Green roofs in Vienna: which roofs could actually carry one
Of Vienna’s roof area, 3,300 hectares are flat enough for greening — 54.9 %. Calculated here from the city’s own roof model, building by building, all 23 districts.
A green roof is not a matter of taste, it is a matter of pitch. Substrate stays on a flat roof and slides off a steep one, and that single fact decides which of Vienna’s roofs could ever carry one. The City of Vienna drew the line years ago — up to 5° an intensive build-up works as well as an extensive one, up to 20° an extensive one does. What was missing was a current answer for the whole city, roof by roof.
So we calculated it. The City publishes its generalised roof model as open data: every building, every roof face, as geometry. For each of the 192,785 buildings in it, every face becomes a 3D polygon whose tilt against the vertical is its pitch. Sorted into the City’s own classes and summed: 6,006 hectares of roof carry 3,300 hectares that are flat enough — 54.9 %, and about 8.0 % of Vienna’s entire surface.
An own calculation without an outside check is a claim, so here is the check. Measured the way an aerial photograph sees a roof — horizontally projected — these roofs add up to 5,533 hectares. The City surveyed its roof area in 2009 from infrared aerial imagery and arrived at around 5,242 hectares. Two methods, more than a decade apart, under ten per cent apart, and the direction is the expected one: Vienna has built since 2009. The second check runs on the inside — the projected roof area has to equal the footprint area, and the ratio comes out at 1.0002. A roof counted twice would push it above one.
Three classes, and what each one carries
All 23 districts — how flat their roofs are
Ranked by share, not by area: the absolute figure mostly says how large a district is, the share says how its roofs were built. Simmering leads with 69 % — industrial estates and post-war housing roof flat. Josefstadt comes last with 35.6 %, and its Gründerzeit roofs are the reason.
Own calculation from the City of Vienna’s generalised roof model (LOD2.1, CityGML, CC BY 4.0), 1,459 map sheets, 192,785 buildings. Suitability by pitch using the City’s own thresholds. Hectares mean true roof surface, not footprint. 2,434 buildings on the city boundary could not be assigned to a district and are missing from the ranking.
Roof shape follows the building era, not the climate
The obvious headline would be that the hottest districts have the least greenable roof. Measured against our own heat index, that holds as a tendency and not as a law: the six most heat-prone districts average 42.8 % greenable roof, the six coolest 51.1 %, at a correlation of -0.43. The exceptions are the interesting part. Landstraße is among the hottest and still reaches 53.4 %; Währing is among the coolest and only reaches 40.9 %.
What decides a roof is when it was built, not how warm its district is. Around 1900 Vienna roofed steeply; after 1960 it roofed flat. The two maps overlap at all only because the Gründerzeit belt is also the dense, sealed, hot one — the classic urban heat island, where sealed surfaces hold the warmth and the night barely releases it.
Concluding from this that the roof is the wrong surface in the hot districts would be a fallacy from share to area, and it is demonstrably wrong. The same density that pushes the share down stacks the roofs up: in Mariahilf the greenable roof faces amount to 25.8 % of the district’s entire area, in Hietzing to 3.7 %. Where least ground is free, most greenable roof per hectare of city sits overhead. What follows from that is not this page’s recommendation: courtyard and façade areas are nowhere measured here, and what a single roof carries is for a structural engineer to say.
The rain that would stay up there
Vienna gets 626.3 mm of rain in an average year — the 1991–2020 normal at the Hohe Warte station. On a flat gravel roof almost all of it reaches the sewer within minutes of a downpour. A green roof holds a large share and gives it back to the air. Austria’s drainage standard puts numbers to it: a runoff coefficient of 0.8 for gravel, 0.5 for an extensive build-up from 8 cm. Across the whole greenable area that is on the order of 6 million cubic metres a year that would not reach the drains. Holding back more than that takes a retention roof — a build-up with a storage layer and a throttled outlet, designed for the job. The figure here assumes the plain extensive build-up, not that one.
This is the point of the „sponge city“ (Schwammstadt), the idea Vienna and many European cities are planning towards: rain should be held where it falls, seep away and evaporate, instead of being piped off a sealed surface within minutes. A roof is the one surface in a dense district that is available without taking anything away from anyone — no parking space disappears, no lane is narrowed. Rainwater retention on the roof is also one of the figures a sustainability report asks for — under ESG as much as under the EU taxonomy — alongside the microclimate effect. The number here is a starting point for that, not a certificate.
The same layer also cools, and it is worth saying how: water evaporating over the build-up takes heat out of the air, and the substrate buffers what would otherwise travel through the top floor slab into the flat below. That is why a green roof sits next to trees and unsealed ground in the city’s heat plans. How much it amounts to on a given day depends on the build-up, its depth and how moist it is — so no figure in degrees appears here.
The coefficients are design values from ÖNORM B 2501 for sizing drainage pipes, not measured annual averages. Applied to a yearly volume they give an order of magnitude, not a second digit. Rainfall: Statistics of the City of Vienna, measurements by GeoSphere Austria. For a building permit or a sustainability report, a planner recalculates this against the actual build-up.
What this calculation does not know
Three limits, and they matter more than the headline figure. It does not know the stock: pitch is not planting, so a roof that has been green for twenty years looks exactly like one that never was. The City’s 2009 survey found roughly 5 % of Vienna’s roof area green; per roof that number does not exist, and it is not invented here. It does not know the usable area: the roof model is generalised, so plant rooms, skylights, chimneys, lift heads and any photovoltaics already up there are not subtracted. And it does not know the statics — whether a roof’s load-bearing capacity takes the 80 to 150 kg per square metre of an extensive build-up is a structural engineer’s answer, not a geometry model’s.
Rules and funding
Vienna’s building-code amendment of 2023, promulgated on 13 December 2023, made greening easier rather than compulsory: a green roof may exceed the permitted building height by 15 cm, and specialist bodies point out that this now applies in combination with a thermal insulation system. Façade greening can be written into a development plan. New residential buildings have to carry photovoltaics — 1 kWp for every 150 m² of conditioned gross floor area — which is why a flat roof has become a contested surface, and why it matters that the two can share it. The city’s environmental department funds roof greening with up to 30,000 euros per roof. What applies to one specific building is for the building authority to say.
Frequently asked
How much of Vienna’s roof area could be greened?
By roof pitch, 3,300 hectares — 54.9 % of the city’s roof area, or about 8.0 % of Vienna’s surface. Of that, 2,479 hectares lie almost flat (up to 5°), where a deeper build-up with beds and shrubs is possible; the rest is gently pitched, where a thin sedum and grass layer works.
Who calculated this, and how?
We did, from the City of Vienna’s official generalised roof model (LOD2.1, CityGML). For each of the 192,785 buildings in it, every roof face is a 3D polygon, and the angle between its normal and the vertical is the pitch. The suitability thresholds are the City’s own — up to 5° intensive and extensive, up to 20° extensive, minimum 5 m² per contiguous face — so we apply their definition to newer data, nothing more. The built-in check: the horizontally projected roof area must equal the footprint area, and it does (ratio 1.0002).
Does Vienna not have a green-roof potential cadastre already?
It does, and it is good — but it rests on an airborne laser scan from 2007, and it is not published as an open data layer; it lives inside the city’s Umweltgut map viewer. The roof model used here is newer and describes every roof face geometrically. Both answer the same question; this calculation can simply be run again when the data changes.
Which districts have the least greenable roof?
Josefstadt with 35.6 %, followed by the other Gründerzeit districts. The reason is the roof itself: a steep pitched roof from around 1900 sits well above 20°, and no build-up stays on it. Where the flat roofs are — industrial estates, Gemeindebau, post-war and new development — the share is far higher, up to 69 % in Simmering.
Is my roof already green?
This calculation cannot tell you. It knows the pitch, not the planting. The only stock figure is the City’s own survey from 2009 (infrared aerial imagery): around 5 % of roof area was green then. Per roof that figure does not exist, so nothing is claimed per roof.
What does a green roof do for rainwater?
It holds a large part of it up there instead of sending it into the sewer. Austria’s drainage standard (ÖNORM B 2501) works with a runoff coefficient of 0.8 for a gravel roof and 0.5 for an extensive green roof from 8 cm build-up. Applied to Vienna’s annual rainfall of 626.3 mm and the whole greenable area, that is on the order of 6 million cubic metres a year. The coefficient is a design value for sizing pipes, not a measured annual average — the order of magnitude holds, the second digit does not.
How do you calculate the rainwater retention of a flat roof?
Three figures give the order of magnitude: the roof area in m², the annual rainfall and the runoff coefficient of the build-up. For Vienna that is 626.3 mm on the 1991–2020 average (Hohe Warte station) and the coefficients of ÖNORM B 2501 — gravel roof 0.8, extensive greening from 8 cm build-up 0.5, intensive from 25 cm 0.1. A 300 m² gravel roof therefore sends around 150 m³ a year into the sewer, an extensively greened one about 90 m³; the difference of roughly 60 m³ is the retention. This is a back-of-the-envelope figure: the coefficients are design values for sizing pipes, not a measured annual average. For a building permit, a sustainability report or sponge-city planning, a planner recalculates it against the actual build-up, with its retention layer, slope and emergency drainage.
What does the law say, and is there funding?
Vienna’s 2023 building-code amendment (promulgated 13 December 2023) made greening easier: a green roof may exceed the permitted building height by 15 cm, and specialist bodies note that this now applies in combination with a thermal insulation system. New residential buildings must carry photovoltaics — 1 kWp per 150 m² of conditioned gross floor area. The city’s environmental department (MA 22) funds roof greening with up to 30,000 euros per roof. Which rules apply to a specific building is for the building authority to say.
Solar potential · Cool living · All districts
Sources: own calculation from the City of Vienna’s Generalised Roof Model LOD2.1 (CityGML) and the district boundaries — data.wien.gv.at, CC BY 4.0. Suitability thresholds (5°, 20°, minimum 5 m²) as published for the City’s green-roof potential cadastre. Runoff coefficients: ÖNORM B 2501, design values. Rainfall: Statistics of the City of Vienna, measurements by GeoSphere Austria. Roof stock 2009 and funding: City of Vienna, environmental department (MA 22). No planning and no offer: statics, monument protection, the owners’ association and the building authority all have a say.