The EurActive Roofer project undertook numerous wind tunnel tests using scale models of roofs with panels, as well as testing theoretical calculations that can, in time, form the basis of a European standard. Although the findings are complex they can be summarised as follows:
The effects of wind speed, pressure and loading on solar (PV) panels
The wind loading needs to consider the maximum recurring wind speed and not, as is used for micro wind turbines, average speeds. This map shows UK maximum speeds in metres per second and, in broad terms, no normal solar panel properly fitted is likely to add an unsafe wind loading to a typical UK roof in zones I to III so a simplified calculation method can be used. Special attention needs to be paid in Zones IV and V, principally north and west Scotland and the Northern and Western Isles.
Where more detailed calculations are required, the Eurocode [Note1] for wind loading on building structures and the related UK National Annex should be used. The calculation process has six steps:
- Determine Site wind speed Vs
- Determine Effective wind speed Ve
- Determine Dynamic pressure qs = 0.613 Ve2
- Determine external surface pressure pe
- Determine internal surface pressure pi
- Determine net load on the PV module P = (pe - pi) A
This page will not attempt to show all the factors brought in, but the information required includes the building height, altitude, distance from the sea, the distance from edge of town, site altitude, topography, distance to and height of surrounding buildings and other factors for all 12 wind directions. BRE Digest 436 gives more information on this process and there is a BRE program BREVe. The National Annex gives external pressure coefficients for a wide range of roof shapes including flat roofs, monopitch roofs, traditionally pitched roofs and hip roofs, but does not specifically address PV or other solar panels.
This diagram shows a schematic plan of a pitched roof with wind coming from two directions (west and south if north is the top of diagram), showing the various wind pressure zones on the roof for which calculations may need to be undertaken.
In zones I to III, it is often possible to use a simplified formula for the wind force: F = qsCpnetCaA where:
- qs is the dynamic wind pressure.
- Cpnet is the net pressure coefficient.
- Ca is a size effect factor that reduces to a value of 1 for an array under 5m diagonally across.
- A is the loaded area of the PV module.
In this case, the dynamic wind pressure (qs) can be found from reference tables that consider just the building height, altitude, zone and whether local topography is significant or not. There are two values needed for the net pressure coefficient - one for potential uplift and the other for the downward force.
Panel height and design
The EurActive Roofer project had earlier identified five basic arrangements of panels. In essence, for traditionally mounted panels on the surface of a roof, Cpnet can also be found from tables, with the values varying on whether or not they are within 300mm of the roof edge - so this becomes an important design feature. The type of fixing is also crucial for traditional stand-off panels. Hook fixings must not be so flexible that they significantly lift the surrounding roof tiles. The maximum gap between tiles with a hook installed should be less than 6mm and it is recommended that maximum deflection of the hook should be less than 70mm at the design wind load. This can be checked by applying the design wind load to a hook or dummy roof using weights and a pulley system and measuring the residual deflection.
Integrated designs that are nominally airtight can be treated like any other roof cladding in accordance with the Eurocode (providing they do not protrude from the roof by more than 100mm). Air permeable roof tiles or slates also use a variant on the simplified calculation method, with some allowance for whether or not they fit onto the main roof battens directly.
Situation of PV modules on flat roofs
This is a little more complex, depending partly on whether they are mechanically fixed or freestanding. Both will need to resist wind uplift but freestanding modules will also need to resist sliding. Wind loads will depend on the location of the module on the roof, whether the roof has a parapet and whether the PV support structure is open or fully clad. Calculations will also need to consider where the modules are on the roof, with different net pressure coefficients applied for those near the edge, in corners or at the centre of the roof. On flat roofs, a key calculation can be to determine whether there is a risk of overturning. For this, the mass of the unit and ballast must exceed the wind force (moment), which in turn is affected by the angle of inclination of the panel.
Note 1: BS6399: Part 2 (Code of practice for wind loading on Building Structures) was withdrawn in March 2010 and replaced by BS EN 1991-1-4:2005 Eurocode 1: Actions on structures: Part 1-4: Wind actions and the NA to BS EN 1991-1-4:2005 - UK National Annex to Eurocode 1. Actions on structures. General actions. Wind actions (September 2008).
We would like to thank BRE and the other partners in the EurActive Roofer project for their assistance with this page. The information given is for guidance only and should not be used in place of proper engineering calculations in accordance with the relevant British Standards.
This information is based on work undertaken by the EurActive Roofer project which ran from 2005 to 2008 and was supported by the European Union's programme for Horizontal Actions involving SMEs.