2.5 ROOF
The choice of stadium roof can be influenced by environmental requirements, spectator experience, the project construction budget, and the desired aesthetics/image. The inclusion of a roof can help to retain noise/atmosphere within the stadium bowl, and can protect spectators from the weather to provide a more comfortable viewing environment.
“PROVIDE A MORE COMFORTABLE VIEWING ENVIRONMENT”
GENERAL GUIDANCE
When including a roof to protect spectators from the elements, this should be done in a way that takes the local climate and stadium orientation into account (see Section 2.2). When reviewing rain cover, it should be remembered that due to wind – prevailing or otherwise – spectators can and most likely will get wet beyond the drip line of the roof, defined as a line drawn directly downwards from the edge of the roof (see Figure 2.5.1). A general guide for the protection from rain offered by varying degrees of roof coverage is as follows:
• Seats located beyond an angle of 30 degrees from the drip line will be well protected, and spectators here will remain dry in all but the most severe rain conditions.
• Seats located within an angle of 15 degrees from the drip line offer little protection, and it is highly likely that spectators here will get wet when it rains, depending on the direction of the wind.
• Seats located within an angle of 15-30 degrees from the drip line will be moderately well protected, however, it is likely that spectators here may get wet when it rains, depending on the wind direction.
Figure 2.5.1
Weather protection from roof and possible viewing obstructions
It is recommended that all VVIP, VIP, broadcast and media tribune seating be located beyond the 15-degree line and preferably beyond the 30-degree line.
The extent of the required roof coverage over the seating may be dependent on local weather conditions and the project aspirations.
When looking at the design of the roof, the roof coverage, the height of the roof above the pitch and the stadium orientation, the shadowing of the pitch from direct sunlight and therefore the impact on growing conditions for the grass should be considered (see Section 2.4). The shadowing of the pitch may also affect TV broadcast, with the potential for extreme levels of light and shadow on the surface of the pitch to compromise the quality of the broadcast. Both issues can be mitigated with the use of transparent roof materials over the whole or parts of the roof. The degree of coverage of transparent roof materials required to achieve the desired levels of shading can be evaluated as part of a sun trajectory/path study (see Section 2.2). Different materials, such as glass and polycarbonate, for example, will perform better than others in the transmission of sunlight. The cost of the long-term maintenance of these materials will vary, and this may affect the selection and extent to which the roof design features transparent portions.
TYPES OF ROOF DESIGN
When considering the structural approach to the roof, the overall size of the stadium may help to determine the type of design that is needed. Possible requirements for future-proofing the roof structure and achieving the desired aesthetics and meeting the project costs should also be considered in this process. Sightline restrictions caused by the roof support columns are not recommended and do not represent best international practice for stadium roof design.
There are three main types of stadium roof structure (although derived structural types can also be used).
• Cantilevered roof structures – where the roof structure is supported at one end, usually the rear of the stand, and projects forward over the stand to create a column-free cover. The main advantage of this type of design is that it can be installed bay by bay. This can be useful in the phased development of a project and also allows for future changes to the stadium. This method can mean that a lot of the structure ends up at the rear of the stands, which can impact both the appearance of a stadium and operational flows behind the stand.
• Simply supported trusses – where the roof structure is supported at two ends, with a large truss spanning either the length of a stand or over the stadium. This structural method can be cost-efficient, particularly when designing individual stands, and can also reduce the amount of structure to the rear of the stadium. The location of the columns/cores supporting the trusses needs to be well planned in order to avoid sightline restrictions and impeding any future stadium expansion.
• Tensile structures – these are stabilised by tension rather than compression and are usually constructed with a tensioned cable-net system, on which the roofing membrane sits. This form of roof is usually more applicable to larger stadiums and can be difficult when it comes to future stadium expansion or development. The advantage of this type of system is that it can appear very light and elegant, often with a minimal amount of structure required for support.
As well as affecting the transmission of sunlight to the pitch and TV broadcasting, the choice of roof materials can impact the stadium atmosphere. Architectural fabrics can have an elegant appearance, but generally allow a greater amount of the sound generated in the stadium to escape, which can impact on the atmosphere and create noise pollution. The use of materials that reflect sound to the roof soffit can mitigate these issues.
A retractable roof in place at the Johan Cruyff Arena, Amsterdam
MOVING/RETRACTABLE ROOFS
Moving/retractable roofs can be effective in widening the use of a stadium for year-round football and other events such as concerts, particularly in regions with extreme climates. In built-up urban areas, the ability to close a roof may be beneficial to mitigate noise breakout, especially if the stadium is to be used for concerts. The implementation of such systems should be considered in line with the business plan and overall aspirations for the project, due to the large costs often incurred (see Section 1.6).
TECHNICAL USE OF THE ROOF
The roof structure is often used to support a host of technical infrastructure required for stadium operations, in particular lighting, communications and IT systems. These items need to be considered in the early design stages to account for structural load and performance requirements, and to ensure that no sightline restrictions are caused by the installation of this equipment. In particular, the high-ball line may be restricted by equipment suspended from the roof, including large TV screens and speakers (see Sub-Section 5.6.2 and Section 5.7).
Other opportunities when designing a stadium roof include its potential to collect rainwater to be used around the stadium site as grey water and landscape irrigation. Stadium roofs also provide an ideal location for the installation of solar collectors to generate energy and aid the stadium’s sustainability profile (see Section 2.7).