Installing PV Arrays on Buildings

Array fixed over an existing pitched roof covering

Most PV installations are installed over the roof covering by clamping the PV array to a pair of rails fixed to the roof. The mounting rails are fixed to the roof rafters by roof anchors. The irregular or handmade construction of many older roof coverings may make it more difficult to make regular fixings to the roof structure. The type of roof anchor will depend on the existing roof covering, and the height and spacing of the roof battens. A sufficient area must be left free of panels to allow access and maintenance work to be carried out safely.

When fixing over a roof covering, consideration should be given to the PV array’s lifespan, which is typically much shorter than the life span of the roof covering. It should therefore be capable of being removed without causing long-term damage to the permanent structure. Care must be taken to ensure that the installation of PVs over the roof covering does not lead to long-term structural or moisture issues, both during and beyond the system’s expected lifespan.

Tiled roofs

On tiled roofs, the roof anchors are typically fixed to the rafters. This is done by lifting the existing tiles and notching the underside of the tile above the anchor, so that gaps are not opened up in the roof covering. It is important to ensure that the design and installation does not disrupt the way the tiles lie as this could allow water ingress.

Slate roofs

On slate roofs, the roof anchors are also fixed to the rafters, but a proprietary clamp or lead flashing component is required to weatherproof the connection and prevent water penetration. Installers should never drill through slates to install fixings as this risks cracking the slate and will result in water ingress.

Lead roofs

Lead sheets undergo considerable expansion and contraction as the temperature changes, so joints in lead roofs are designed to accommodate this movement. Custom-built clamps to fix around the lead rolls are not ideal as the clamps do not allow the lead to expand and contract naturally with changes in temperature.

The Lead Sheet Association (LSA) advises using a simple raised timber block system, capped with lead sheet to support and fix typical handrail systems and other mechanical equipment to roof structures. This approach could also be used to fix PV panels to lead roofs without anchoring through the lead to the rafters underneath. The support blocks should be positioned to allow for drainage on either side of the support, and away from any cross-joints, such as steps or laps, in the leadwork.

Advice will be needed on the fixings required to secure the blocks to the substrate and provide stability and resistance to wind pressure. The blocks should be co-ordinated with the structure below, to avoid overloading the lead sheets.

Structural loading

The mounting of panels on a roof needs to be carefully considered to avoid damage. Because the panels are fixed through the roof covering and into the primary roof structure (for example, the rafters), this creates a route for moisture ingress into the structure. This needs to be carefully detailed to avoid leaks. Ideally, access to the underside of the structure should be provided to allow regular inspection, post installation.

The typical weight of a panel is 15kg/m². This is a small increase in the overall static load of the roof. For example, a timber flat roof has a load (permanent and imposed) of 150 kg/m². In some circumstances it may be that strengthening work or replacement of roofing members will be required. For example, this might be needed if:

• The strength of the roof structure has been reduced by decay (such as significant timber rot or steel corrosion)
• The structure has a flat roof, and it is proposed to use ballast to hold the PV panels in place, significantly increasing the load on the roof
• The roof structure is already highly loaded because the original roof covering is very heavy such as Cotswold stone slates or has been replaced with heavier roof covering than it was originally designed for
• The roof covering is a lightweight material and the PV load is a significant increase in the overall static load
• The engineer will also need to consider the effect of additional wind uplift forces on the roof structure and fixings. The magnitude of the uplift force will vary depending on the layout and orientation of the PV panels and the location of the building. This is likely to affect the design and detailing of the fixings into the roof structure

Arrangement, reflectivity, colour, and finish

Careful selection and design of the colour, reflectivity, framing, size and symmetry of PV installations can reduce their visual impact. The colour and finish of roof-mounted installations should be chosen carefully to complement the colour of the existing roof covering. PV panels tend to be a dark blue or black, although there are different finishes and tones available.

Anti-reflection coating (ARC) can be applied to PVs to reduce glare and reflection. In many instances, ARC results in increased optical performance and energy yield, because a greater proportion of the incoming radiation is absorbed and not reflected back into the environment. It is increasingly being specified as standard . It also helps to reduce the visual impact as the panel appears less shiny. Customisable wraps can also be applied to the PV panels to mimic the finish of the roof but this will reduce the yield of the PVs.

The metallic supporting frames for PV arrays are usually anodised and can be finished in black or silver to complement the roof covering.

The way in which panels are laid out in relation to one another can make a huge difference to the appearance of the system. Installations that are arranged symmetrical and evenly, with all panels either portrait or landscape, work much better visually than panels arranged randomly. Its also important to consider how the panels align with windows and roof lights.

The existing roof covering replaced with solar slates or tiles

Roof-integrated PVs function both as a roof covering and an energy generator. Whilst these have the potential to offer a reduced visual impact, they do not fully replicate the appearance of slates or tiles. They also have a very short lifespan compared to slates and tiles. This means they are generally not suitable for use on prominent roof slopes of listed buildings and the advantages of mounting them in less visible locations is very limited.

Their performance as a roof covering must also be carefully considered and the risk of compromising the weather-tightness of the roof. It is important to consider how the PV tiles/slates will weather and change over their lifespan, which may alter their visual appearance and performance as a roof covering.

PV slates are integrated with the roof tiles or slates with flashing, in much the same way that flashings are used to weather the abutment with a roof light. However, these are only compatible with certain types of existing roof coverings, so, unless the whole roof covering is being replaced, it is essential to check compatibility with the manufacturer. It is also important to ensure that ventilation can be maintained behind the array to ensure there is not a build-up of heat that will affect the array output.

Ballasted or fixed onto a flat roof

On flat roofs, where tilted PV panels might have an undesirable visual impact, they can be mounted at low pitch angles (as low as 10 degrees) or even flat. This allows the array to be hidden from view at ground level behind a parapet wall. However, this is a compromise, and will be less productive, due the array not being positioned at the best angle.

When installed on a flat roof, PV panels are normally fixed to a frame which is angled to capture the maximum sunlight. The frame can be held in place with ballast or, if the roof cannot take the extra weight, fixed with screws through to the roof structure. Where it is necessary to penetrate the roof covering, it is important to ensure that it is properly sealed to prevent water ingress. The lifespan of a felt roof covering can be as short as 10 to 15 years before the felt must be replaced whereas other roof coverings typically last much longer. As a PV array has a useful life of over 25 years, it may be sensible to consider re-covering the roof at the same time as the installation, or the PV array may need to be temporarily removed later when the roof needs re-covering.

Another method of fixing to a flat roof without the need to penetrate the roof covering is to brace across the roof if there is a parapet wall. A lightweight framework can be fixed to the parapet walls to support the panels. In all cases, a structural engineer should be consulted in the design process to ensure the roof or parapet walls can take the additional weight of the PV array.

As a PV array has a useful life of over 25 years, it is sensible to consider whether a flat roof needs renewing at the same time as the installation, otherwise the array may need to be temporarily removed later to enable the roof to be re-covered.