Best Fixing System for Exposed Location

[DickiePhitt]

I'm considering a Solar PV system on a property in an exposed location in the Pennines. We've had regular instances of tiles being lifted and displaced on our (concrete tile) roof by the winds we get here so a critical part of the installation is ensuring the panels dont get twisted, or ripped off the roof.

Are there specific upgraded fixings for such applications? If not, what is the most resilient fixing system in people's experience?

 

[solarsavings]

You'll need to look for modules that can take high wind loads as well as the mounting system design.

 

[Worcester]

@DickieFitt You will almost certainly need a proper Structural Engineers report, covering all aspects, mountings, panels and the method of fixation, or else your household insurance could be null and void.

There is a potential big problem with the fixation method as some of the current practices don't meet best practice from the BRE (e.g. edge distance from timber edge to fixing etc)

Every installer is supposed to do the calcs, we supply them to our customers as part of their handover pack, since they have paid for them. Check that your installer has done them / had them done, will do them, I would also get them independently checked, and include a break clause in your contract if there is a problem, also check the appropriate professional indemnity insurance of whoever they get to do the calculations. Far better to get it done right now, than trying to pin blame on an incompetant installer if it all goes wrong later. In fact since you are aware of the problem, if you don't you may actually find out that you are accepting the risk.


Also bear in mind that German derived recommendations may not actually be correct for the Uk due to different construction practices, one of the biggest problems in the size of timber used in the rafters, ours in the UK are generally much smaller.

 

[DickiePhitt]

Thanks, Worcester, for the solid advice. I'll be interested to see what my 2 shortlisted contractors have to say, and whether it comes close. Especially as one is a roofing company that does accreditation of solar installers.

 

[Worcester]

In a VERY windy location, I would expect them to use a system screwed to the rafters. (Not a Hanger bolt, see discussions elsewhere!). The click-fit system can work, though as below, get them to produce the calcs.

It actually doesn't take a lot to lift a tile off a roof, just a combination of right angle, right velocity. Most tiled houses will exhibit tile rattle at some time or other during a windy period, it is a common occurence.

By they way, before they quoted, did they do a structural survey of your roof, and collect angles, timber sizes, spacing etc?

Measure the width of your rafters yourself.

Ask them the bolt / screw size, (they will probably say 8mm)

Then ask if they are galvanized or s/s (we always use s/s)

If it's 8mm, ask them how they are going to fix it. The guidelines (BS 5658) gives minimum distance from the edge of the timber to the screw, though in practice you should only use an 8mm coach screw if your rafters are at least 48 mm wide. On anything less, use a 6mm coach screw, and in that case they will have to put in at least 3 coach screws per roof hook (to get the pull out load). If your rafters aren't wide enough and they want to use an 8mm screw, they will have to strengthen the rafter at each location by adding an extra peice to the side (from inside the roof void) and fixing it securely with bolts. If the rafters are less than 35mm wide, they will have to go smaller still and install more coach screws.

Then ask what spacing the rafter / roof hooks will be if they say 1200mm, ask them how they came to that.

As a rule of thumb, I would expect it to be nearer 900mm, if your rafters are at 600 c/c then they should put one on each rafter along the bottom and top rail, it may be permissible to open it up on the intermediate rails as the lowest uplift will be in the centre.

The strongest uplift occurs in the bottom corners of the array of panels, so a good design in a windy location, if you are at 450mm c/c rafters, then they should put a roof hook on adjacent rafters in the corner and then open up the spacing to every other rafter.

Also ask them what the maximum overhang that they will use from last roof hook to end of rail (it should be around 1/4 - 1/3 of the distance between the roof hooks - 1/2 is too much)

They should have the calcs to back all this up, it is not nescesary for them to do it for the quotation, they should do it before they start work tough, don't let them start without seeing the calcs. It is an MCS requirement to check the structural integrity which can only be proven by producing the calcs.

So also ask them who they are MCS certified with and which parts of the Building Regs are they allowed to operate as a 'Competent Person' as, once again, they should know. Not all MCS accreditors approve companies as a 'Competent Person' for the structural elements of the Building Regs, so in that case they will need to pre-notify. Ask them and see if they even know what you are talking about, I can assure you, most won't!

MIS 3002
4.4.1 All contractors shall make their customers aware of all permissions and approvals required for the installation. The contractor shall assess the building using a qualified professional, experienced in photovoltaic systems as defined under item 5, to ensure that the site is suitable for the installation and that the building will meet the requirements of the building regulations and other applicable regulations applicable to their work during and following installation. Where required planning and/or building control approval should be obtained before work is commenced.
4.4.3 External metalwork used for bracketry or mounting frames shall be constructed to correctly support the imposed static and wind loads, must be adequately ballasted or fixed into a suitable structural member and shall be adequately protected from corrosion for a typical life to first maintenance of at least 20 years. (for example, stainless steel number 1.4301 or 1.4401 to EN 100881 or galvanised coating on mild steel as specified in EN ISO 14713:1999 for the appropriate environment, or equivalent)
4.4.4 The contractor shall ensure that the roof structure is capable of withstanding the loads (static and wind loads) that will be imposed by the PV modules and their mounting arrangements. If there is any doubt, a structural engineer must be consulted. Guidance on the mechanical installation and wind loads are given in BRE Digests 495 and 489 or BS 6399.
Note: where a new roof incorporates new trussed rafters, the designer of those trussed rafters shall be advised of the position, number and weight of the PV modules to be mounted onto the roof structure.

It's a lot more complicated than a lot of electricians realise, and most roofers aren't structural engineers. (We have one structural engineer full time on our staff and one part time.)

 

[DickiePhitt]

I've tried hard to avoid the guys 'graduating' from d'glazing but none of the 5 companies looked in the loft, trusting to ground measurements, tile counting and Google earth to give me a quotation. Of course this is the unique problem with PV - there is electrical specialism and roofing specialism and your average rep aint an expert at either, let alone both. To be fair 3 companies were quite clear that a full survey would be done at the next stage but structural engineers' calcs not mentioned.
As a retired (mech) engineer with 16th Edition qualification I want to know more about the detail than most householders I suspect.
Anyway, armed with what you've posted I'll rejoin the battle next week when I've meetings set up with the technical specialists from the 2 front runners.
Pity you dont have an operation 'Oop North'

 

[SRE]

We're in the North, though not where you are, as Worcester says we use structural calcs, we tend to increase the number of hooks and use SEN SOL 50 - it's more expensive but worth it. I have it on our roof just in case we'd have real success with a wind turbine in our area, it seems to be blowing a gale a lot of the time

I'm sorry but I couldn't agree to buy soemthing so expensive from someone who hasn't even looked in the loft. Try local rather than national installers you'll usually be dealing with the person installing not selling and they should be able to answer all your questions.

 

[solarsavings]

Worcester I tried to send you pm but your mail box is full.

 

[DickiePhitt]

Good advice SRE. I've come to the same conclusions - am staying local and ensuring I'm speaking with the specialists in the companies, not just the sales guy. I'll take a look at SEN SOL

 

[Worcester]

@solarsavings, Should be space now