How to know if your roof is sound enough for solar panels
Posted by Chris Rudge on 19 September 2011 at 3:29 am
A: You are right. It is very important to ensure your roof is going to be sound enough to take the added weight of a Solar (PV or thermal) system. Most PV systems will add another 14kg to the dead load of the roof, plus another factor to bear in mind is that the system is only going to be mounted on one side of the roof, therefore possibly unbalancing the structure if it's insecure.
In my experience, most recent roofs are designed to take a load of at least 100kgs a square metre. Structural publications refer to Kilo Newtons, but for the purposes of this brief post I have done a rough conversion to Kg, which most people can relate to.
There are various weights to the type of roof covering, as examples: 25Kg sq mtr for slates, 50 Kg sq mtr for Roman tiles and 75Kg a sq mtr for large concrete tiles. A 15% tolerance for additional load as well (such as snow) is generally allowed, thus even with concrete tiles, the added 14Kg sq mtr for PV, just slips in as OK.
Even though your local planning office will probably give you the green light with regard to permitted development, no more than 200mm above the roof and not above the ridge, this does not automatically mean your roof is good to take the weight of added load.
If there is any doubt on either the security of the roof joists/ purlins etc, the solar installation surveyor should pick this up, as this is a critical part of the survey. The surveyor will need to inspect the roof structure and probably take some photos, so will need access to the loft to check all these items during the survey.
Anyone that tells you they have looked at the Google aerial view and your roof is perfect is not professional! Even if the solar installation is a 'free' version, it’s still your roof and you really don't want it collapsing in the autumnal gales.
Going back to your original responsibility part of your question. If the roof did collapse due to the PV system overloading, it would definitely go back to the PV installation company, whose insurance company MAY not support the payout due to general incompetence of the installer.
Wind uplift is a smaller, but not to be ignored, problem. Situations where roof plates have not been secured and movements in high winds have resulted are not unknown. There is a BRE guideline document PV installers are able to use that addresses wind uplift, but there are no firm calculations, simply guideline calcs to go by. It is definitely down to the experience of the installer here.
As a side note, some of our guys went to a trade day event recently, and were astounded to hear that installers are simply screwing PV support brackets to the tile batten! These substantial structurally designed brackets need to be secured to the joists with substantial screws that relate to the timbers concerned.
As mentioned above, wind uplift is a small issue on a correct installation, but a very, very big issue if the system has been incorrectly installed using simply the tile batten, or even worse using the 'click' system of hanging the whole array onto the tiles only with no screwed fixings. There may be 'budget' companies doing this in the UK. If so, strong wind will displace the whole array easily.
Photo by Pellaea
About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.
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