BRE Digest DG 489 is the authoritative reference for wind loading on solar panels. It is pointed at by both the MCS PV Guide (for solar PV installations) and MIS3001 (for solar thermal installations). Solar installers are instructed to use it to demonstrate that their proposed solar installation is proof against the wind loads it will encounter.
This document matters.
Ten years on from its original issue, the BRE Digest 489 has been comprehensively extended and updated by its highly respected author, Dr Paul Blackmore, based on the latest research. In this article I outline the most significant changes and discuss what they mean for the solar industry.
- Solar thermal explicitly included
Although the wind behaviour of flat plate solar thermal and solar PV panels is very similar, the new version of DG489 explicitly acknowledges this with a change of title to show that its scope includes solar thermal panels. A new section outlines how to calculate the wind load for evacuated tube format collectors to take into account the fact that wind can pass in between the tubes. For tubes without rear reflectors the wind pressure is calculated based on the projected area of tubes and manifold rather than the overall area and the negative pressure coefficient is reduced to a third of the equivalent flat plate.
- New wind map
In the time between the first and second revision, BS6399 has given way to Eurocode 1 and the wind map in the document has been updated to reflect this change. The new map is now very close to that in the MCS PV Guide (which was based on an earlier version of EN1991-1-4).
Caption: Spot the difference - old (left) and new (right) UK wind speed map
- New categories added
The scope of the document has been increased with the sections that offer guidance on how to calculate wind loading for the following situations:
- In-roof mounting systems for pitched roofs with an integrated under-tray below the solar panel
- Panels installed above a pitched roof but not parallel to the roof
- Panels that stick up above the ridge line of a pitched roof
- Evacuated tube collectors on pitched and flat roofs
- Panels installed above a flat roof but parallel to it
- Thin film modules bonded directly to the roof covering of a flat roof
- Introduction of a safety factor
In theory designers should have taken the wind loads calculated in the original version and applied a safety factor to them. In practice many designers ignored this requirement. The new version explicitly applies a safety factor of 1.35 to all wind loads.
- Reduced wind loads
Arguably the most eye-catching change is a significant reduction in wind loads for the two most common installation situations. At the time of the original version there was little evidence available, so BRE had to take a conservative line and choose values that gave higher rather than lower wind forces. This time aroundnewly available published research has allowed the BRE to reduce the wind pressure coefficients for both above pitched roof and above flat roof installations. We’re not talking about tinkering here either - the changes in some cases are very, very big.
Above pitched roof
The pressure coefficients for panels installed above pitched roofs are reduced by nearly two thirds. For example an above-roof panel mounted in the central zone of a duo pitch roof now has a pressure coefficient of 0.5 for wind uplift. The corresponding value was previously 1.3.
Before you sell off your stock of roof hooks, you need to remember that manufacturers also have guidelines on the maximum span between fixings for their support rails. It is now more likely that this factor, rather than the wind resistance of the hooks, which is likely to determine the number of fixings to use.
Above flat roof
For panels mounted on support frames above flat roofs the assessment of wind loads has increased in scope, with guidance covering a much wider range of panel pitch angles. In particular the figures given for panels pitched up between 5 degrees and 10 degrees is a welcome improvement that supports an approach that has become common practice.
In addition there is recognition that structurally connecting larger arrays of panels improves the overall wind resistance because wind gusts do not act equally across the whole array at the same moment. The larger the area of the array, the more significant the effect; wind pressure can be reduced by up to 30% for the largest arrays.
Finally, the pressure coefficient for enclosed panel support structures has been reduced to a level similar to that for open structures. Somewhat counter-intuitively the first issue had much higher wind uplift values for enclosed (so-called low ballast) systems.
The outcome of these changes is that on the whole wind uplift values for flat roof arrays will lower when calculated using the new document.
The new document should be welcomed by the industry as it clarifies a number of situations the original had left open to interpretation or for which it simply offered no guidance at all.
Reduced wind loads will lower the cost of some installations and open up more buildings to solar without the need for expensive structural strengthening. (Although, if you weren’t using a safety factor before, its addition could completely offset the gains).
However, even the very large reduction in wind load has not rescued thermal panels tested to EN12-975 (used for the Solar Keymark) which, unless tested to above the minimum required by the standard (1000Pa), are still not suitable for large swathes of the UK – see my blog on this issue here.