8. How do I stop my sourdough bread from flattening?

A very important aspect of making sourdough is the amount of starter
used in the recipe and how long it has been since the starter
matured. Typically, about 20-40% of the total flour should come from
the starter. The higher the percentage of starter, the less proofing
time it will stand. In other words, if 40% of the flour comes from
starter, you may only be able to proof 3-4 hours before the loaves
flatten excessively, depending on the starter and degree of maturity.
I've never used Carl's starter, but since people like it I assume it
has fairly low levels of enzymes which make it more tolerant to
various baking procedures.  Different lactobacilli have different
capacities to degrade flour and to make acid and therefore they act
differently in bread.

The standard methods to keep bread from flattening excessively
include reducing water, increasing kneading or adding ascorbic acid
(100-200 mg per 5 pounds of flour), making sure the starter is not
overly mature, and doing some of the fermentation as a "bulk"
fermentation.  Bulk fermentation simply means that after mixing the
dough you let it sit for 2-3 hours at proofing temperature before
shaping the loaves.  That will give the bacteria/yeast time to make
flavor and gas without having to worry about the loaves flattening.
Then the loaves are shaped and a final proof of 3-4 hours results in
a fantastic loaf with a more interesting internal and external
texture.

One other important reason why sourdough loaves may flatten is that
the starter is not fresh enough.  When you feed your starter use the
smallest amount of old starter that you can while still getting a
very active ferment by the time you need to mix your dough.  If the
old starter is very active I would use only 5-10% by weight as an
innoculum.  Starters that are not fresh produce extremely slack
doughs.  The type of flour you use will help, but will not completely
overcome the problem.  If 20-30% of the flour in your dough comes
from starter you should be able to proof a free standing loaf for
many hours without flattening. I typically mix a dough, let it sit
for 3 hours, shape into loaves, and give up to 5 hours of final proof
with little flattening.

Water content for this type of loaf is 56-60% on flour.

-Troy