Designing new flat roofs with PV from the start

[low_and_there]

Hi all, I’ve done a bit of googling and forum searching on this topic and come up short, so hoping you can help us: we want to understand how best to factor in Solar PV to the design of our loft extension (mid-terrace house c.1905)

 

Subject to planning, the aim is to have one dormer on the rear roof slope and another on the outrigger roof; facing NW and W respectively. We’ve been advised we could get 6 panels for the main dormer and 3 on the rear. We will also seek permission to put one on the front elevation (all we can fit because of Velux and a gable), which gets the morning sun (SE facing).

 

We also have a tiny little strip of pitched roof to play with at the very rear, NW again and totally unshaded, which could host three smaller panels (200w).

 

I’ve so far had one person come to take a look, and when I asked about how we best plan this into the design of the dormers, they just said they’d use trays with ballasts for the flat roofs and mount the smaller panels atop the new roof at the rear, so no need to consider it in the design.

 

I find this a bit odd - presumably there is a flat roof material that makes more sense when fitting PV, or a way to build a frame for PV into the new roof during construction, and weight considerations we ought to plan for? 

 

I had thought about in-roof for the rear (a small pitched strip), to avoid paying for new roofing tiles, only to have them covered up by solar panels… I’ve been told in-roof is a bad idea as the panels need ventilation. But isn’t there anyway supposed to be a ventilated gap between the roof and the internal felting/insulation etc?

 

Also wonder about the angle - we could angle the roofs more (3 degrees is current design) to slope more towards the ideal pitch - but maybe this complicates the build and the trays with ballasts are just as good?

 

Suggestions appreciated.

 

 

 

 

[Gus Potter]

59 minutes ago, low_and_there said:

and weight considerations we ought to plan for? 

Good point.

 

Often if converting an attic you run into head room problems and you want to try an avoid having to strengthen / deepen rafters if you can.

 

PV panels have become much lighter as time has passed. SE wise you check the roof for the following:

 

1/ The self weight of the tiles / rafters and so on plus an access load of about 60kg per square metre.

2/ If a low pitched roof.. access plus wind loading.. sometimes critical if a big flat roof

3/ Snow loading plus roof self weight.

 

Now in many parts of the UK the snow loading is much smaller than the access load. And under normal conditions you would not expect folk to be standing/ placing scaffolding etc on your panels (this would be what we call an accidental loading case). Thus you can primarily design for the self weight of the roof plus the panel weight plus snow which could be less than self weight plus access and you may find that works to give you head room inside if you get stuck.

 

You could go further and go for an in system (if you can detail the ventilation ok) where you just have lightweight trays. Load is reduced further (no tile weight etc) and you save a bit on the tiling which offsets the cost of the trays.

 

The difference is small but if you are struggling for head room have a chat with your design team.

 

 

 

 

 

 

 

 

[Thorfun]

9 hours ago, low_and_there said:

I’ve been told in-roof is a bad idea as the panels need ventilation

this should not be an issue. if it is then I'm in big trouble!

 

 

as are many others on this forum as the in-roof trays are very popular if building a roof from scratch.

[SteamyTea]

Have you looked at how little generation you will get from the NW facing modules?

https://re.jrc.ec.europa.eu/pvg_tools/en/

[JohnMo]

The issue with ventilation is, to get full output you need some airflow at the rear for cooling.  GSE trays do include ventilation.  Even without cooling it is at max output when you may loose a few percent in performance, so 20 or 30 days a year, for a couple of hours a day if facing south.  Not something to worry about.

[low_and_there]

 

13 hours ago, SteamyTea said:

Have you looked at how little generation you will get from the NW facing modules?

https://re.jrc.ec.europa.eu/pvg_tools/en/

 

hi @SteamyTea I had looked at that simulator previously and noted it was lower, but still thought it may be worthwhile. I've just re-run the numbers, and pasted them below for review. Whaddya think?

 

Roof	Type	PVGIC Azimuth angle	Roof slope	Yearly PV energy production [kWh] - based on 1kWp per elevation	% of the 'optimum' angle
Front Roof	Pitched	-45 (SW)	35 degrees	1,061	93.34%
Main Dormer	Flat	-90 (W)	3 degrees	956	84.05%
Rear Dormer	Flat	0 (S)	3 degrees	985	86.61%
Kitchen rear	Pitched	135 (NW)	35 degrees	702	61.72%

 

 

[low_and_there]

22 hours ago, Gus Potter said:

You could go further and go for an in system (if you can detail the ventilation ok) where you just have lightweight trays. Load is reduced further (no tile weight etc) and you save a bit on the tiling which offsets the cost of the trays.

 

Thanks for your notes @Gus Potter, appreciate the breakdown of things to think about in terms of weight. In terms of the lightweight trays you refer to, are these something like the "GSE" trays that @JohnMo mentions? Are there any such items for flat roofs you could point me in the direction of?

[Gus Potter]

22 hours ago, low_and_there said:

Are there any such items for flat roofs you could point me in the direction of?

Not really but others may know more. The thing is that that any advantage gained by way of load reduction when retrofitting PV would be offset by the cost of trying to make a flat roof water tight if the carrier system for the PV needs fixed down to the underlying structure. Any leaks could mean the panels need to be lifted and that could be the start of a nightmare.

 

On a flat roof if the PV panels are to be pitched then the carrier system has some self weight.. which again reduces any benefit of the proposal I make above in terms of justifyable load reduction.

 

 

 

 

 

 

 

[low_and_there]

13 hours ago, Gus Potter said:

Not really but others may know more. The thing is that that any advantage gained by way of load reduction when retrofitting PV would be offset by the cost of trying to make a flat roof water tight if the carrier system for the PV needs fixed down to the underlying structure. Any leaks could mean the panels need to be lifted and that could be the start of a nightmare.

 

On a flat roof if the PV panels are to be pitched then the carrier system has some self weight.. which again reduces any benefit of the proposal I make above in terms of justifyable load reduction.

 

 

Thanks again for the explanations @Gus Potter, super helpful!

 

Based on this thread, I'm now thinking:

 

1) For the existing roof where we're (hopefully) not re-tiling, we'll go over the existing tiles - if we have to re-tile the roof then the in-roof trays are an option.

 

2) For the two new flat roofs we're creating, we need to factor in the additional weight considerations of the trays, ballasts and panels, along with the various loading scenarios that you outlined in your earlier post - that is the simplest and safest way rather than building in any frames or any screwing into the roof that could compromise weather tightness.

 

3) For the new pitched roof we're creating, we could go for in-roof trays so long as we ensure decent ventilation behind them.

[SteamyTea]

On 29/06/2022 at 22:05, low_and_there said:

Whaddya think

Sorry, missed this earlier.

Seems reasonable production.

Flat roof systems do perform quite well when cloudy (scattered light).

Did you take into account some slope to allow water to wash dirt off?

[Ronski]

Here's some info you may find useful.

 

https://www.singleply.co.uk/fixing-solar-panels-to-flat-roofs-why-we-recommend-an-engineered-solution/

 

https://www.nicholsonsts.com/products/rooftrak  Apparently these are about £50 each.

[low_and_there]

8 hours ago, SteamyTea said:

Did you take into account some slope to allow water to wash dirt off?

 

good point @SteamyTea, I recall reading something like 10 degrees is needed. For the projections, I just inserted the roof slopes angles, but I'll see what happens when I increase it to account for that.

 

And thanks @Ronski - have you got any experience / heard anything about these systems in use?

 

[Ronski]

No I haven't, I have seen similar used on the roof of a new local building for what I presume is safety cables, the building roof is level with the cliff top path (Royal Sands Ramsgate)

 

It was a local installer that told me they were about £50 each, I'm looking to put a system on a flat roof garage.

[NandM]

@low_and_there - what did you end up doing for your flat roofs?