I have been doing a literature review into green roofs as I am thinking of covering the topic in my final year dgree project. One area I was interested in was how practical is it to turn a trad pitched roof into a extensive green roof.

Anyone seen this done before????

Thanks J

If it is steep enough it could be thatched.

Doubt any existing rooves will take the load.
Have seen one where they used a series of small terraces to deal with the slope.

James, it just happens i was reading an article on public perceptions to green roofs in refurbishment of existing premises. Is in Sustain magazine v08 i05 (?) Can scan and email the article if you want. refers to a scheme in liverpool and has before and after photos of the roofs. the planted element appears to be in square 'tiles' that are like eggboxes containing the necessary soil material rather than a monolithic surface of soil as the other posts appear to refer to. i'm sure you know what i mean.

me again,
do you have a copy of 'green roofs and facades' by gary grant, bre,2006.
also remember reading somewhere that because the chosen plants only need shallow beds the weight is negligible, even when saturated, compared to tiles or slates which are bloody heavy, will try to reference that comment if i get a chance.
toms photos are great but appear to be a turf roof. an extensive sedum roof is quite different.
greenfix.co.uk has an illustration of a solution for >25 degree pitch.

Well how come these obviously thrive then?
http://i30.photobucket.com/albums/c301/fostertom/ReNatur_Norwegian_House.jpg
http://i30.photobucket.com/albums/c301/fostertom/Optigrun_Meadow_House.jpg
http://i30.photobucket.com/albums/c301/fostertom/norway040s.jpg

J..M - Green rooves have their uses when having to work within silly planning limitations. They do have technical issues and are not necessarily as green as some would like to believe, but they do look good. That said, the slate solution is simple, tried and tested, effective and also looks good. All we need to do now is land that 747 on the planning office and shake them up a bit. Got any spare landing lights?

You can call me old fashioned, but roofs are primarily for protecting a building against weather, right?
"Green roofs" are not sustainable, the roof element has to hold an amount of water and I don't care how good the materials are or how excellent the workmanship is, it will fail.
Pitched roofs will shed water quickly and effectively (even with tiles missing) ,they work and work very well.
'rant over'

Ecoworrier does have a point Chocolatepixie

If the introduction of a green idea leads to the least green solution, then we should all be worried by it as a failure of concept could lead to future green ideas being rejected. I worry that some of the construction methods suggested for green roofs (not all) may lead to a relatively high annualised embodied content spend (particularly associated with high maintenance costs) for relatively little gain.

didn't think my question would cause such a stir.

thanks for all the input.

Roofkrete wd seem ideal as membrane under green roof.

I heard yesterday that some kinds of single layer flat roofing are vapour permeable. Hard to believe - anyone know?
If so, that's radical, as then the massiveness of the green roof can be in contact with the interior, or at least the top of the insulation, not separated from it by a through-ventilated space above insulation.

Anyone know if, given the above, it's possible to rely for insulation on a good (sufficient) thickness of soil? Or would wetting destroy the soil's insulation? What if good drainage prevented the soil from getting too wet/waterlogged?

Ah damn, so single layer roofing isn't a breather, just a little bit permeable?

Posted By: fostertomAnyone know if, given the above, it's possible to rely for insulation on a good (sufficient) thickness of soil? Or would wetting destroy the soil's insulation? What if good drainage prevented the soil from getting too wet/waterlogged?

Come to think, leave "given the above" out of the above question -

Anyone know if, using no membranes except the one directly under the soil, it's possible to rely for insulation on a good (sufficient) thickness of soil? Or would wetting destroy the soil's insulation? What if good drainage prevented the soil from getting too wet/waterlogged?

This is an interesting discussion, and as an architect who's done some work with green roofs, I'll pipe up with my own understanding of things. One caveat: I'm reading this from the northern US, so some of my points may be directed towards realities here -- but I believe the same general concepts apply in the UK.

To address the initial question, sloped green roofs are indeed very possible. Generally speaking, there is a "sweet spot" where the slope is steep enough to allow proper drainage, but not so steep that erosion becomes a significant problem. If your roof is shallow, you will likely need to incorporate a drainage layer, and if it's steep, you'll likely need to add a stabilizing structure of some sort to prevent the assembly from sliding off the roof. In either case, the roof will be more complicated, and more expensive. (In general, once the plants have established themselves, erosion becomes less of a problem, as their roots tend to knit everything together.)

Also, many existing buildings can support the weight of a fully-saturated green roof without structural reinforcement; it largely depends on the thickness of the soil (properly referred to as "growing medium"). The thinnest systems weigh no more than the ballast on a conventional membrane roof. A structural engineer or architect should always be consulted, however, to see if an existing building is capable of supporting the loads of a green roof – and it’s not just the roof structure that matters. The loads have to be carried down through the building structure and into the foundations.

Green roofs can be built up in layers and planted in-place, or delivered to the site in pre-grown mats or trays. Sedums are known performers and the most common species used, but natives are increasingly preferred. Sedums alone provide very little habitat benefit, and are sometimes considered invasive.

Green roofs improve water quality and air quality, decrease cooling loads in the heating season, and can even replace certain habitats for birds and beneficial insects. Slate, which was mentioned earlier, is an excellent roofing material, but has few environmental benefits beyond being a natural, long-lived product. Moreover, a green roof will last several times longer than most roof systems used today because the additional layers of the green roof assembly protect the membrane from UV degradation, as well as expansion and contraction due to freeze-thaw cycles.

The most important thing to consider when specifying a waterproofing membrane is that most roofing products are not suitable. Roofing and waterproofing are two very different concepts: roofing is meant to shed water as quickly as possible, whereas waterproofing is meant to tolerate constant exposure to water. Since most of the environmental benefits of green roofs depend on water being sequestered on the roof for use by the plants, proper waterproofing design is critical.

And finally, a few directed comments to previous posters:

@J..M:
I have a hard time considering green roofs a "fad," since there’s good evidence that they’ve been in use for the past 2,000 years in a wide variety of climates.
@Terry:
In what ways are green roofs "not as green as some would like to believe"? True, some green roofs are “greener” than others, depending on the design and implementation, but I’m curious about this blanket statement.
@ecoworrier:
In what way are green roofs "not sustainable"? And as for failure, any roof will eventually fail if not properly maintained -- but as I mentioned above, green roofs actually fail less often than most roofs.
@jon:
The maintenance costs for green roofs are not appreciably higher than for other roofs. There is a bit more work to be done for the first year or two (during the plant establishment period) but not in the long run.
@fostertom:
I wouldn't rely on saturated growing medium for thermal insulation. Most of the thermal benefit from a green roof is seen during the summer, when the plants shade the building and cool it via transpiration.

@fostertom:

I actually haven’t ever heard cold-weather insulation being touted as a significant benefit; not of contemporary green roofs, anyway, and not in this part of the world. Maybe that was true historically, before the advent of materials that are cheaper, lighter, and far more efficient insulators than wet dirt. However, if you’re talking about an earth-sheltered home, or an “intensive” green roof with considerably more than the 4-6” of growing medium commonly used, you probably would see a significant wintertime thermal benefit. Most new green roofs are relatively thin, though, so “massive” probably isn’t the most apt adjective.

Whether or not you need to ventilate the roof is completely dependent on the nature of the roof assembly, but it has very little to do with the fact that it’s a green roof: all the same general principles apply as do to conventional roofs. Roofs are often built with rigid insulation on the roof deck, no ventilation required.

As far as what the point is of green roofs, there are many: improved water quality and air quality, decreased cooling loads, mitigation of the urban heat island effect, improved building acoustics, habitat restoration, beautification, urban agriculture, construction waste reduction. I don’t know of any other sustainable building technology with such direct and varied benefits.

Personally, I’ve never done a ventilated green roof, so I admittedly haven’t worked out all the implications – but I certainly wouldn’t say that a ventilated green roof offers no internal thermal benefits. It certainly would offer a cooling-season benefit through the shading and transpiration effects (although you could make the case that those benefits would be greater with an unventilated roof). I would also surmise that a ventilated green roof would offer a small heating-season benefit as well: the volume of air between the membrane and the insulation would be kept warmer with the addition of a green roof, therefore increasing the R-value of the entire assembly – but as you allude, I’d want to figure out exactly where the dew point falls so I wouldn’t have any condensation problems.

In my mind, it’s just easier to insulate above the membrane and eliminate the ventilation altogether. There’s another benefit to this sort of “inverted” roof, especially in the context of green roofs. If the membrane is applied directly to the roof deck, and the insulation is above that, there is far less chance of an eventual leak migrating horizontally through the roof assembly. Therefore, if you see evidence of a leak on the underside of the deck (which is likely exposed, or easily exposed, since the insulation is above), you can pinpoint the location of the leak with some certainty. That’s a boon with any leaking roof, but particularly with a green roof, considering the amount of overburden that has to be removed to locate a leak and make a repair.