Thursday 24 July 2014

Short of the Mark?

If you’re relying on a Solar Keymark wind resistance it turns out that there are large parts of the UK where you shouldn’t install

Viridian Solar has recently launched the latest version of its wind load calculator to partnered solar installers.  The calculator has been updated to take into account new guidance in the latest version of BRE digest DG 489 “Wind loads on roof –mounted photovoltaic and solar thermal systems”.

Read my blog on the changes to DG 489 here.

I thought I’d take the calculator for a test-drive, and decided to use it to try to answer the question “Is the Solar Keymark wind resistance test to 1,000 Pa adequate for the UK?”
The Keymark requires solar thermal panels to be tested for wind resistance, but historically it was a pass-fail test set at 1,000Pa (100 kg/ m2).    With the recent introduction of the new standard for solar thermal panels (ISO 9806), Keymark testing will require a pass-fail test at 2,400 Pa (putting it more in line with the level required for solar PV panels in EN61215).
However, the vast majority of solar panels currently on the Keymark and MCS database have been tested to the old standard EN12-975 at 1,000Pa.  While it is possible for the manufacturer to request that the test continues beyond the minimum, most are content to achieve the ‘gold standard’ of Keymark.

So how well does a single test to 1,000Pa cover us in the UK? 

In assessing that a structure has sufficient resistance to wind, Eurocode 1 requires you to apply a partial safety factor to the tested strength.  The safety factor you choose depends on how the solar panel fails.  For example a solar panel that has its failure in a metal component has a lower safety factor (divide by 1.1) than one where the failure is in wood (divide by 1.44).  This is to take into account the natural variability in the strength of wood and the consistency in the strength of steel. 

Since Keymark doesn’t require a test to find the failure level, there’s no way of knowing if the panel would have failed at 1,001Pa or would have gone on to 5,000Pa.  You also don’t know what the failure mode would have been.  Unless the manufacturer has tested beyond 1,000Pa the only safe assumption is to use 1,000Pa and, if they haven’t tested to confirm the failure mode, to reduce this by the highest partial safety factor of 1.44 for failure in a timber fixing.

Consequently, to be in compliance with UK Building Regulations and MCS, a panel tested to only 1,000Pa should not be used if the calculated wind pressure is higher than 694Pa, being 1,000Pa divided by 1.44.

I ran the wind calculator for the common situation of a two storey building with a duo-pitch roof at various wind speeds.  I assumed that flat plate solar panels are fixed above the roof in a position to avoid the edge-zones (which have higher wind loads), that the building location was 75m in altitude and relatively close to but not right on the sea (2-20km) and that there were no special topographical features (the building is not on a hillside).

It turns out that it’s really rather easy in the UK to achieve wind speeds that produce uplift pressures exceeding those for Keymark tested products.  The table below shows the calculated wind pressure (including partial safety factor) and the map summarises the results.

Some solar manufacturers will to tell you to just put a few extra fixings on if the winds are high.  A few moment's consideration reveal this is completely inadequate.  If you haven’t tested to failure, you cannot know that the fixings are the weakest part.  It could just as easily be the cover glass that comes away at 1,001 Pa.

The only installation that meets building regulations and therefore the requirements of the MCS installation standards is one that uses solar panels and fixing kits that have been tested to a level that exceeds the wind pressure.

It is a welcome development that the new test method in ISO9806 is to 2,400Pa, though it is regrettable that the authors of the standard did not take the opportunity to resolve other shortcomings of the wind pressure test that I have written about before.
I don't anticipate that many products will struggle to achieve this higher pressure level, after all people have been installing all over the UK for years.  The fact remains that until they’ve been re-tested to a higher wind pressure there will be many locations in the UK where the Keymark alone falls short of the mark as far as building regulations are concerned.  The responsibility is clearly with solar installers to use solar panels with a tested pressure resistance high enough for the location.