Managing falls on greenroof?

[dwtowner]

I am building a single-storey extension with a green roof. I want to understand the best way of managing the falls. The attached image shows the proposed extension against my existing house. I already have a downpipe in the angle of the main house on the right hand side with all the associated manholes etc beneath it, so I want that to be the lowest point of the roof with a suitable drain to take the water from the flat roof. There are a couple of other small gutters which would need to discharge onto the roof and be directed into the same drains. I think that there are 3 ways of laying the falls:

 

- have a channel (with fall) along the edge of the house (orange), taking the existing downpipes to the drain in the corner, and have the rest of the roof laid to fall in the yellow direction.

- have a channel (with fall) along the front of the building (green), with the rest of the roof laid to fall in the pink direction. 

- lay the falls in the blue direction between the left hand corner of the roof and the opposite corner where the drain is. 

 

Is there an obvious answer of which is best, please?

 

Is it worth using tapered insulation boards, or stick to firrings with a board overlay?

 

Ancilliary question: In the past I've seen something like a downpipe that goes through a flat roof, taking water from above and guiding it through the roof, while also allowing water from the flat roof itself to enter the downpipe from a hopper in the flat roof, but I now can't find anything like it. Does such a thing have a name? I've drawn what I think it should roughly look like in my diagram.

  

thanks,

 

dan.

[JohnMo]

With the proposed falls are you simply inviting trouble. Any blockages from autumn leaves etc could lead to water accumulation at the house interface. Possibly water in the house. You would never slope a patio towards the house, so why would you do that with a roof?

 

I would be sloping away from the house, simple and safe.

[SteamyTea]

Darcy's Equation is all you need, probably.

[saveasteading]

Firstly let me say that I don't favour green roofs at all. Had to build one once, but otherwise have always argued successfully with the planners against them. ( ie that there were better sustainable solutions.)

 

However, let's assume you are determined.

 

What slope are you using?

 

It's a very heavy thing. Make sure your roof is designed for it. Wet substrate, plants, people on it, snow. Ice.

 

In extreme weather there is no capacity to absorb rain and it will pour off, or through, your roof.

 

So I say, have a significant slope  to one side and fit a normal external gutter.

Your little downpipes should also connect by a pipe just above the roof and get the water off there.

 

I presume you have a spec for the roof waterproofing. Remember that it has to be special to withstand roots but also resist sunlight at any exposed spots.

 

Also remember that you will need safe access for weeding without risk, replacing dead plants, removing litter and dead birds etc.

 

I'd be interested to hear why you favour it. If it is for climate change resistance then ok but do add a water barrel which will have more immediate benefit.

 

 

[Marvin]

1 hour ago, SteamyTea said:

Darcy's Equation is all you need, probably.

You couldn't think of anything simpler to explain then? 😂

[saveasteading]

1 hour ago, SteamyTea said:

Darcy's Equation is all you need, probably

That would be rather complex. You've got me thinking:  I think I'd want some guaranteed water path beneath, eg perforated pipe and gravel, but then it would probably silt up and need cleaning... so just assume the rain is not being absorbed and runs off as in a normal flat roof.

 

@JohnMo rightly says to avoid a slope towards the house, and the channel. 

if you do nothing else, take that advice and slope it away.

what happens if an external gutter is blocked? It overflows to ground.

If it leaks? It drips on the ground.

With a channel that is a risk. When I have been made, as a contractor,  to use a channel I've added overflows /weirs at my own cost.

Edited Sunday at 08:59 by saveasteading

[Marvin]

I have never seen a green roof that falls towards a higher side of a building. I would try to find another way. At least falling away from the building and altering the drainage. It's even more of a concern when you consider that all the rain from the house roof in that area will also be draining there.

 

[Mr Punter]

It looks like you have enough space under the first floor windows for a pitched hipped roof.  The design you have is destined to fail.

[SteamyTea]

2 hours ago, Marvin said:

You couldn't think of anything simpler to explain then? 😂

All it is really saying is that between known limits, the total mass and the total flow will vary disproportionately.

As the mass increases (more water soaked into the soil), then, until total saturation happens, the flow decreases. At total saturation, the mass is known (soil + plants + water) the flow will increase in proportion to the rainfall.

If it freezes, which will probably happen when the weather is drier, ice can build up if it does rain.

Heavy snowfall it probably the biggest structural risk (flat roofs are not as good as pitched roofs for distributed solid loads, why they have deeper joists).

 

All in all, best avoided, or just create a small patch of greenery on it.

[saveasteading]

1 hour ago, SteamyTea said:

just create a small patch of greenery on it.

The problem with that, unless products have improved since I did one, was that UV resistance and root resistance were not available in the same product.

 

It's noticeable that green roofs no longer seem to be promoted  at construction industry exhibitions. I think they were targeted at architects, "sustainability consultants", and planners and had their few years of use.  They were top of the planners' hierarchy of rainfall solutions: they couldn't explain who decided that.

I pointed out, and presumably others did too, that  ponds would run dry and newts would die. Better wildlife ponds, meadows and rainwater harvesters.

 

@dwtowner I know this sounds a bit of a downer, but if you would  respond with your aims we can try to be more constructive. It's just that I know the sheer cost of these in construction and msintrnance ( many times more than the sellers say)  and my priority is always keeping the weather out, as that is what a building is for.

[SteamyTea]

27 minutes ago, saveasteading said:

The problem with that, unless products have improved since I did one, was that UV resistance and root resistance were not available in the same product.

A GRP tray, which is the same material that fishponds are often made from will last decades.

 

28 minutes ago, saveasteading said:

 

It's noticeable that green roofs no longer seem to be promoted  at construction industry exhibitions

Probably because the environmental rules have changed a lot and it is easier to just buy credits and let some farmer turn his unproductive land into a weed patch.

[saveasteading]

16 minutes ago, SteamyTea said:

GRP tray, which is the same material that fishponds are often made from will last decades.

At the time I was doing it, that was not the case. The roots were, I think, the issue. 

Re credits, how true and what a con./ colour. 

How do they measure 'unproductive'. I'm aware of one such farm, where they produced sheep and hay. Now being planted with trees and meadows. Surrounding farms are identical to my eyes.

It's a business. I feel it's misguided but haven't studied the subject.

But preferable to green roofs which would better have solar panels on them and lagoons and ponds below.

[SteamyTea]

1 hour ago, saveasteading said:

How do they measure 'unproductive'.

Generally by the grade/classification of the land.

[Gus Potter]

20 hours ago, dwtowner said:

I am building a single-storey extension with a green roof. I want to understand the best way of managing the falls.

For me the starting point is to look at the underlying structure. While I'm an SE and will design beams and slappings all day long, if you pay me it's not that great as it's your money. 

 

I think. What kind of opening are we making into the existing house. More load from the flat roof can = more money on structure you don't get any benefit from. 

 

Once you nut that out you can then look at the insulation levels you need. Roughly your glass area does not look too large once you take into account the area of existing openings you are building over, relates to the U value calculations. 

 

I would aim for the simple stupid. Modern PVC type roofings are much more forgiving than traditional felt roofs, there are a few pitfalls, but lets run with it for now. 

 

Given the shape of your extension the main run of joists and fall is likely boundary wall to glazing line. So concealed gutter above the glazing. 

 

Run that back as you have shown to the internal corner. 

 

I like green roofs, but they need looking after. Also, on warm roofs they are great for sound proofing.

 

To resolve the issue of excess water collecting against the house introduce a water gate, so if the puddle gets more than 25mm deep the water will pour over the side of the building, you'll see that and know that there is a blockage. This is standard fare on industrial buildings with valley gutters.

 

Your model is a good start. Add to it and put a 1.0 degree fall on the roof and see what it looks like.

20 hours ago, dwtowner said:

Is it worth using tapered insulation boards,

 

It does work but if the builder makes one mistake then you have a problem. Timber firrings are much more forgivable and can accomodate,up to a point late, design changes, could be a roof lantern in your case.

 

 

 

 

 

 

 

 

 

 

[dwtowner]

Thank you everyone for the comments. I take on board the various points made. I'm having fun trying to do the entire extension myself, from planning to construction, so I welcome your help in understanding what to do better.

 

@saveasteading asked for my reasoning for some decisions:

- I will be looking out onto the roof from my office windows (the pair of Velux Elements), and the rest of my garden is much higher than the house so will be looking down on the roof. I wanted something attractive to look at, and a green roof is better than 24m^2 of EPDM or GRP.  My planning application decision notice did mention that the green roof was a positive point (although they didn't say why!) - would they hold me to it?

- My neighbours house is considerably lower than mine and I want to keep the height of the extension as low as possible to avoid having too big a mass on the boundary, and avoid too much overshadowing on winter days. I can see a pitched roof could be a better solution, but in the interests of neighbourly relations I'll do what I can with a flat roof.

- All the existing drainage (soil pipes, inspection chambers, gutter down pipe) are already in the internal corner of the house, which is why I routed everything back there.

 

If I pitched the roof towards the boundary with my neightbour, I could put a concealed gutter along that edge, and a down pipe to the left-most corner. It would put the fall at its lowest too, which minimises the height for my neighbour, and might allow me to have a greater fall too as I can tweak my parapet to hide the bigger height. It would mean having to run another drain pipe around the entire perimeter of the building to get back to the existing drains though, but I'm landscaping the surroundings anyway so maybe that isn't a problem.

 

I do like @Gus Potter 's solution too - pitch toward the glazing with a hidden gutter, and make the internal corner the drain with an overflow. It keeps everything on my side of the house rather than my neighbours (maybe not an issue though), and ties into the existing drainage provision. Downside would be that the roof would be at its maximum height on my neighbours side where I really want to minimise the height.

 

 

 

 

 

 

[saveasteading]

Thanks @dwtowner. So you really want one.

So ignore my sustainability debunking  opinion but do take on board my practical points.

 

You've told the planners you are doing it so you must do it , or ask for permission without due to new information received.

 

I seldom disagree with @Gus Potter. But I hate internal gutters as  they are very risky unless you maintain very, very  regularly.

 

And do get the roof designed for the very heavy load. 6x2s will become 8x2s or v close centres.

And have a ladder fixing point as you will be up there rather a lot.

 

Keep us informed of progress on gutter design (have an overflow as well as the dp outlet) membrane choice etc.

 

 

 

[Gus Potter]

On 26/10/2025 at 22:20, dwtowner said:

Thank you everyone for the comments.

Thank you for posting and asking questions. It keeps BH alive! In no particular order.

 

On 27/10/2025 at 07:56, saveasteading said:

I seldom disagree with @Gus Potter.

True enough. We are both experienced in design, have much common ground. @saveasteading has more than a decade of design experience than I. But it's ok to sometimes disagree. You present / argue your case. You bring to bear specialist knowledge and sometimes that sways the case. Good designers learn every day, age is not a barrier, new materials are on the market so the choice of material is more varied. It's a great time to be a designer!

 

But if we never disagreed that would be unheathy.

 

On 26/10/2025 at 22:20, dwtowner said:

My neighbours house is considerably lower than mine and I want to keep the height of the extension as low as possible

Check the privacy regulations for your neighbours windows. Often you need to add 2.0m plan distance for every 1.0 m change in level. Just check even if to rule out as a consideration. 

 

I've attached a handy guide that is easy reading with lots of visuals that explain daylight and sunlight. Don't discard this on the basis that it is from a Scottish council.. the principles apply UK wide. 

Daylight and Sunlight guidance 17-06-21.pdf