Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Drain, reserving 1 cup of the cooking water.
Meanwhile, heat 3 tablespoons olive oil in a separate large pot over medium-high heat. Add the sausage and cook, stirring and breaking up the meat with a wooden spoon, until browned and crumbled, 2 to 3 minutes. Add the clams and 1/4 cup water. Cover and cook, removing the clams to a bowl as they open, 5 to 8 minutes. Discard any unopened clams.
Add the pasta, 3/4 cup reserved cooking water, the roasted red peppers and remaining 2 tablespoons olive oil. Simmer, stirring, until the liquid is slightly thickened, about 2 minutes, adding more cooking water as needed; season with salt and pepper.
Stir in the parsley. Add the clams and gently toss.