Flat roof and solar

[LSB]

Our DIY new build has a flat roof (12 degrees).  This is huge at 24m * 9m and will be metal.  It's a barn conversion so we are limited to this design.

I'm being told that you can't have in-roof solar on flat roofs, is that really the case or have I just not found the right supplier.

Our planning does not allow us to go any higher than the top roof height, but also says we must have solar and we don't have space for ground mounted.

The roof is pointing south with nothing stopping the sun so is an idea location.

I don't want to cover the whole roof due to the size.

 

Thanks

 

[JohnMo]

1 hour ago, LSB said:

flat roof (12 degrees)

So not a flat roof (for insurance purposes).

 

You can have them on a 12 Deg roof, but winter performance will be utter rubbish. So I would tilt on stands to to a higher angle. Depending on width of roof the panel could go on the low side and still be below the ridge height and be tilted up.

[saveasteading]

12 degrees is far from flat. Not just for insurance but anything.

Most steel buildings I built were 6 degrees (10%). That required careful detailing and sealing.

12 degrees is fairly normal. steeper than that is very steep for metal.

 

So yes,  you have the wrong supplier. 

 

Do you really mean 'in-roof'. It is easier, safer  and cheaper to have it mounted on the surface.

[LSB]

17 hours ago, saveasteading said:

12 degrees is far from flat. Not just for insurance but anything.

Most steel buildings I built were 6 degrees (10%). That required careful detailing and sealing.

12 degrees is fairly normal. steeper than that is very steep for metal.

 

So yes,  you have the wrong supplier. 

Do you really mean 'in-roof'. It is easier, safer  and cheaper to have it mounted on the surface.

 

I appreciate it's easier to mount, but I don't want to pay for a huge metal roof to promptly cover it up with panels.

We are very limited to our planning, the difference in height between low and high is 2 blocks high so 430mm.

Needless to say the planning and SE just specify solar without any details about mounting etc.

 

 

 

[JohnMo]

GSE trays can go on a 12 Deg roof I believe, they would displace the steel roof where installed. You may need to clean the panels regularly as the self clean function wouldn't work well at 12 degs.

[JohnMo]

Datasheet confirms 12 Deg is ok

gse-in-roof-datasheet.pdf

[saveasteading]

Whether mounted over or inset, you will see solar panels, not metal roof.

As you imply it is expensive I assume it is standing seam. I would never, ever put any opening into standing seam: it's sole purpose is appearance and its main problem is sealing at joints and interfaces. 

I avoid openings in any metal roof but SS is an issue.

 

Hang on... 12 degrees is about 20% isn't it, or one in 5. So a 9m span has a rise of 1.8m, not 430mm. Am I getting confused?

 

[-rick-]

31 minutes ago, saveasteading said:

As you imply it is expensive I assume it is standing seam.

 

It's never been the look for me (though I like the look). The whole appeal of standing seam for me is (assuming it's continuous on a single surface) nowhere that can leak. All the joins overlap and crimp in place. Therefore, once you add junctions and weird shapes the appeal fades.

[LSB]

53 minutes ago, saveasteading said:

Whether mounted over or inset, you will see solar panels, not metal roof.

As you imply it is expensive I assume it is standing seam. I would never, ever put any opening into standing seam: it's sole purpose is appearance and its main problem is sealing at joints and interfaces. 

I avoid openings in any metal roof but SS is an issue.

 

Hang on... 12 degrees is about 20% isn't it, or one in 5. So a 9m span has a rise of 1.8m, not 430mm. Am I getting confused?

 

It's probably me getting confused.   I took what I was told by roofing bod, but maybe I misunderstood.  Back to algebra to work it out properly.

[Mr Punter]

We put standing seam on a roof and the customer had solar added.  The were fixed by brackets that clamped onto the standing seams.  The roof actually had a slight fall to north but it still worked fine.  The standing seam was Kalzip type.

[saveasteading]

34 minutes ago, -rick- said:

All the joins overlap and crimp in place.

Except at the ridge. You know not to put anything through it, but lots of people don't, including Architects. I only have horror stories and dire warnings to go on , as I never did one. We had it in our portfolio of options, and no client ever chose it, based on cost and the warnings about function. We also offered it with sliding fixings to allow for movement as otherwise it can ping.

 

17 minutes ago, LSB said:

Back to algebra to work it out properly.

Yes do it yourself. Forget degrees and stick to the more useful horizontal length , height difference and slope distance.  you can do this yourself with a tape and a long stick.

 

430mm height difference over 9m ( 1 in 20 or so) would indeed be effectively  'flat' and not a great idea in metal cladding.

Come back when it's worked out.

[JohnMo]

5 minutes ago, saveasteading said:

1 in 20

Google says 2.86 degs, long way off 12 degs.

[RobLe]

It’s important not to have panels too horizontal - I think 10 degrees is the typical minimum - in order water drains off the edge and doesn’t pool, reducing performance as dirt is left behind.  Maybe some glass only bifacial panels avoid the slight edge lip so allow for shallower angles.

[Gone West]

17 hours ago, LSB said:

Our DIY new build has a flat roof (12 degrees).

A roof is classed as a flat roof up to 10 degrees. I am currently working on a small area of roof on my place and although it is 11 degrees, I'm treating it as a flat roof, for covering material.

 

https://ikogroup.co.uk/news-advice/what-is-flat-roof-compared-to-pitched-roof/

 

https://en.wikipedia.org/wiki/Flat_roof

 

[Gone West]

13 hours ago, saveasteading said:

Hang on... 12 degrees is about 20% isn't it, or one in 5.

 

Gradient Degrees to Percentage.pdf

 

[LSB]

This is replacing the previous barn roof which was in situ since built in the 60's.

That was metal and drained fine, albeit lots of water when it rains due to the size.

Very much flat.

It is being replaced with one roof covering both sections seen below.

In this picture we have already removed gutters and part of the lower roof.

It won't be this profile of sheets.

It is facing south so I was hoping to put solar on top.

the dead trees are long gone so nothing to stop light.

 

 

 

Picture taken from other end, but this is what it looks like now.

 

 

Edited 10 hours ago by LSB

[LSB]

Better picture of the roof being replaced.

 

 

Apart from it becoming one roof our planning only allows this height so we can't raise the higher side for angle or reduce the lower side as limited head room inside.

[saveasteading]

OK so it is rather flat. Metal will have to be very well detailed and built as water will drive uphill in strong wind.

That needs serious thought and close supervision.

Screwed panels are OK, despite what some people say IF done well with top quality screws Hilti etc.

I've known more problems with felt/ fibreglass as it's often done badly. 

[saveasteading]

1 hour ago, Gone West said:

A roof is classed as a flat roof up to 10 degrees

Where does that definition come from?

 

I must have done 250,000m2 at 10% using steel.

It didn't feel flat when walking on it.

[Gone West]

2 hours ago, saveasteading said:

Where does that definition come from?

From the links I put up and from the roofers who put the Sika membrane on my extension.

With all due respect, I don't think how it feels when walking on it, determines how it's classed.

[saveasteading]

1 hour ago, Gone West said:

how it feels when walking on it determines how it is classed

It does to me. If it feels steep then the rain runs freely down it, and doesn't drive far up in wind.

If it feels flat then water sits, especially at any dips and laps.

It depends of course on what material and on workmanship, and it is sensible to assume a roof may not all be as designed.

I've looked at your links now: thanks for back referring. I see they are one manufacturer's definition for their cold applied product. Maybe they need it.

But to me 10° is steep. That's 18% or 1.8m over a 10m width. The tools and materials are in danger of sliding down that on the metal roof of the original question. I've seen drills sliding down that sort of slope.

 

On a road i think 'steep' is defined as about 10% and a pedestrian ramp at 5%.

I do prefer % as the sums are easier.

 

 

 

[ADLIan]

Flat roof definition of up to 10 degrees is in a number of British Standards on roofing.

[Tetrarch]

23 hours ago, JohnMo said:

So not a flat roof (for insurance purposes).

 

You can have them on a 12 Deg roof, but winter performance will be utter rubbish. So I would tilt on stands to to a higher angle. Depending on width of roof the panel could go on the low side and still be below the ridge height and be tilted up.

 

To put things in context

 

From the calculator on https://solarenergyconcepts.co.uk/post/panels-tilt/ for a notional 10kw system

 

If optimal 36 deg for South East gives 1,026 kwh

 

Then 15 deg gives 963 kwh and zero deg gives 940kwh

 

My understanding (from a previous insurance question) is that flat is defined as anything less than 15-deg

 

Regards

 

Tet

[JohnMo]

37 minutes ago, Tetrarch said:

1,026 kwh

1026 kWh are you sure, that is correct - I have 9-11kW (added another 1840W just recently) of panels and 7kW of inverter in NE Scotland, so far this year at nearly 4000kWh. Plus most my panels are vertical, so optimised for winter production not summer.

[saveasteading]

In summary then.

Flatness is in the perception of the user specifier

For solar we want it to tilt a long way up.

To keep rain out it depends entirely on the construction and product.

For insurance, Insurers make their own rules without understanding construction. I've certainly never declared anything at 10 to 15%.

 

In real life, I regard my walk to the shop as far from flat.

 

It's been interesting. For fun I asked AI how steep Ben Nevis is  (not direct via the cliffs)

 

The average gradient of the standard tourist route (the Ben Nevis Mountain Track) is approximately 10% to 15% (or roughly 1 in 10 to 1 in 7 overall), with a maximum gradient capped at 1 in 5 (20%). [1, 2]

Route Gradient Details

• Overall Average: Roughly 10% to 15% grade across the entire ~8 km horizontal distance (1,345 m total ascent). 

• Maximum Gradient: 1 in 5 (20%), which was specifically engineered when the path was originally built for ponies supplying the top observatory. 

• Steepest Sections: Parts of the lower slopes and the upper zig-zags approach that maximum 1 in 5 limit. 

So, @LSB back to the original post: from the pictures it looks as if your estimate of 440mm or whatever is about right. So you have 2 challenges. 1. You have to detail and build the roof very well to keep the rain out, esp at edges and interfaces.

2. That is very flat and inefficient for solar so maybe play the game, using the newly allowed balcony solar. 

Later you might go back  to planning for a higher system. A few hundred mm would likely be allowed.

 

in fact I'd want to reclad with double skin, insulated. That will be higher but nobody will question it unless you have an enemy with binoculars.