Best practices for good flat roof design

Roof design requires the careful balance between functionality, sustainability, structural integrity and aesthetics. Flat roofs offer a lot of versatility. Among many advantages, they provide easier accessibility than pitched roofs and offer a great canvas for solar and green roof installations. Here are some best practices to consider during the design and specification phases of a flat roofing system:

A roof’s primary function is to provide shelter and protect the building from external elements. Therefore, ensuring its structural integrity and proper drainage are paramount. 

Assessing the roof’s load-bearing requirements, including the weight of the roofing materials, potential snow loads, wind loads, and any additional features such as solar panels, green roofs or HVAC units is critical.

Flat roofs require a minimum slope to allow water to drain properly. Incorporating enough drainage solutions, such as internal drains, scuppers, or gutters, are essential to direct water away from the roof and prevent its accumulation in puddles (also known as ‘ponding water’). Roof slope must of course be integrated into the design and can be further enhanced by using tapered insulation boards.

Is the building in a particularly windy area? Is it close to the sea? How warm does it get in the summer? How much snow does it fall? Local weather patterns and environmental concerns should be considered early in the design process to choose the appropriate roofing system and materials.

The building’s function is another key factor that needs to be addressed early on. An industrial warehouse, an indoor swimming pool and offices all have very different roofing needs. This hugely impacts the design of the roof, choice of materials and attachment system.

Roofs significantly affect a building's thermal performance. The choice of insulation layer (material type and thickness) is therefore crucial to obtain a durable roofing system, and should always meet and even exceed the minimum local requirements.

Implementing green roofs also enhances insulation, reduces the urban heat island effect, and improves air quality and biodiversity. Integrating photovoltaic panels into the roof design is another effective method for contributing to the building's energy needs and promoting sustainability.

How roofing materials have been manufactured, their packaging, performance requirements, the way they are installed, service life, recyclability, and overall impact on the building’s life span all affect sustainability. 

The waterproofing layer is a crucial component of the roofing system. Single-ply membranes are particularly suitable for flat and low slope roofs as they are durable, lightweight, quick, safe and easy to install, cost-effective, are compatible with green and solar roofs, and have an overall low environmental impact. The main types of single-ply roofing membranes are EPDM (Ethylene Propylene Diene Monomer), TPO (Thermoplastic Polyolefin), PVC (Poly Vinyl Chloride) and PIB (Polyisobutylene).

On refurbishment projects, it is estimated that up to 80 percent of single-ply membranes can remain in place as an underlay. If the roof does require stripping, it is possible to recycle the old single-ply membrane, although the level of recyclability will vary depending on the type of membrane selected.

To maximize sustainability, it is vital to assess the full lifecycle of roofing materials, from production to end-of-life disposal or recycling. The durability, longevity, adaptability, and resilience of single-ply membranes are key factors that contribute to their alignment with circular economy principles, ensuring they minimize environmental impact while extending the roof’s service life.

From a life-cycle cost perspective, it is also a smart move to protect the insulation layer with cover boards. Extreme weather conditions, such as hail or heavy snowfalls, or frequent foot traffic (e.g. because of the installation and maintenance of a solar roof) can damage the insulation layer, resulting in a system failure and/or reduction of the roof’s thermal performance, which in turn would cause repair works and/or an increase in energy costs. 

Ensuring the correct installation of the roofing system is key to achieve the best performance and extend the service life of the roof. Even on the best designed roof, poor workmanship may cause the whole roofing system to fail. 

Properly following the guidelines of the roofing system manufacturer and carrying out seams and flashings thoroughly and efficiently is critical to ensure watertightness and minimise the risk of water ingress through the roof assembly. Junctions between different materials are a common problem area, so contractors are advised to seek advice from the membrane’s manufacturer to check if different materials are compatible with each other.

When specifying and installing a roofing system, it’s important to take advantage of the manufacturers’ technical expertise and trust on the available technical approvals, including the pertinent fire and wind-uplift tests. When selecting a waterproofing membrane, for example, you should expect to receive information such as local specifications, wind load and thermal calculations, regulatory compliance, and confirmation of national third-party accreditations.

The technical advice of the manufacturer should also ensure the most appropriate roofing system is developed and the membrane is applied in accordance with their guidelines. Once the system is approved by a technical services team and executed by a trained and certified contractor, a long-term system warranty can be provided.

For the best peace of mind, it is best to seek a manufacturer that can supply all the different components of the roofing system, such as the vapor control layer, insulation and waterproofing membrane. This not only ensures material compatibility but also provides a single point of responsibility throughout the roofing system’s life span.

A well-designed flat roof should allow roofing contractors to be able to access the roof safely for inspections and eventual repairs. This is particularly important on roofs with solar panels or green roofs, as they will naturally require more frequent maintenance.

It is always advisable to provide extra protection on the roof areas that may receive increased foot traffic. This can be done by providing a maintenance path on the roof using walkway pads

While functionality is key, the roof should complement the building’s style, proportions, and material palette. Consideration of the visual impact of roof elements such as skylights or solar panels can enhance the building's visual appeal. Considering how the roof will be viewed from different angles, both from within the building and from the surrounding area, is important to create a harmonious visual effect.