Posted by: @david999

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does the inverter have to be configured for these extra panels.

I doubt it. Both of the MPPT inputs will be ready to accept panels.

Posted by: @david999

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Do the panels have to match the specs of the ones already on the other string.

No. The MPPT inputs operate independently.

Provided each PV string outputs a voltage between 120v – 550v and less than 13.5A, the inverter can't tell what sort of panels you've actually got.

Posted by: @david999

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do I have to upgrade any fuses.

No.... although I'm unsure where you might already have fuses for the existing installation.

Personally I connect my (four) strings of PV panels using fuses, DC-trips and a DC rotary isolator.
But it's rare for others to have all three types of protective isolation in series.

Posted by: @david999

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going to use boxes since it’s a flat roof and weigh them down, do they need screwed down as well.

Imagine a solar panel is like a sail. It will be affected by wind-shear forces.

Some people place concrete blocks in their mounting boxes which adds 19kg per block.
Others fill boxes with water, which adds 1kg per litre.

Yes you could decide to screw the boxes into the flat roof.
It depends on whether than roof is attached to the walls below using roof-straps.

If not, then a winter storm will happily carry away the entire roof!

Posted by: @david999

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should I use 4mm or 6mm cable

4mm cross-sectional diameter will happily carry current in excess of the 13.5A maximum input allowed by your inverter.

Posted by: @david999

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The panels will be configured two panels and three in front. Will I have to make up an extension lead to join the forward ones to the rearmost ones.

That will be fine.

Just make sure you properly fit the MC4 connectors.

I prefer to use a type that uses a crimp tool on the conductor. You want to achieve as low a contact resistance as possible, so they don't overheat.

Those made by Amp (Amphenol) are compatible to join with those from other manufacturers, but the plastic spanners you use to do them up are different.

Posted by: @david999

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any other tips appreciated, I’m an engineer so electrically trained but not in solar.

Take photos of progress.
You may need to post them here if you have further questions.

Be careful that the cables don't rub against anything, especially at the point where they go over the edge of the roof.
I've use a 3D printer to make clips which secure cables just above my plastic guttering.
That avoids abrasion from the edges of my slates.

@transparent the roof joists are 3 inches by 9inches and not strapped to the walls but built slightly into the brick.  They quote 100kg per box to secure them and not sure how much wind it would take to move that if they are fitted landscape. It’s a double garage so the joists span 14 foot spaced at 600mm centres. The garage isn’t too exposed

OK. That garage roof looks corrugated to me.

I'm not concerned by the size of the roof joists themselves. 225x75mm seems over-spec for such a lightweight roof, even at 600mm spacing.
The SPAN-tables from Part-A of the Building Regs would easily permit 5x 100kg+ load spread across the roof.

But I don't see how you could add such weight to a corrugated substrate.
It's probably not intended to support that mass.
What material is it?

There are twisted roof straps which you could use to tie the joists to a concrete wall.
But if those walls are made from relatively thin steel reinforced concrete panels then you may need to drill through the entire panel and use through-bolts.
There may not be sufficient thickness to take an expanding wall plug with a screw.

An alternative approach is to use aluminium fixing rails which you screw through the corrugated roof into the joists below.

You would be migrating from a weight-based solution to one which uses screw fixings and lightweight framing:

To spread the weight (and lift from wind-shear) across multiple joists, you may wish to fabricate your own aluminium frame rather than buy off-the shelf adjustable angles.

I've used C-section channel lengths in some places. It can accept t-nuts to hold panels in place.

PS. Can I avoid comment on the pipe insulation for your solar-thermal panel at the moment?
That might benefit from being a separate topic.

@transparent it’s insulation on top of insulation to mitigate heat loss. Doesn’t look great but works.  How bad is it transparent, you are an expert, don’t hold back

Your pipe lagging efficiency depends on the thickness of dry insulation, which traps air within closed cells in the foam.

If water can get in

... or you compress the thickness with nylon tie wraps

... or the cells become 'open' to the atmosphere through damage from UV radiation

then it's compromised.

@transparent Its 10mm pipe in there so well insulated.  I’m thinking on your comment on that box section steel roof. Although it’s mounted on plywood it may be an idea to place something in the hollows to spread the load off the peaks.   Do these tubs tend to move with that weight in them and fitted landscape ?

Thanks for confirming that the roof is steel box-section and supported by plywood.
That makes a difference.

We obviously haven't got enough data to assess whether weighted tubs will move, cause flexing or damage.

For that we'd need calculations based on thickness of steel and plywood, plus knowing what weight you envisage adding to each tub.

What happens if you walk on the roof? Does it flex?

It you were to jump and then land on both feet, is it likely the steel would be dented?

It's not just the possibility of a PV tub moving horizontally. But it is also going to bounce slightly in the vertical direction due to wind.

@david999 I have four panels on a flat roof and those mountings are a Schletter design in aluminium. I was to have ‘buckets for the 10 ground mount panels in the back garden and these would have been weighted down with gravel or similar. Due to post Covid 19 shortages, these were substituted with Van der Valk bolt together ali frames and weighted slabs - they have not shifted at all. The flat roof installation was subject to a survey of the construction and the fixtures were bolted through into the roof beams and waterproof components were used to seal the whole caboodle. Regards, Toodles.

@transparent the box section can be dented if you walk on the roof so as mentioned likely need to fix spacers so the tubs sit on the hollow and thus the plywood and joist directly.  I think they would squash the box section at 100kg per tub. The other issue is them sliding on the smooth section so likely screwed down as well.   But I’m begining to see doom as in my roof getting ripped off.

Posted by: @david999

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But I’m begining to see doom as in my roof getting ripped off.

Indeed. There's lots of people get ripped off when installing solar panels!  😋 

You're caught between two conflicting concepts.

Either you use too little weight and risk the tubs moving,
or you use too much weight and end up damaging the roof.

If I was in your situation, I'd be looking more seriously at lightweight aluminium frames.

@transparent need to look at that and perhaps fit them flat