Binding things together entails more than one thing! Also, the hyssop (or "eizov"), and its uses in the process of purification. Also, materials that might usually be used for food (and therefore be able to be rendered impure) and use them for roofing, there are ways to use them in ways that won't be rendered impure. For example, grapes are obviously food, but the branches that they're on are never eaten. Once there's more inedible than edible, them you can use it for schach.