Solar PV panels & roof weight

I am neither a roofer nor an engineer, so plz correct me if my figures are wrong!

I rang building control on the advice of my Tesco PV solar quote. Building control explained that solar fit was a strange one as the work was self certified but that the installers may not be qualified structurally. BC say that they get involved if the load changes by more than 15%.
The installers all say that weight is never an issue.
I have concrete tiles and I think that would be upto 53Kg m2, solar panels would add 13Kg / m2 (320Kg total). This is 24% increase in areas where panels are fitted. Suppose that you could average out over the whole roof (only one side has the sun and so the panels!) and/or over a side (not all of the side is fitted).
Q) Whats the min/max weight of my roof?
Q) Is the 15% over the whole roof or any one section?

1970's house, my roof looks like a W truss, 600mm centres, 70*40mm rafters and ceiling joists with interlocking loft boards loose on the ceiling joists. Width of the house is about 6.5m. Don't seem to have any supporting walls upstairs.

I understand that in addition to the roof weight I have wind loads etc on my side. From my windsurf days I remember that wind force is a square of speed so if my roof used to do 100mph then 25% would lower it too 90mph and so on.
Also my water tank must weigh more than 300Kg and thats just on two W sections ( has a couple of additional joists along the inner part of the W, guess to move the load away from the centre of unsupported ceiling joists).
My concern is however that come some big storm in 10 years time will the insurance companies have worked out a way to avoid paying because I fail some regulation?

Some simple suggestions of what and how to beef up the roof and likely costs so I can consider doing it as a 'no brainer' would be appreciated. Ideally solutions would not need to lift the loft boards and declutter the loft! I don't think that I have supporting walls upstairs either.
Also any thoughts on the true need.

 

I've never heard of solar panel installation needing any beefing up of the roof timbers, bear in mind that the load is spread quite well across the panel frame.

If you are that concerned you could get a structural surveyor to take a look, wouldn't think it would cost too much in comparison to the current costs

 

Thanks Surrey.
I wonder what the installer checks during inspection though & what the MCS regulations ask for. Wonder if there is a standard saying what to expect from each roof type.
Also in a roofer's experience is my style of roof standard or are the timbers smaller than usual/liked?
You're right the surveyor cost is peanuts compared to the install but it is still a few beers.

 

Having helped a local electrician installed around 40 pv installations, i will say he always employs a roofer[me] and a structural engineer as part of the package and we have beefed up the rafters on the majority of these roofs.

 

I'd second SRE comments..
Your roof construction was not uncommon.. Then.
I hope your panels are still running in 10 years....