
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