Not only do PV modules need to be fixed securely to the roof in a fashion that will be secure for the lifetime of the system through a wide range of weathers, but they also need to be held at an angle that is optimal for generation of electricity and do so in an attractive, aesthetically pleasing manner. There are a large number of solutions for this available, and the most appropriate will depend upon the underlying roof type and the building use.

In general, roof mounting systems fall into one of the following categories:

• 'Retro-fit' / Non-integrated
• Integrated
• Flat-roof
• Flexible panels
  
• Ground-mounted

Retro-fit Mounting Systems

A 'retro-fit' mounting system (also referred to as 'on-roof' and 'non-integrated') is the simplest and most cost-effective way of installing a PV system on a building. Despite the name, retro-fit systems are suitable for both old and new buildings, and there are solutions for almost all pitch angles and roof coverings. Retro-fit systems are installed on top of a conventional roof so there is no need to replace or remove the roof tiles, while standing seam roofs (and other roof coverings) do not need to be penetrated to fix the system in place.

The mounting system typically consists of a series of horizontal rails, to which modules are fixed by a series of clips. The rails are then held to the roof by regularly-spaced mounts, chosen to suit the roof type. For example, on a tiled roof hooks are run underneath tiles and bolted to the rafters, while on standing seam roofs brackets are bolted to clamps fixed to the seams. This means that the original roof remains intact (and no warranties are voided). Sometimes secondary rails are required to make adequate fixing to the underlying structure.

The installed retro-fit system runs parallel to the original roof covering, with a small gap (approx. 50-100mm) between the roof and the modules. An entry point for the field cables is required at some point in the building, but this can be in the most suitable location for weatherproofing.

The retro-fit style of mounting system, with some bespoke modifications, is often the technology used for vertical building cladding.

Roof Integrated Systems

A roof integrated mounting system essentially replaces some or all of the roof covering with the PV system. The PV system can replace almost all of a roof or just a section, depending on the building requirements. This type of  mounting system is most cost-effective when installed during the installation of a new roof. As the system needs to provide a weatherproof seal, integrated systems are more expensive than some other mounting systems, but as the name suggests they are integrated into the roof and so can be less visually obtrusive than a retro-fit system.

The most cost-effective method of integrating a PV system is one that uses standard modules mounted in line with the roof. Overlapping modules (to minimise pooling of run-off) are held in place by clips or a frame fixed to rails running alongside the battens for tiled roofs, or by a similar arrangement for other roof types (depending on the underlying structure).

There are also several models of PV tiles, which are PV modules sized and shaped to fit alongside the native tiles in the roof. These give a very flush finish, but due to the current manufacturing cost of each tile, and because as each tile must be invidually electrically connected and fitted, this style of roof mounting is comparitively expensive. The PV tile used must match up to the standard roofing tile chosen, and this can preclude some tiled roofs (especially for older buildings).

Integrated systems also include more bespoke mounts, such as cells fitted into glass panes in to form a glass / glass laminate. These often give incredibly attractive or novel finishes, but the bespoke nature does increase the cost.

Flat-roof Mounting Systems

Flat-roof mounting systems are one of the simplest mounting systems available, and are the most suitable mounting system for flat roofs (roofs with pitches of less than 10°). Depending on the roof requirements, it is possible to install this type of system without any penetration, temporary or permanent, into the roof. The main design feature is that the systems raise the incident angle of the PV modules with the sun to a more efficient angle, maximising output for a given system.

The basic premise of the system is a pre-made slanted top 'bucket', onto the top of which the modules are bolted. The principle fixing method is ballast placed inside each unit to counter uplift and dragging wind forces; this can introduce significant loads (sometimes over 100kg per module), which can be reduced if necessary by the use of adhesives and roof-bolts. It can sometimes be necessary to link the units together once installed to increase resistance to wind loading.

This type of system is most often employed on taller, flat buildings, which bring with them a number of design issues. The ballast and/or fixing method must be able to withstand significant wind loads, as the units are raised above the building surface. This type of building often has number of objects, such as parapets, skylights and lift mechanisms, which create shadows on the roof, while the raised units also create a shadow; this can limit the feasible roof area and design work must account for this.

Due to the way flat-roof mounting systems are held in place, they can only be safely used on flat roofs, and cannot be fixed directly to a standing seam. Retro-fit mounting systems can be used on almost all flat roofs, but the low angle reduces the output per module (although it can be more space-effective) and the modules require regular cleaning to stay functional.

Flexible Panel Systems

By using the mechanically flexible properties amorphous silicon, it is possible to create panels that can flex sufficiently to follow the almost any curved roof. Specially designed panels can be bonded to membrane roof coverings with a long-term adhesive during the installation of the roof.

The panels must be specified with this application in mind and often the panels are specific to certain suppliers and roofing types.  In addition, the roofing system must be penetrated to allow enable access to the distribution board and in some instances to conceal the wires.

Ground Mounted Systems

Increasingly popular in Continental Europe, but has yet to be mainstream in the UK, PV arrays can be mounted on the ground to cover an area limited only by the land available. These types of mounts are often used for large generating plants set up to sell power, or for off-grid systems where the roof cannot support sufficient quantities of modules.

Typical ground-mounted systems consist of a framed rack maintained at an optimal angle, with the structure held in place with solid foundations. The ground between rows of modules is generally shaded, but can be used for some agricultural purposes (such as grazing) providing that the electrical equipment is sufficiently protected. As noted above, this style of mounting is aimed at installations with a large amount of space; most urban sites are likely to be shaded at certain types of day.  Some of these large arrays have tracking systems ensure the array follows the sun.