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How to know if your roof is sound enough for solar panels

Posted by Chris Rudge on 19 September 2011 at 3:29 am

Q: I've built a few houses and know how important it is to have calculations for roof trusses, considering the weight and "sail" potential of PV (I'm considering a 3.99kw 21 panel Schuco system in excess of £10k). Who takes responsibility if this lot blows off or you get some structural damage? The insurance company I'm sure will ask for calcs and building regs approval or structural engineers report and his insurance details. Also of course you've added over £10k to the rebuilding costs. I'm aware it's permitted development but I understand that covers planning not building regs or insurance. Have I got it wrong or are you deep in it if your roof blows off and you've not sorted this out?

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.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

7 comments - read them below or add one

Icarus

IcarusComment left on: 11 September 2013 at 7:49 am

Based on my experiences, I'd check that your roof is in good condition, before installing panels.

http://www.solarpowerdiary.co.uk/2013/09/hows-your-roof-coping-with-solar-panels.html#more

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Sims Solar Ltd

Sims Solar LtdComment left on: 3 December 2012 at 5:39 pm

Picking up on a few points raised in the entries. As installers we need to confirm that the roof can take live and dead loads imposed by the panels. We as a company use a qualified structural engineer and while we have never not been able to proceed we have had to stiffen truss blades, install diaphram floors where truss cords have been cut for loft hatches etc. At the end of the day the customer has to take a view as to whether the cheaper offer is the best offer to accept.

Regarding slate type roofs we uses a product from Dulas which is flashed in. Our Twitter feed @simssolar has some recent examples.

One of the problems we have in this country is that a lot of the roof fixing readily available are from the German market where roof timbers ar 70mm or more in width and bracket fixings are designed to suite. Trying to apply this solution to a truss blade only 35mm wide is not going to work, hence the thickening up described by Paul 128.

Hope this helps

 

When we use ,

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Rebecca Hall

Rebecca HallComment left on: 3 December 2012 at 12:28 pm

There is some really great advice in this post and equally in the comment made by Icarus. For the first time I have actually had a look at my roof as I was considering solar panels myself. I dont know how you would go about roof access but I opted to hire an access platform because I was not confident using a ladder. Due to my surroundings I needed to find one which would allow me to get access in a narrow area. This guide pointed me in the right direction

http://www.universalplatforms.co.uk/docs/Universal%20Access%20Guide%202012.pdf 

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Gilly Jones

Gilly JonesComment left on: 28 September 2011 at 4:51 pm

Q: I've tender for the supply and fix of a P.V. array of panels to my slate roof (Cambrian slate by Redland). I am concerned about the method to be adopted for fixing the support framework for the panels to the roof. Information from the rep. was somewhat vague and "optomistic" to say the least.. I have been in touch with Redland technical Dept.. but they equally are vague about fixing methods---I suspect they leave it to the sub-contractor ! Are you aware of any "preferred method" of fixing particularly for Cambrian slates which is a lightweight reconstituted slate, interlocking and fixed with two clips ?

A: “Until now we have used a standard slate bracket and lead flashing on slate roofs.  This has been the strongest and most watertight solution available.  However, K2-Systems who we use for mounting systems have just released a specially designed bracket for interlocking slates.  We shall be introducing this for slate roofs as soon as this bracket is available. “

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Paul128

Paul128Comment left on: 24 September 2011 at 10:48 am

I have had 2 surveys so far.  One installer insists if I go ahead on having a structural engineer report before he will proceed.  He inspected the rafters in my roof which are 50mm wide and said that the size and position of the screws to adequately secure the brackets may cause splitting and reduce structural integrity and to avoid this they would install additional wider wood alongside existing rafters and fix the brackets into the wider wood instead of the existing rafter (This installer is relatively new but is MCS/REAL accredited and buy with confidence listed).

I've also had round a long establised company (long in the context of solar - nearly 10 yrs so local council work and domestic) MCS/REAL accredited.  The installer did a visual inspection and some measurement in the loft and said the installation would be fine - no structural report and no mention of any extra wood.

 Whom am I to believe and what's with the extra wood thing?


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Icarus

IcarusComment left on: 22 September 2011 at 6:01 pm

Having PV panels on your roof can be a worrying experience,

I'm reasonably certain that the Schuco fixing rails which are attached to the 14 Schuco  MPE 190 MS 05 PV panels on my roof are secure. However, I did some research online to discover which home insurance policies specifically include panels, inverters and related wiring in their cover. Amazingly my existing company offered total cover and gave me a warm and cosy feeling even if there was a problem.

If you're interested if YOUR policy offers you the same cover take a look at the relevant page on my website: http://www.solarpowerdiary.co.uk/2011/07/solar-panels-insurance.html

Hopefully, none of us will ever need to make a claim.

Good luck. Icarus

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David Hunt

David Hunt from Comment left on: 20 September 2011 at 12:48 pm

Great article Chris.

 As suggested beware cheap and cheerful installers that don't survey your roof correctly, just want to sell you a system. And beware of installers that just screw through your tiles. Good for them, cheap and quick, but not for you and your roof. And are they likely to still be there when you have a problem?

 Use YouGen to check your installer, and ask for customer testimonials. 

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