Roof design for solar PV

ed110220

Hi all,

I'm coming at this from a slightly unusual angle in that our roof covering needs replacing in the near to medium term and I'd like to combine this with solar PV so I may need to make a few design choices rather than just trying to fit something on once it is done.

At the moment the roof is covered in the original 1948/9 corrugated asbestos cement sheets. Last year they got slightly cracked in repairing a leak to the chimney flue, which was probably inevitable even with proper equipment given their age and fragility, so need replacing as they have a slight leak.

The house is semi-detached and it's a simple pitched roof (gable, no hip etc) of 22 degrees, back of the house faces SSW (10 degree azimuth from South). Each roof 'face' is a minimum of 6.97 m long and 4.13 m from gutters to ridge (measured internally, so probably a little more externally).

The main choice is about the chimney which is on the SSW face but up against the ridge, reducing the usable distance from gutter to chimney to about 3.7 m. There is also a soil vent pipe near one corner. The chimney is used by by a back boiler which needs replacing and possibly re-siting. It is not a chimney in the traditional sense, it is just a galvanised steel box through which the flue pipe passes.

So the main question is when replacing the boiler, does the flue need re-siting to give a better usable area? My original idea was keep it in situ as there is room in the chimney box for the soil vent pipe to exit next to the flue.

A few doors down in an identical house they have installed 8 panels facing SW and 8 facing NW, but from the look of it they should have been able to fit more on the optimum side even without moving the vent.

Ed

ed110220

Here's a photo, I didn't take it for the roof but for the rainbow so you can see more of the neighbours' identical roof (conifer will probably have to go).

Ed

Ectophile

Unless you want to make your entire roof out of solar panels, I don't see either the chimney or the soil stack being a problem. The chimney should not be casting any shadows over the panels - unlike the conifers.

On my SSE facing roof, the panels are in a block in the middle, with the ends of the roof free - including the end where the soil stack is.

It's worth getting an installer to have a look at it. They will tell you how much of the roof needs covering in panels. If you go for too many, you'll get more generation but you'd be over the limit for the domestic Feed In tariff.

ed110220

Ectophile wrote: »

Unless you want to make your entire roof out of solar panels, I don't see either the chimney or the soil stack being a problem. The chimney should not be casting any shadows over the panels - unlike the conifers.

On my SSE facing roof, the panels are in a block in the middle, with the ends of the roof free - including the end where the soil stack is.

It's worth getting an installer to have a look at it. They will tell you how much of the roof needs covering in panels. If you go for too many, you'll get more generation but you'd be over the limit for the domestic Feed In tariff.

Hi, thanks for the reply. I think the roof looks as though it will fit more panels than it really can and it will not be able to get up to the full 4 kW with the ordinary 250 W panels on the SSW face. Moving the chimney would only be to increase the usable area if necessary as it doesn't cause any shading.

I had a quote a while ago for a 2.75 kW system of eleven 250 W panels arranged landscape in a three rows (the 'missing' one where the vent is). They produced this array plan but I wasn't that impressed with their measurements as they did them all at ground level, even though the roof has a significant overhang at the gable and eaves.

Ed

Martyn1981

Hi Ed. Been looking and pondering at your images. I think you might be better off going to higher power panels, than considering the north roof, as you're so close to south.

If that image is correct and too scale, then (by eye) I suspect useable height might be 3.2m, as the pic shows 3 1m panels, and a little bit of space above and below, compared to the gaps on the two sides. That should allow 2 panels height wise of roughly 1.6m each.

Width wise, 4 panels should be about 6.4m in landscape, perhaps 6.6m for Hyundais.

So looks like you have room for 4*3 or 6*2 panels. I'm assuming the soil pipe will get moved, or cut back into the roof, with an AAV added.

Options, as per the image, 12 regular panels, or 12 high efficiency panels, or 16 smaller high efficiency panels.

Using Swithenbanks retail prices purely for comparisons, I get:
YingLi 250Wp black monos at £156 * 12 = £1,872
(for fairer comparison to 4kWp) £156 * 16 = £2,496
BenQ 325Wp higher efficiency £313 * 12 = £3,756 (+£1,250)
Panasonic 240Wp high efficiency £209 * 16 = £3,344 (+£850)

I think you can get the Panasonics on in a 8*2 pattern, as they are only 798mm wide, so total width 8*798mm + 7*20mm = 6.524m.

You could of course use the north roof instead for approx 1kWp of panels, but as it's nearly due north and shallow pitch, I'd expect it generate at roughly the same time as the south roof (so no wider generation, apart from a few scraps very early/late in peak summer). So you'd need to compare overall generation and income loss from the north roof, to the additional investment of the higher powered south roof option.

Last thought, will you being having a more conventional roofing surface as a replacement? How does the weight of the old v's new roof stack up? Will any extra reinforcing be needed / be sensible at the time to allow for PV? [I'm making this bit up, just thoughts, I have no idea how this compares.]

Mart.

ed110220

Hi Mart, traditional roofing such as tiles is out of the question due to the roof structure only being designed for lightweight sheet materials so it will have to be a modern lightweight replacement. This should be more trouble free than tiles as it comes in larger sheets and the fewer joins the better in my opinion.

I hadn't actually thought of the smaller Panasonic panels as an option.

Ed

EricMears

ed110220 wrote: »

. . .roof structure only being designed for lightweight sheet materials so it will have to be a modern lightweight replacement.

Your next question ought to be : "Is the roof going to be strong enough to support any solar panels ?"

OTOH, providing the attic space is empty (and the picture doesn't seem to show any attic conversion) it's not that big a job to strengthen the existing roof timbers with some additional ones alongside them in which case roof could be made suitable for (say) concrete tiles and solar panels.

ed110220

EricMears wrote: »

Your next question ought to be : "Is the roof going to be strong enough to support any solar panels ?"

OTOH, providing the attic space is empty (and the picture doesn't seem to show any attic conversion) it's not that big a job to strengthen the existing roof timbers with some additional ones alongside them in which case roof could be made suitable for (say) concrete tiles and solar panels.

Hi, I assume it will be strong enough as quite a few British Iron and Steel Federation houses have solar panels fitted, including one just down the road from me. Obviously this would need to be checked.

These houses are always reroofed with a lightweight coated steel system, either in imitation tiles such as Decra or some sort of profiled sheet as the roof is a simple steel structure that is not designed for tiles. Unlike a traditional house there is no timber in the roof and the tubular steel trusses transmit all the load to the perimeter walls (there are no load-baring internal walls upstairs as there would be in a traditional house). It's not a problem though as there is no need for a tiled roof.

If strengthening is needed, there is access to the loft. It is only used for storage as it is not tall enough at the ridge to stand up in due to the shallow pitch.

Ed

WestonDave

Ed - bit of a left field suggestion and given your roof structure you'd need to look into it but there are panels now which replace the traditional slates or concrete tiles. There's a new build house in Longwell Green (just beyond the derelict petrol station on the left as you head towards the trading estate where B&Q is etc, that has them in its roof and you can barely see they are there! They may be lighter than concrete tiles and light enough to be used on your roof.

I also wonder whether retaining the existing sub strength structure is worth it in the long run or whether a new timber structure would be worth paying for now. I guess it depends how good the metal structure is and whether you can get something suitable to recover it with. Stick some copper sheets on it and let it go green like Snow Hill!

ed110220

WestonDave wrote: »

Ed - bit of a left field suggestion and given your roof structure you'd need to look into it but there are panels now which replace the traditional slates or concrete tiles. There's a new build house in Longwell Green (just beyond the derelict petrol station on the left as you head towards the trading estate where B&Q is etc, that has them in its roof and you can barely see they are there! They may be lighter than concrete tiles and light enough to be used on your roof.

I also wonder whether retaining the existing sub strength structure is worth it in the long run or whether a new timber structure would be worth paying for now. I guess it depends how good the metal structure is and whether you can get something suitable to recover it with. Stick some copper sheets on it and let it go green like Snow Hill!

Hi Dave, I know the house you mean in Longwell Green I think - if it's the one I'm thinking of they look like slates.

On the roofing side it shouldn't be a problem as there are about thirty thousand houses like this and probably at least half have been reroofed. Certainly all those that are owned by housing associations have had the roof covering replaced, so it is a well established method. Basically the steel structure is fine, but the purlins are far apart because the corrugated asbestos has a high strength to weight ratio. To replace it timber rafters are placed on top of the steel purlins and these hold the new roof covering, so the steel structure still bares the weight, as I don't think a timber structure spanning 6.5m unsupported would be practical.

Ed

Martyn1981

ed110220 wrote: »

Hi Mart, traditional roofing such as tiles is out of the question due to the roof structure only being designed for lightweight sheet materials so it will have to be a modern lightweight replacement. This should be more trouble free than tiles as it comes in larger sheets and the fewer joins the better in my opinion.

I hadn't actually thought of the smaller Panasonic panels as an option.

Ed

Hiya Ed. This is still going round in my head, and I'm having second thoughts. My concern about mixing S&N panels, was that with high summer sun on such a shallow pitch, the north roof would generate at the same time as the south, so going large could mean capping.

But, even with the sun shining over the ridge onto the N panels, I doubt you'd hit 3.68kW assuming S panels at 80% and N panels at 50%.

Also, if I'm even close on the extra costs of those higher efficiency panels then you'd have around £1k extra to spend on regular panels, allowing 5kWp in total - 3kWp South + 2kWp North.

I've lost track of where you are, but for my house in Cardiff, using a 22d roof pitch and SSW (+10d) and NNE (-170d) I get the following:

SSW only 4kWp * 943kWh = 3,772kWh pa

SSW, 3kWp * 943kWh = 2,829kWh pa +
NNE, 2kWp * 639kWh = 1,278kWh pa = 4,107kWh

FiT would be 10% less, which equates to 4,107 * 0.9 = 3,696 (comparable?), whilst export and leccy savings aren't reduced.

The big question is, would there be any/much capping during peak generation, and I suspect not (I was wrong in earlier post).

Actually, there is a second big question, can the roof structure support the extra PV.

Assuming I'm right and that capping would be none to minimal, then you'd also have other options, such as 3kWp on the north, leave the soil pipe alone and shift an extra panel to the north, use 260Wp to 275Wp panels for an additional boost etc etc etc.

Mart.