Structural assessment for rooftop solar PV

[edit] Introduction

A rooftop solar photovoltaic (PV) system adds load to a roof structure. A structural assessment determines whether that structure can safely support the additional load, in combination with the loads it already experiences, for the design life of the installation, which is typically 25 years or more. In the UK, structural assessment for rooftop solar PV is informed by the Eurocodes, the Building Regulations and, for certified small-scale installations, the relevant Microgeneration Certification Scheme (MCS) installation standards.

[edit] Why Structural Assessment Matters

Many roofs that now carry, or are proposed to carry, solar PV systems were designed and constructed before rooftop solar became commonplace. Their original design may not have allowed for the additional permanent load of a PV array or for the changes in wind and snow loading behaviour that panels can introduce.

Installing a PV system without verifying the structural capacity of the roof may overload roof members, fixings or supporting elements. Consequences can include excessive deflection, water ponding, damage to roof coverings or, in severe cases, local or progressive structural failure.

A structural assessment provides documented evidence that the roof can safely accommodate the proposed installation throughout its intended service life and may also be required by insurers, lenders or certification schemes.

[edit] Loads Imposed by a Solar PV Array

A rooftop PV installation introduces several loading effects that should be considered together.

[edit] Dead Load

Dead load arises from the weight of the photovoltaic modules, mounting systems and, in the case of ballasted flat-roof systems, any ballast used to resist wind uplift. Ballasted systems can impose substantial permanent loads on the structure.

[edit] Wind Load

Wind uplift and overturning forces can be significant, particularly at roof edges and corners where local pressures are highest. These actions should be assessed in accordance with the relevant structural design standards and recognised guidance for roof-mounted photovoltaic systems.

[edit] Snow Load

Snow loading should account for local climatic conditions and the potential for drifting or accumulation behind low-pitched arrays or adjacent roof features such as parapets.

[edit] Combined Load Cases

The additional loads from the PV installation should be considered alongside existing permanent and variable actions acting on the roof structure. Structural design should evaluate appropriate load combinations rather than considering each action in isolation.

On flat roofs, the combination of ballast weight and wind resistance is particularly important. On pitched roofs, the capacity of fixings and their transfer of loads into rafters or purlins should also be assessed.

[edit] Roof Types and Structural Considerations

[edit] Pitched Tiled or Slated Roofs

Common on domestic and smaller commercial buildings, these roofs typically require assessment of rafters, battens, fixings and the condition of the roof covering.

[edit] Steel Portal Frame Buildings

Frequently used for industrial and warehouse buildings, these structures may have adequate primary frames, but purlins, cladding rails and fixing systems often determine whether a solar installation can be accommodated without strengthening.

[edit] Flat Roofs

Flat roofs commonly support ballasted systems, making the assessment of additional dead loads and reserve structural capacity particularly important.

[edit] Metal Profiled Roofs

Trapezoidal and standing-seam roofs require careful consideration of fixing methods and pull-out resistance, as the connection details may govern the design.

[edit] Ageing and Asbestos Cement Roofs

Older roofs may have reduced structural capacity due to deterioration or previous alterations. Asbestos cement roofs also require careful management to avoid disturbance during installation.

The age and condition of the roof should always form part of the structural assessment.

[edit] The Assessment Process

A structural assessment for rooftop solar PV typically establishes:

The form and capacity of the existing or proposed roof structure.
The additional loads imposed by the proposed PV array and mounting system.
The structural utilisation under relevant load combinations.
Whether strengthening or design modifications are required before installation.

Where reliable drawings and calculations are available, much of the work may be completed as a desktop exercise. Otherwise, an on-site inspection and measurement survey may be necessary to determine structural capacity.

For portfolios containing multiple similar buildings, desktop screening can identify which properties require more detailed investigation.

[edit] Existing Buildings and Retrofit Projects

Most rooftop solar installations in the UK are retrofits applied to existing buildings. In many cases, original structural drawings and calculations are unavailable, or the building has been modified since construction.

Where documentation is lacking, structural capacity may need to be determined through site measurements, engineering judgement and structural calculations based on the observed construction and condition.

Assessing an existing building is often more complex than incorporating PV loads into the design of a new structure.

[edit] Possible Assessment Outcomes

Following assessment, several outcomes are possible:

The roof is structurally adequate for the proposed installation.
The roof is suitable subject to design modifications, such as reduced ballast or changes to array layout.
Local strengthening works are required before installation.
The proposed system is unsuitable for the existing structure.

Identifying these issues before installation allows the design to be modified and reduces the risk of costly remedial work.

[edit] Regulatory and Standards Context

Structural assessment for rooftop solar sits under two separate frameworks, and which one applies depends on the size of the installation.

The Building Regulations apply irrespective of system size. BRE Digest 489, revised in 2014, identifies the threshold at which they bite: where a retrofit solar system increases the applied loading on the roof by more than 15%, Approved Document A requires that the structural integrity of the roof structure and the supporting structure be assessed. This applies to building works carried out in England, and in Wales in some circumstances; Scotland has equivalent provisions in Section 1 Structure of the Technical Handbooks, and Northern Ireland in Technical Booklet D. Where the roof cannot carry the additional load, strengthening is required, which constitutes a material alteration. The Digest notes that an assessment remains appropriate even where the increase falls below 15%, so the threshold marks where the requirement applies rather than where the risk begins.

For installations certified under the Microgeneration Certification Scheme, MIS 3002 sets additional requirements. Version 6.0 applies to systems up to 50 kWp. The standard does not require a structural engineer in every case, but it does in specific ones. Ballasted or weighted systems on flat roofs require the design to be verified by a structural engineer. Roofs of unusual form, or of uncertain or unknown construction, likewise require structural engineering input. For conventional pitched roofs within defined geometric limits the standard permits assessment by a competent person working to a recognised method, which is not the same as permitting assessment by anybody.

Installations above 50 kWp fall outside the MCS scheme altogether. These are assessed directly against the structural Eurocodes, their UK National Annexes and the applicable Building Regulations, with no certification scheme setting a floor for who carries out the work or what it must establish. In practice this means the largest installations are the least prescribed, which places the responsibility on the client to specify the assessment properly.

[edit] Relevant Standards and Guidance

Commonly referenced documents include:

BS EN 1990 – Basis of structural and geotechnical design.
BS EN 1991-1-1 – Densities, self-weight and imposed loads.
BS EN 1991-1-3 – Snow loads.
BS EN 1991-1-4 – Wind actions.
UK National Annexes to the Eurocodes.
BRE Digest 489, revised 2014, Wind loads on roof-mounted photovoltaic and solar thermal systems. The 2014 revision is written to BS EN 1991-1-4 and its UK National Annex and replaced the 2004 first edition. It divides the UK into five wind zones and gives a partial factor of 1.35 for roof-mounted solar, derived from the wind action factor of 1.5 reduced by a consequence class factor of 0.9.
Building Regulations and associated Approved Documents.
MCS MIS 3002, Requirements for MCS contractors undertaking the supply, design, installation, set to work, commissioning and handover of solar photovoltaic microgeneration systems. Version 6.0 applies to systems up to 50 kWp.

[edit] Conclusion

Structural assessment is a fundamental part of the design process for rooftop solar photovoltaic installations. By verifying that an existing or proposed roof can safely support the additional loads imposed by the PV system throughout its intended service life, engineers and designers can minimise risk, protect building occupants and ensure long-term performance.

Careful evaluation of roof condition, loading, fixings and applicable standards enables informed decisions about whether strengthening or design modifications are required before installation proceeds.