A flat sourdough loaf almost always comes down to one of a few problems: a weak starter, incorrect fermentation timing, poor gluten development, or not enough steam in the oven. Sometimes it’s a combination. The good news is that each cause leaves distinct clues, so you can usually diagnose what went wrong and fix it on your next bake.
Your Starter Isn’t Strong Enough
The most common reason for flat sourdough is a starter that wasn’t ready to leaven bread. A healthy starter should at least double in volume before you use it. White flour starters often peak at 3 to 5 times their original volume, while whole wheat starters may only hit around 2 times. If your starter hasn’t reached its peak, or worse, has already collapsed past its peak, it won’t have enough gas-producing power to lift your dough.
Peak is the moment your starter has risen as far as it will go and is just beginning to flatten before it falls. You’ll see lots of bubbles on the surface and along the sides of the jar. Using a rubber band around the jar at feeding time makes it easy to track the rise. If you’re baking with a starter that peaked hours ago, you’re essentially adding spent fuel to your dough.
Over-Proofing and Under-Proofing
Fermentation timing is the trickiest part of sourdough baking, and getting it wrong in either direction produces the same disappointing result: a dense, flat loaf. Under-proofed dough hasn’t developed enough gas to expand fully in the oven. Over-proofed dough has generated plenty of gas but the gluten structure has weakened to the point where it can’t hold that gas anymore, so everything collapses.
The difference shows up in the crumb. An under-proofed loaf tends to have a tight, gummy interior, sometimes with a few large irregular holes near the top. An over-proofed loaf often has a uniform but flat, almost cake-like crumb with no spring to it. A well-fermented loaf sits tall with an even, airy interior.
Room temperature has a massive effect on how quickly your dough ferments. At 27°C (81°F), you might see a 30% rise in just 4.5 to 5 hours. At 23°C (73°F), a 60% rise takes 6 to 7 hours. Drop below 20°C (68°F) and you’re looking at 12 or more hours for the dough to double. If you’re following a recipe written for a 24°C kitchen but your house sits at 19°C, your dough will be significantly under-proofed at the same time mark. The reverse is just as dangerous in a warm kitchen.
Using an Aliquot Jar
One of the most reliable ways to judge fermentation is with an aliquot jar. When you mix your dough, pinch off about 20 to 30 grams and place it in a small, straight-sided glass jar. Mark the starting level with a rubber band. This sample ferments at the same rate as your main dough, but because it’s in a clear jar, you can see exactly how much it has risen. When your recipe calls for a 50% rise during bulk fermentation, you’ll know precisely when you’ve hit it. Before you shape, just fold that sample back into the main dough.
Weak Gluten Structure
Even with perfect fermentation, your bread will spread flat if the gluten network can’t support its own weight. Gluten is the elastic protein mesh that traps gas bubbles and gives bread its structure. Two things build it: the flour you choose and how you handle the dough.
For sourdough, bread flour with 12% to 14% protein gives you the best chance at a tall, well-structured loaf. All-purpose flour, which typically runs 11% to 12% protein, can work but produces a more tender, lower-rising bread. If you’ve been using all-purpose and wondering why your loaves spread, switching to bread flour (King Arthur Bread Flour at 12.7% protein is a widely available option) can make a noticeable difference.
Flour alone isn’t enough, though. You need to develop that gluten through stretch and folds or coil folds during bulk fermentation. A common approach is three sets of folds in the first 90 minutes, spaced about 30 minutes apart, followed by a couple of gentler folds over the next hour or two. If you skip these, the dough stays slack and can’t hold its shape. Machine kneading for about 8 minutes can replace the folding step, but most home sourdough bakers use the fold method because it’s gentler on the delicate structure that develops during long fermentation.
Too Much Acidity Breaks Down Gluten
Here’s something many bakers don’t realize: sourdough fermentation actively degrades gluten over time. The acids produced by the bacteria in your starter increase the activity of protein-breaking enzymes naturally present in flour. The longer fermentation goes on, and the more acidic the dough becomes, the more those enzymes chew through the gluten network you worked to build.
This is why over-proofed sourdough doesn’t just run out of gas. It literally loses structural integrity. The gluten bonds break, the dough becomes sticky and slack, and it spreads out instead of holding its dome shape. This effect is more pronounced with longer, warmer fermentations and with starters that haven’t been fed recently (which tend to be more acidic). Keeping your starter on a regular feeding schedule and not pushing fermentation times beyond what the dough can handle are the best defenses.
Shaping and Surface Tension
Shaping is where you give the dough its final structure, and it’s a step that directly determines whether your loaf holds tall or pancakes in the oven. The goal is to create a taut skin on the outside of the dough without crushing the gas bubbles inside. Think of it like inflating a balloon: the tighter the outer membrane, the more pressure it can hold before giving way.
High-hydration doughs (those with a lot of water relative to flour) are especially prone to spreading because they’re soft and sticky. The key is working quickly, using just enough flour on your hands and work surface to prevent sticking, but not so much that you create dry seams in the dough. Pre-shaping into a loose round and letting it rest for 15 to 20 minutes before the final shape gives the gluten time to relax, which actually makes the final shaping easier and more effective. If your shaped dough doesn’t hold a firm, rounded form on the counter, it won’t hold one in the oven either.
Scoring Depth Matters
The score mark on top of your loaf isn’t decorative. It’s a controlled weak point that directs where the bread expands. Without a proper score, the dough may burst unpredictably at the sides or bottom, losing upward height. But scoring too deep can also work against you, causing the loaf to open out wide and sit lower.
A depth of about 5 millimeters works well for most loaves. Hold the blade at roughly 30 to 40 degrees relative to the dough surface, with the bottom edge of the blade angled toward the base of the loaf. This angle is what creates the characteristic “ear” that peels back during baking. A straight vertical cut tends to open symmetrically and won’t direct the spring upward as effectively. The score doesn’t need to be perfectly straight either. A slight natural curve still produces good oven spring.
Not Enough Steam in the Oven
Steam during the first 15 to 20 minutes of baking is critical for oven spring. Water vapor condenses on the cool surface of the dough, keeping it moist and flexible so it can continue expanding as the gases inside heat up and push outward. Without steam, the crust sets hard within minutes, locking the loaf into whatever size it was when it went in. The result is a smaller, denser bread with a thick crust.
A Dutch oven is the simplest way to create steam at home. The lid traps moisture released from the dough itself, creating a self-steaming environment. If you bake on a stone or steel instead, you’ll need to generate steam separately, usually by pouring boiling water into a preheated pan on a lower rack. Either way, remove the lid or steam source after about 20 minutes to let the crust brown and crisp.
Putting It All Together
Flat sourdough is rarely caused by a single mistake. More often, it’s a chain: a slightly sluggish starter leads to extended fermentation, which degrades gluten, which makes shaping difficult, which means the loaf has no tension going into the oven. Breaking the chain at any point helps, but the highest-impact fixes are using your starter at peak activity, tracking fermentation with an aliquot jar instead of guessing, and making sure your flour has at least 12% protein. Start there, and the other pieces tend to fall into place.