BRE DG495 PDF

Introduction

There are many different types of roof mounted photovoltaic (PV) systems in use in the UK. Some have been designed specifically for UK roof construction and practice, whilst others are imported systems originally designed for use in other countries. This can lead to confusion and in some cases inadequate installation has resulted in failures under wind action or rain penetration. Guidance exists for electrical installation of PV systems but there is no equivalent guidance for mechanical installation.

The PV system should be fully defined at the planning stage, including all system components. This includes co-ordination of the assembly sequence for individual system components. The overall design, assembly sequence and detail design solutions should be mutually consistent. Special consideration should be given to the adequacy of the fixings or anchors to ensure that they can withstand wind forces. It should be noted that wind forces vary throughout the UK and are affected by roof height, roof pitch, orientation, etc.

Therefore a design that is suitable for one roof shape might not be suitable for a differently shaped roof, or a fixing system used in the London area might not be adequate for use in northern England or Scotland. Digest 489 gives further information on calculating wind loads^[1].

The specifications of the roof covering, roof weatherproofing system or external substrate should be taken into account when planning the installation of a PV system. In particular thermal insulation, structural stability and weathertightness of the existing roof should not be compromised by the installation of a PV system. Installation should not reduce the air space beneath the roof covering to such an extent that it affects roof ventilation. The ideal free air gap beneath the roof covering is 50 mm, although 25 mm is acceptable in some situations: a similar air gap should be maintained beneath PV modules (see BS 5534^[2] and Thermal insulation: avoiding risks^[3] for more information).

The PV modules should be located optimally on the roof with respect to the sun in terms of both compass orientation and angle of inclination (ideally south facing and at 30° to 40° for the UK). Avoid shading where possible. The PV system supplier will be able to provide further advice.

Since there are no harmonised EN Product Standards or Technical Approvals for PV systems they cannot be CE marked under the Construction Products Directive rules, although a PV-specific CE mark is currently in development. Design and installation requirements for many PV systems are covered by the British Board of Agrément or other third party certificates or by the manufacturer's recommendations.