The tent contains 2 separate layers, which are placed one on top of the other on its roof or upper part. Usually, the first layer is waterproof, while the second is a layer that allows air to pass through and offers better ventilation to the tent.

Waterproofing in a tent shows how much water the outer fabric can repel. It is measured in millimeters (mm) and refers to how many millimeters of a vertical column of water (and therefore pressure) a square centimeter of fabric can withstand without being penetrated by water.
The higher the degree of waterproofing, the less breathable the fabric is, and therefore the warmer the tent. Common waterproofing values range from 500mm to 10000mm. Theoretically, any fabric with waterproofing >1000mm is considered waterproof. For example, a summer tent may have waterproofing from 500 to 1500mm.