Positioning PV panels horizontally flat isn't recommended.

They need to be at an angle sufficient to allow rain to wash them clean.

There's a static electricity attraction between the top glass layer and air-borne dust particles. It's worse than you'd expect because the electrical activity of the silicon wafers and their interconnecting foil tracks keeps the glass 'charged'.

Commercial solar-farms usually carry out an annual cleaning regime, even though their PVC panels are tilted at around 35°, and the aluminium frames are earthed to help reduce static attraction.

@transparent I need to think this out carefully, my first choice is tubs and at 100kg and screwed to the roof joists I doubt they would move.  In terms of the roof coming off I think all that weight should keep it down. And as mentioned fit joist clips to the internal walls although one side had units on that wall so will need to modify them if that’s the route.  It’s solid brick walls but the joists are sat on the wall and could lift off but those joists are beasts and the weight of the tubs surely would not allow that.

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Having seen that latest photo, I'm inclined to agree with your strategy.

I can't see how you could fit any additional packing beneath the raised portions of the corrugated sheets.
There's no access.

So your better strategy is to simply ensure that load from the the five tubs are spread across the underlying beams.

@transparent ordered my panels, Aiko 510, going to install them on the tubs screwed through the roof and ballast.  One question though, should the cables be in conduit.  I read they should be protected from the sun, not laying directly on the roof and open to the air so they don’t overheat. Not sure how you do that if I have to run them along that pipe insulation, and do the cables need to be kept a distance apart from one another?

Cables run through conduit have to be 'down-rated' to a lower current capacity.
They'll be fine in open air, where any heat buildup gets radiated away.

You can run the PV cables alongside each other. They're high-voltage, low-current.

The outer insulation is UV resistant. It should be fine for the next 100+ years.

@transparentransparent I guess there is nothing you don’t know, I’m fair impressed, you are one of them blokes that an apprentice would love. I will take some photos of it as I progress and post them for your thoughts.  In all honestly though this forum is incredible and blessed with several great minds.  

@transparent do these boxes have to be further back do you think?

and what distance should I position the boxes for the ones in front 

Posted by: @david999

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meant to add they are getting bolted down and ballast.  Do they need as much as 100kg 

I doubt my cables will plug into the front ones, do you simply make extension cables?   Taking my time I want a good job done. 

@transparent do these boxes have to be further back do you think?

and what distance should I position the boxes for the ones in front 

Since you're positioning the boxes directly above joists, and then screwing them down, I can't see any reason why you'd need to position them further back from the garage door opening.

There's a minor/dubious infraction of the planning regulations.

If you were to exercise permitted development rights and build an outbuilding this close to the boundary with your neighbour, then it must be no more than 2.5m above finished ground level.
The box closest to that boundary might be above that limit, especially if the neighbour's garden is higher than yours to start with.

Under planning rules I would expect the fixed boxes to be regarded as part of the roof structure.
They can block light and cast shadows, which would be valid reasons for a complaint.

What's preventing you from moving these boxes closer to each other?

Are you concerned about overloading the weight capacity of the roof beams?

BTW, If I had an apprentice, then I'd never get anything done.

I'm a self-builder, still working through the 5-year renovation project which started when we moved to this Devon farmhouse a quarter of a century ago!

We just posted simultaneously.

If the boxes are to be bolted down, then they're going to need 100kg of ballast in addition.

Simply put large washers beneath the bolt heads to ensure they can't be pulled through the plastic base in a Force-10.

There's no problem with you making up MC4 extension cables.
But in general an electrical installation is more robust with fewer connections.

@transparent the panels are 190 long and thus the way I have spaced the boxes spreads the load over the boxes.  They are hammered down with big screws and square washers so won’t move  but not sure if I need an entire 200kg. It took several hours to get them up today, being old, on my own and the heat.  I was expecting to have to move them because of bind sheer but let’s hope nobody else says I have to 

 the position of them is hidden behind the neighbors garage so no loss of light for them. 

@transparent I stuck these under the boxes in the low sections off the roof so as not to crush the material over time