What are the Different Layers of a Roof?

The structure of a roof is composed of several layers, each playing a crucial role in protecting the building beneath from the elements. Understanding these layers is essential for anyone involved in building maintenance, construction, or those simply looking to enhance their knowledge of how homes are built and protected. This article will explore the various layers that make up a typical roof, detailing their purposes and importance.


The Structure of a Roof

A roof’s complexity goes beyond its outermost layer, with each component working together to offer insulation, waterproofing, and durability. From the decking to the covering, roofs are meticulously designed to withstand weather conditions while providing thermal comfort to the building’s occupants.


The Key Layers of a Roof

  1. Roof Decking (or Sheathing):
    • The foundation layer upon which all other layers are built. Usually made from plywood or oriented strand board (OSB), the decking provides structural support and a base for the roof components.
  2. Underlayment:
    • A crucial barrier against water and moisture, this layer sits atop the decking. Typically made from felt or synthetic materials, it offers additional protection from rain and snow.
  3. Roof Covering:
    • This is the outermost layer, visible from the outside, and can be made from a variety of materials, including tiles, shingles, metal, or slate. It’s the first line of defence against the elements.
  4. Insulation:
    • Placed either directly below the decking or above the ceiling of the top floor, insulation helps maintain the building’s temperature, keeping it warm in winter and cool in summer.
  5. Ventilation:
    • Essential for regulating temperature and moisture levels in the attic space, ventilation helps prevent the build-up of condensation, which can lead to damp and mould problems.
  6. Flashing:
    • Made from materials like metal or plastic, flashing is used to seal and protect joints, valleys, and edges where water might infiltrate, such as around chimneys, vents, and skylights.
  7. Drainage:
    • The shape, layout, and slope of a roof are designed to direct water away from the building, minimising the risk of water damage.


The Importance of Each Roof Layer

  • Structural Integrity: The decking provides a solid foundation that supports the weight of the entire roof structure.
  • Moisture Protection: The underlayment acts as a secondary waterproofing barrier, safeguarding the roof from water ingress.
  • Weather Resistance: The roof covering is the primary shield against weather, including rain, snow, and sunlight.
  • Energy Efficiency: Insulation is key to preventing heat loss, contributing to the overall energy efficiency of the building.
  • Longevity: Proper ventilation extends the life of a roof by preventing the accumulation of heat and moisture.
  • Leak Prevention: Flashing directs water away from vulnerable areas, preventing leaks and water damage.
  • Water Management: Effective drainage ensures that water is efficiently channelled away from the roof, reducing the risk of leaks and structural damage.



The roof of a building is a complex system made up of several layers, each with its own specific function. From providing structural support to offering protection against the elements, these layers work in harmony to ensure the durability and efficiency of the roof. Understanding the role of each component is essential for proper maintenance and repair, ultimately contributing to the longevity and performance of the roofing system. Proper attention to each of these layers during construction and renovation can significantly impact a building’s comfort, energy consumption, and resilience against weather conditions.