Mix 1/2 cup of water and the sugar and yeast in a large bowl or the bowl of a stand mixer and set aside about 10 minutes until the yeast froths.
Mix the sourdough starter into the remaining water. Add to the yeast.
Add 4 cups of flour and salt and mix with a spoon or with the dough hook if you're using a mixer.
Continue adding flour, half a cup at a time, until you have a sticky and soft dough. It should have the consistency of a ciabatta dough. I needed all of the six cups on a rather moist day in my town, but you might need less if you're working in a dryer climate.
Spray a large bowl with some oil, pour the dough into the bowl, and spray some oil on top to coat and seal the dough. Cover tightly with a plastic bag or plastic wrap and set the dough in a warm place to rise for two hours.
After two hours the dough should have risen all the way to the top of the bowl. Turn it out onto the kitchen platform and fold it over once or twice. You don't need to knead it. Place the dough back in the bowl and cover again and let it rise for another two hours.
Line two loaf pans with parchment paper or aluminum foil and spray with some oil.
Remove the dough onto the kitchen platform and divide into two. Shape each half into a loaf, making the top as smooth as you can. I do this by tucking the sides underneath. The dough is still quite sticky, so work fast or it will be hard to shape.
Place each loaf in a loaf pan. Dust some flour on top of the loaves and cover loosely with a kitchen towel. Set the loaf pans in a warm place and let the loaves rise for an hour or until they begin to dome over the top of the loaf pans. Remove the kitchen towel as the dough rises over the tops, or the towel will stick and deflate the dough as you remove it.
Preheat the oven to 500 degrees Fahrenheit.
Place the loaves in the oven and bake 10 minutes. After 10 minutes, reduce the heat to 350 degrees and bake another 30 minutes.
Remove the loaves to a rack and let them stand about 15 minutes or until the pans are cool enough to handle. Unmold by turning out the loaves to a rack. Let them cool, right side up.
Slice and serve.