Types of cool roofs

[edit] Introduction

A roof is a structure forming the upper covering of a building or other shelter. Its primary purpose is generally to provide protection from the elements, but it may also contribute to safety, security, privacy, energy conservation and so on.

Cool roofs are roofs that stay cooler than traditional roofs in the sun by minimising solar absorption and maximising thermal emittance. This reduces the need to control indoor air temperatures with active systems.

The colour of roofs and their material composition can significantly influence the temperatures they reach. Temperatures can also be influenced by building design type, roof pitch, materials and structural components and application techniques.

[edit] Low-pitched (or flat) roofs

This type of roof has a flat, or near flat pitch with just enough slope to facilitate drainage. Typically they are designed to have a minimum fall of 1:40, which given on-site inaccuracies should result in a minimum fall of at least 1:80 in the finished construction.These types of roofs are frequently found on large industrial structures.

For low-pitched roofs, cool roof options may include:

• Coated roofs. These roofs are treated with a number of different applied finishes. See below for specific types of coatings and their characteristics.
• Foam roofs. These are roofs topped with a foam-like substance that is generally made from two liquid chemicals that mix together to create a solid, flexible and lightweight material.
• Built-up roofing (BUR) systems. These are roofs made from multiple layers that commonly include: a base sheet, fabric reinforcement layers and a protective surface layer. Some BUR systems embed thermally reflective materials in the layers while others are coated with sheets of reflective materials or are treated with suitable coatings.
• Modified bitumen (Mod-Bit). This type of roof is much like the BUR system, but it has a higher degree of elasticity. It is asphalt based and is suitable for both cold and warm climates.
• Single-ply membranes. See below for specific types of single-ply membranes.

[edit] Steep-pitched roofs

This type of roof has a more significant incline and is generally found on domestic buildings. Steep-pitched roofs are generally made from materials that differ from low-pitched roofs, which limits the number of suitable cool roofing application options.

One option is cool asphalt shingles or other materials (such as wood or polymer) that have been coated with substances that make them more reflective. Most asphalt shingles have a relatively low Solar Reflective Index (SRI); white shingles and other colour tiles can be even lower. SRI can be calculated based on properties including the pitch of a roof, its weight, its solar reflectance, thermal emittance (emissivity) and its ageing characteristics.

For steep-pitched roofs, slate, clay or concrete tiles are another common option. Tiles are most frequently found in warmer climates, but they can be seen all over the world. These materials can be treated with coatings to improve their reflective properties. Both clay and concrete tiles come in many colours and can be durable under extreme weather conditions.

[edit] Metal roofs

Low- and steep-pitched metal roofs are available with several finishes that improve solar reflectance and thermal emittance, allowing it to satisfy basic cool roof criteria. Cool reflective coatings can also be applied to low-pitched metal roofs.

[edit] Cool roof coatings

Cool coatings can be added to many different types of materials, including asphalt, gravel, metal and other single-ply materials. These can include:

• White roof coatings, that are opaque and reflective, consisting of polymeric materials and some types of white pigment.
• Pigmented coatings are made from similar materials and come in darker shades such as red, green and blue. These coatings are more commonly used for aesthetic purposes on residential buildings.
• Aluminum roof coatings are made from a resin containing flakes of aluminum leaf. This coating masks the composition of the material layer underneath it while providing effective solar reflectance and thermal emittance.
• Elastomeric roof coatings (ERCs) are acrylic-based materials that can help protect roofs from extreme weather conditions and can be applied directly over an existing roof, depending on the condition of its surface. ERCs can be used on various types of flat or low pitched roof materials and some are specially formulated to dry quickly.

[edit] Cool membrane roofing

Membrane roofing is primarily used on flat- or low-pitched roofs in commercial settings, but it is also sometimes found on residential structures and other buildings with steeper-pitched roofs. The membrane serves to protect the structure from extreme weather and can provide insulation as well. It is often made from felt, fiberglass, polyester or other materials and can then be covered with pigments that increase solar reflectance. It can also be covered with roofing gravel.

One type of single-ply cool membrane roofing is Ethylene Propylene Diene Monomer (EPDM), which is often found on low-pitched commercial structures. EPDM is available in black or white.

There are also single-ply cool thermoplastic membranes, which are flexible sheets made of polymers such as polyvinyl chloride (PVC) and thermoplastic polyolefin (TPO). These sheets are heat welded and are most frequently manufactured in white (however, other pigments can be added).

[edit] Related articles on Designing Buildings

• Albedo in the built environment.
• Cool roofs.
• Domestic roofs.
• Flat roof.
• Green roofs.
• Metal roofing.
• Solar Reflectance Index in the built environment.
• Types of roof.
• Urban heat island.