Tiles installed on roofs having a slope of less than what ratio are considered decorative and should have a built-up roof or waterproof membrane installed beneath the tile?

Tiles installed on roofs with a slope of less than a 2 and 1/2 to 12 ratio are classified as decorative. This classification is important because it indicates that the roof's slope is insufficient to facilitate proper drainage, which can lead to water pooling and increased risk of leaks or structural damage over time. To mitigate these risks, it is essential to install a built-up roof or a waterproof membrane beneath the tile.

The purpose of the waterproof layer is to provide an additional line of defense against water infiltration, ensuring that if any water does penetrate the tile layer, it will be captured by the membrane or built-up system instead of entering the structure. Roofs under such slopes rely heavily on the performance of these underlying materials, hence the emphasis on proper installation to prevent future issues.

Choosing the appropriate ratio for determining when tiles are merely decorative versus functional is crucial for maintaining the integrity and longevity of roofing systems. The other ratios provided do not align with industry standards stipulating when installations should transition from decorative to functional requirements involving water resistance.