I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm putting
together. It'd be an 'extensive' type with about 6 inches of growing medium
(gravel base - weed control membrane -compost/soil).

As ever, too much advice on the web, and it's more guidance on the materials
and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've got
anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed drainage to
water butt.

About right?!

--
Cheers, Rob, Sheffield UK

I'd have thought the problem was the weight of the wet soil and plants on
the poor old shed, unless you are going to use concrete posts all around it
to take the weight, and something very strong as the base to stop it sagging
in the middle.
Brian

--

--:
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
briang1@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"RJH" <patchmoney@gmx.com> wrote in message
news:tj59u8$1k07d$1@dont-email.me...
> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm
> putting
> together. It'd be an 'extensive' type with about 6 inches of growing
> medium
> (gravel base - weed control membrane -compost/soil).
>
> As ever, too much advice on the web, and it's more guidance on the
> materials
> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've
> got
> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed
> drainage to
> water butt.
>
> About right?!
>
> --
> Cheers, Rob, Sheffield UK

Brian Gaff <brian1gaff@gmail.com> wrote:
> I'd have thought the problem was the weight of the wet soil and plants on
> the poor old shed, unless you are going to use concrete posts all around it
> to take the weight, and something very strong as the base to stop it sagging
> in the middle.
> Brian
>

I have seen a few of these around on extensions and frankly they all seem
to end of as a mossy mess and for most of the time looking anything like
living or green

Richard

Brian Gaff <brian1gaff@gmail.com> wrote:
> I'd have thought the problem was the weight of the wet soil and plants on
> the poor old shed, unless you are going to use concrete posts all around it
> to take the weight, and something very strong as the base to stop it sagging
> in the middle.

Yes. Let's say the roof holds just water. Water weighs 1 tonne per cubic
metre, so a 6" depth would weigh 150kg per square metre. Then there's
gravel and soil up there as well. Density of rock is 2-3 tonnes per cubic
metre, so let's guestimate 2 (there will be pores filled with water, and
plant material has density <1). So we need to design for 300kg/m2 + margin.
And then if somebody might be standing on it they might weigh 150kg, so
that's SWL 450kg/m2 (which probably covers the margin).

If the roof is about 5m2 that needs to hold 2.5 tonnes, not including the
weight of the roof structure itself.

You can build a deck to handle that (do the beam loading calcs - probably
need to space the joists quite close together) but if this is a timber shed
I would question how strong the walls are?

(Without doing the calcs I think you can do it with timber framing, but
would need chunky timber posts not typical garden shed battens)

One resource that might be worth trying is 'snow loading' calculators,
because this is roughly the same problem as having a load of snow dumped on
the roof in the winter. And that's something many roofs have to be designed
to cope with.

Theo

On 24/10/2022 07:04, RJH wrote:
> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm putting
> together. It'd be an 'extensive' type with about 6 inches of growing medium
> (gravel base - weed control membrane -compost/soil).

Unless it is very very strong your shed will fall down then.
Do the load calculations! This isn't a bad guide:

https://www.rhs.org.uk/garden-features/green-roofs

You have to use the lightest weight growing medium possible and as
little of it as you can get away with and still have plants grow.
>
> As ever, too much advice on the web, and it's more guidance on the materials
> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've got
> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed drainage to
> water butt.
>
> About right?!

More or less perfectly wrong.
(unless your shed roof is incredibly well reinforced)

The living roof plants that will work reliably tend to be semi arid
desert or alpines that need very little soil ~1" and will grow in pumice
or perlite/perlag with a trace of topsoil or humus.

Sempervivums, sedums and that sort of thing.

Beware that some of them will set seed and will also colonise your
gutters and house roof in addition to where you want it to grow. eg.

https://www.etsy.com/uk/listing/823022026/sedum-alba-white-stonecrop-drought

Others like sedum acre are less invasive.

https://www.etsy.com/uk/listing/1244303447/mossy-stonecrop-goldmoss-sedum-biting

--
Regards,
Martin Brown

In message <tj5jb2$1kpk8$1@dont-email.me>, Tricky Dicky
<tricky.dicky@sky.com> writes
>Brian Gaff <brian1gaff@gmail.com> wrote:
>> I'd have thought the problem was the weight of the wet soil and plants on
>> the poor old shed, unless you are going to use concrete posts all around it
>> to take the weight, and something very strong as the base to stop it sagging
>> in the middle.
>> Brian
>>
>
>I have seen a few of these around on extensions and frankly they all seem
>to end of as a mossy mess and for most of the time looking anything like
>living or green

Adnams bottling building at Southwold has this. Never seen it anything
other than khaki/brown:-(

--
Tim Lamb

On Mon, 24 Oct 2022 06:04:56 -0000 (UTC), RJH <patchmoney@gmx.com>
wrote:

>I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm putting
>together. It'd be an 'extensive' type with about 6 inches of growing medium
>(gravel base - weed control membrane -compost/soil).
>
>As ever, too much advice on the web, and it's more guidance on the materials
>and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
>15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've got
>anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed drainage to
>water butt.
>
>About right?!

As many others have said, they are heavy! After a few months, weeds
and grasses start to germinate from seed blown in or shitted out by
birds, and they start to look a mess and just get worse. In very dry
weather they go brown and look dead. A new-build near my previous
bungalow had one. Looked fine for a few months when new but then just
deteriorated into a weed patch and looked awful.

Good luck.

--
Chris

On 24 Oct 2022 at 10:28:53 BST, Martin Brown wrote:

> On 24/10/2022 07:04, RJH wrote:
>> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm putting
>> together. It'd be an 'extensive' type with about 6 inches of growing medium
>> (gravel base - weed control membrane -compost/soil).
>
> Unless it is very very strong your shed will fall down then.
> Do the load calculations! This isn't a bad guide:
>
> https://www.rhs.org.uk/garden-features/green-roofs
>
> You have to use the lightest weight growing medium possible and as
> little of it as you can get away with and still have plants grow.

Thanks - I'll do some calculations and see what comes up. The shed itself will
be strong in compression - it's more the 'foundations' that'll be the unknown
(at the moment large bits of postcrete left on the site).

>>
>> As ever, too much advice on the web, and it's more guidance on the materials
>> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
>> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've got
>> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed drainage to
>> water butt.
>>
>> About right?!
>
> More or less perfectly wrong.
> (unless your shed roof is incredibly well reinforced)
>
> The living roof plants that will work reliably tend to be semi arid
> desert or alpines that need very little soil ~1" and will grow in pumice
> or perlite/perlag with a trace of topsoil or humus.
>
> Sempervivums, sedums and that sort of thing.
>
> Beware that some of them will set seed and will also colonise your
> gutters and house roof in addition to where you want it to grow. eg.
>

This is a shed roof on an allotment.

> https://www.etsy.com/uk/listing/823022026/sedum-alba-white-stonecrop-drought
>
> Others like sedum acre are less invasive.
>
> https://www.etsy.com/uk/listing/1244303447/mossy-stonecrop-goldmoss-sedum-biting

Thanks - I'll take a look

--
Cheers, Rob, Sheffield UK

On 24 Oct 2022 at 11:16:14 BST, Chris Hogg wrote:

> On Mon, 24 Oct 2022 06:04:56 -0000 (UTC), RJH <patchmoney@gmx.com>
> wrote:
>
>> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm putting
>> together. It'd be an 'extensive' type with about 6 inches of growing medium
>> (gravel base - weed control membrane -compost/soil).
>>
>> As ever, too much advice on the web, and it's more guidance on the materials
>> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
>> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've got
>> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed drainage to
>> water butt.
>>
>> About right?!
>
> As many others have said, they are heavy! After a few months, weeds
> and grasses start to germinate from seed blown in or shitted out by
> birds, and they start to look a mess and just get worse. In very dry
> weather they go brown and look dead. A new-build near my previous
> bungalow had one. Looked fine for a few months when new but then just
> deteriorated into a weed patch and looked awful.
>
> Good luck.

Thanks to you and others - mild rethink in progress! But I still want to give
it a go . . .
--
Cheers, Rob, Sheffield UK

On 24/10/2022 11:16, Chris Hogg wrote:
> On Mon, 24 Oct 2022 06:04:56 -0000 (UTC), RJH <patchmoney@gmx.com>
> wrote:
>
>> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm putting
>> together. It'd be an 'extensive' type with about 6 inches of growing medium
>> (gravel base - weed control membrane -compost/soil).
>>
>> As ever, too much advice on the web, and it's more guidance on the materials
>> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
>> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've got
>> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed drainage to
>> water butt.
>>
>> About right?!
>
> As many others have said, they are heavy! After a few months, weeds
> and grasses start to germinate from seed blown in or shitted out by
> birds, and they start to look a mess and just get worse. In very dry
> weather they go brown and look dead. A new-build near my previous
> bungalow had one. Looked fine for a few months when new but then just
> deteriorated into a weed patch and looked awful.

Some of the plants I have suggested can survive and remain green in the
toughest of conditions (but they are potentially a bit invasive).

The house leeks aka sempervivums will thrive on a living roof as will
quite a few of the hardy succulent sedums (but some set too much seed!).

Calendula will grow OK up there. But then so will many unwanted weeds.

--
Regards,
Martin Brown

On 24/10/2022 09:45, Tricky Dicky wrote:
> Brian Gaff <brian1gaff@gmail.com> wrote:
>> I'd have thought the problem was the weight of the wet soil and plants on
>> the poor old shed, unless you are going to use concrete posts all around it
>> to take the weight, and something very strong as the base to stop it sagging
>> in the middle.
>> Brian
>>
>
> I have seen a few of these around on extensions and frankly they all seem
> to end of as a mossy mess and for most of the time looking anything like
> living or green
>
> Richard
>

I've yet to see a living roof live up to all the hype. The local council
built a "green" building in one of the local parks with one of the
living roofs. The problem here in Essex is possibly the lack of rain, or
more importantly enough rain in a timely manner. We had no rain for
around 3 months which included the record temperature summer days. The
roof on the building isn't green and now that we have had some rain
seems to be germinated blown in weed seeds.

As for Brains comments, it is a factor to be considered even on a
traditionally build house roof or balcony. It's the weight of a fully
saturated living roof bed that needs to be considered and not just the
weight of what you put up there.
--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On 24/10/2022 13:22, RJH wrote:
> On 24 Oct 2022 at 11:16:14 BST, Chris Hogg wrote:
>
>> On Mon, 24 Oct 2022 06:04:56 -0000 (UTC), RJH <patchmoney@gmx.com>
>> wrote:
>>
>>> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm putting
>>> together. It'd be an 'extensive' type with about 6 inches of growing medium
>>> (gravel base - weed control membrane -compost/soil).
>>>
>>> As ever, too much advice on the web, and it's more guidance on the materials
>>> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
>>> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've got
>>> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed drainage to
>>> water butt.
>>>
>>> About right?!
>>
>> As many others have said, they are heavy! After a few months, weeds
>> and grasses start to germinate from seed blown in or shitted out by
>> birds, and they start to look a mess and just get worse. In very dry
>> weather they go brown and look dead. A new-build near my previous
>> bungalow had one. Looked fine for a few months when new but then just
>> deteriorated into a weed patch and looked awful.
>>
>> Good luck.
>
> Thanks to you and others - mild rethink in progress! But I still want to give
> it a go . . .

Think bunker. make the shed out of a concrete or a brick arch

Then just leave it.

Weeds will arrive

And then you will be green and happy

--
"I am inclined to tell the truth and dislike people who lie consistently.
This makes me unfit for the company of people of a Left persuasion, and
all women"

The Natural Philosopher <tnp@invalid.invalid> wrote:
> Think bunker. make the shed out of a concrete or a brick arch
>
> Then just leave it.
>
> Weeds will arrive
>
> And then you will be green and happy

I had a green flat roof. Half an inch of gravel that hadn't been touched
for years. It was enough to collect windblown soil and moss to grow. Of
course the moss absolutely loved the rain in the winter. In summer it dried
out, given the lack of rain.

Roof loadings weren't too bad, and it reduced gutter overflows in heavy
downpours.

You don't want to do this if you have leaky felt, though.

Theo

On 24/10/2022 17:42, The Natural Philosopher wrote:

>>
>> Thanks to you and others - mild rethink in progress! But I still want
>> to give
>> it a go . . .
>
> Think bunker. make the shed out of  a concrete or a brick arch
>
> Then just leave it.
>
> Weeds will arrive
>
> And then you will be green and happy
>

Question for which I don't have a definitive answer...

Could something like a living roof that may remain wet/damp (or frozen)
for months in the winter actually cause condensation on the underside of
a un-insulated wooden shed roof?

I occasionally see some of those make-over garden shows and wonder how
long some of that wooden decking, replacement fencing and pergolas are
going to last - especially when seeing cut timber being inserted into
holes and after the postcrete being backfilled with earth. As for earth
retaining walls and raised being built of softwood sleepers....

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

alan_m <junk@admac.myzen.co.uk> wrote:
> Question for which I don't have a definitive answer...
>
> Could something like a living roof that may remain wet/damp (or frozen)
> for months in the winter actually cause condensation on the underside of
> a un-insulated wooden shed roof?

Possibly. It would depend on a source of warm air from inside the shed.
Warm moist air hitting the underside of the cold roof would condense. To
make moist air you'd need some source of dampness inside.

Possible sources:
- people
- heaters
- appliances (eg freezer, tumble dryer)
- hot power tools
- sunlight through walls/windows
- soil
- plants

If this is just a tool shed without power, you wouldn't really have the
first four (nobody will stay in there for long).

You would get sunlight, but it would only evaporate moisture that's already
in there. For example, if you put in a lawnmower covered in damp grass,
that would contribute moisture. If the shed is well ventilated then the
moisture should disperse. Although if the shed is bedded on soil or is not
watertight you could get moisture ingress that way.

If it's a potting shed and full of plants in damp soil, that would be a
problem I agree.

However this is easily solved by insulating the roof, because you get
exactly the same problem in a house: people breathe out moisture which hits
the cold ceiling, causing condensation and mould.

Theo

On Tue, 25 Oct 2022 17:16:54 +1100, alan_m <junk@admac.myzen.co.uk> wrote:

> On 24/10/2022 17:42, The Natural Philosopher wrote:
>
>>>
>>> Thanks to you and others - mild rethink in progress! But I still want
>>> to give
>>> it a go . . .
>> Think bunker. make the shed out of a concrete or a brick arch
>> Then just leave it.
>> Weeds will arrive
>> And then you will be green and happy

> Question for which I don't have a definitive answer...

> Could something like a living roof that may remain wet/damp (or frozen)
> for months in the winter

Can't see it staying frozen for months in the
UK unless its very high and quite northern.

> actually cause condensation on the underside of a un-insulated wooden
> shed roof?

Only if the underside surface is below the dew point which is unlikely.

> I occasionally see some of those make-over garden shows and wonder how
> long some of that wooden decking, replacement fencing and pergolas are
> going to last - especially when seeing cut timber being inserted into
> holes and after the postcrete being backfilled with earth. As for earth
> retaining walls and raised being built of softwood sleepers....

On 25/10/2022 07:16, alan_m wrote:
> On 24/10/2022 17:42, The Natural Philosopher wrote:
>
>>>
>>> Thanks to you and others - mild rethink in progress! But I still want
>>> to give
>>> it a go . . .
>>
>> Think bunker. make the shed out of  a concrete or a brick arch
>>
>> Then just leave it.
>>
>> Weeds will arrive
>>
>> And then you will be green and happy
>
> Question for which I don't have a definitive answer...
>
> Could something like a living roof that may remain wet/damp (or frozen)
> for months in the winter actually cause condensation on the underside of
> a un-insulated wooden shed roof?

Actually isolated from the ground it will probably warm up pretty
quickly although being damp it will always be marginally colder than
ambient. It depends how close to saturated the air is whether or not
there will be condensation on the underside of the roof.

A comparatively thin layer ~3mm of polystyrene insulation under the DPM
would be more than enough to prevent it being a problem. I used the thin
foil backed stuff on the inside of my S facing garage door to make the
interior cool enough to work in during the summer. Without it the whole
space would end up like an oven by nightfall. The only downside was that
the metal door would be too hot to touch at midday in mid summer.

It didn't make much of a difference in winter.

> I occasionally see some of those make-over garden shows and wonder how
> long some of that wooden decking, replacement fencing and pergolas are
> going to last - especially when seeing cut timber being inserted into
> holes and after the postcrete being backfilled with earth. As for earth
> retaining walls and raised being built of softwood sleepers....

Show gardens are designed to survive for about 6 weeks.

As are some of the very expensive garden makeover with buy and die very
expensive large trees that really don't like being moved and probably
won't be looked after adequately by their new owners.

--
Regards,
Martin Brown

That is because they need a lot of maintaining, since the depth is poor the
soil needs to have its nutrients constantly renewed. The way forward is
hydroponics. This does work, but does or can get clogged and is prone to
damage from the elements as there is little to hold the plants solidly.
Brian

--

--:
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The Sofa of Brian Gaff...
briang1@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"Tricky Dicky" <tricky.dicky@sky.com> wrote in message
news:tj5jb2$1kpk8$1@dont-email.me...
> Brian Gaff <brian1gaff@gmail.com> wrote:
>> I'd have thought the problem was the weight of the wet soil and plants on
>> the poor old shed, unless you are going to use concrete posts all around
>> it
>> to take the weight, and something very strong as the base to stop it
>> sagging
>> in the middle.
>> Brian
>>
>
> I have seen a few of these around on extensions and frankly they all seem
> to end of as a mossy mess and for most of the time looking anything like
> living or green
>
> Richard
>

One idea is a kind of terracing, effectively making the roof into ascending
gutters draining into each other. Trouble is the more complex it is the more
things to go wrong and more heavy things get.
Maybe in the end you would just use it to generate power instead?
Brian

--

--:
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
briang1@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"Martin Brown" <'''newspam'''@nonad.co.uk> wrote in message
news:tj5lsm$1ig$1@gioia.aioe.org...
> On 24/10/2022 07:04, RJH wrote:
>> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm
>> putting
>> together. It'd be an 'extensive' type with about 6 inches of growing
>> medium
>> (gravel base - weed control membrane -compost/soil).
>
> Unless it is very very strong your shed will fall down then.
> Do the load calculations! This isn't a bad guide:
>
> https://www.rhs.org.uk/garden-features/green-roofs
>
> You have to use the lightest weight growing medium possible and as little
> of it as you can get away with and still have plants grow.
>>
>> As ever, too much advice on the web, and it's more guidance on the
>> materials
>> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
>> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've
>> got
>> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed
>> drainage to
>> water butt.
>>
>> About right?!
>
> More or less perfectly wrong.
> (unless your shed roof is incredibly well reinforced)
>
> The living roof plants that will work reliably tend to be semi arid desert
> or alpines that need very little soil ~1" and will grow in pumice or
> perlite/perlag with a trace of topsoil or humus.
>
> Sempervivums, sedums and that sort of thing.
>
> Beware that some of them will set seed and will also colonise your gutters
> and house roof in addition to where you want it to grow. eg.
>
> https://www.etsy.com/uk/listing/823022026/sedum-alba-white-stonecrop-drought
>
> Others like sedum acre are less invasive.
>
> https://www.etsy.com/uk/listing/1244303447/mossy-stonecrop-goldmoss-sedum-biting
>
> --
> Regards,
> Martin Brown
>

You know what they say, the difference between plants are weeds is that
weeds are plants in the wrong place. I was kind of thinking about grass and
wild flowers during the year.
Still heavy though.
I recall the hanging strawberry farms of Guernsey inside a greenhouse and
you could sit and eat below the growing containers. So maybe the answer is a
shed with an upstairs greenhouse.
Brian

--

--:
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...
briang1@blueyonder.co.uk
Blind user, so no pictures please
Note this Signature is meaningless.!
"Martin Brown" <'''newspam'''@nonad.co.uk> wrote in message
news:tj61l9$1sac$2@gioia.aioe.org...
> On 24/10/2022 11:16, Chris Hogg wrote:
>> On Mon, 24 Oct 2022 06:04:56 -0000 (UTC), RJH <patchmoney@gmx.com>
>> wrote:
>>
>>> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm
>>> putting
>>> together. It'd be an 'extensive' type with about 6 inches of growing
>>> medium
>>> (gravel base - weed control membrane -compost/soil).
>>>
>>> As ever, too much advice on the web, and it's more guidance on the
>>> materials
>>> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
>>> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've
>>> got
>>> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed
>>> drainage to
>>> water butt.
>>>
>>> About right?!
>>
>> As many others have said, they are heavy! After a few months, weeds
>> and grasses start to germinate from seed blown in or shitted out by
>> birds, and they start to look a mess and just get worse. In very dry
>> weather they go brown and look dead. A new-build near my previous
>> bungalow had one. Looked fine for a few months when new but then just
>> deteriorated into a weed patch and looked awful.
>
> Some of the plants I have suggested can survive and remain green in the
> toughest of conditions (but they are potentially a bit invasive).
>
> The house leeks aka sempervivums will thrive on a living roof as will
> quite a few of the hardy succulent sedums (but some set too much seed!).
>
> Calendula will grow OK up there. But then so will many unwanted weeds.
>
> --
> Regards,
> Martin Brown
>

On Monday, 24 October 2022 at 07:05:00 UTC+1, RJH wrote:
> I'm looking into a living roof on an allotment shed (2m x 2.4m) I'm putting
> together. It'd be an 'extensive' type with about 6 inches of growing medium
> (gravel base - weed control membrane -compost/soil).
>
> As ever, too much advice on the web, and it's more guidance on the materials
> and any tips/tricks I'm after. For now I'm thinking of a mild pitch (say
> 15cm?) along the 2.4m, divide the roof into two trays, use some DPM I've got
> anyway on top of 18mm OSB (would supported 12mm cut it?), enclosed drainage to
> water butt.
>
> About right?!

did this long ago. As folk have said, soil, water etc are very heavy, so one uses as thin a layer as possible. That neans it needs very frequent watering unless you're only growing desert plants. Which is hardly ideal for something away from home. In the end I found a climber grown from the ground made a better roof covering.