What distinguishes waterproofing from damp-proofing?

Waterproofing and damp-proofing serve different purposes in protecting structures from water intrusion, which is captured in the correct answer. Waterproofing is designed to resist hydrostatic pressure, meaning it can effectively prevent water from passing through under varying levels of pressure, such as groundwater. This is crucial in areas where water is likely to accumulate, such as basements and below-grade applications.

On the other hand, damp-proofing primarily addresses moisture control, preventing dampness from seeping into a structure but without the need to withstand significant hydrostatic pressure. This makes it more suitable for use in above-grade applications, where the risk of high water pressure is lower.

While the other answer choices contain elements that might seem relevant, they do not accurately capture the fundamental distinctions between waterproofing and damp-proofing. For instance, the idea that waterproofing is exclusively for roofs and damp-proofing for walls overlooks the fact that both methods can be used in various contexts depending on water exposure risks. Additionally, the notions about resistance to air or limitations to indoor or outdoor applications do not accurately reflect their respective functionalities, which revolve around water and moisture control. Thus, option C clearly provides the most precise distinction.