A green roof is a waterproofed flat roof with a garden on top of it, and the price divides in two: the roof, which a flat roofing contractor builds much as they would any other, and everything above the membrane, which is a different trade's materials and a load the structure has to carry. The plants are the cheapest part. What decides whether the quote is reasonable or alarming is the weight of a saturated growing medium and whether the deck underneath was ever designed to hold it.
What a living roof is, layer by layer
Underneath everything is an ordinary warm flat roof: deck, vapour control, insulation and a waterproofing membrane, and that membrane has to be rated for the job because it is about to be buried. On top of it goes a root barrier, a drainage and water retention layer, a filter fleece, the growing medium and then the planting. An extensive roof is the shallow kind, sedum and low plants in a thin substrate, light and close to maintenance free. An intensive roof is a real garden with deeper soil, shrubs and access, and it weighs several times as much. Almost every domestic green roof worth quoting is the extensive kind.
Why the structure is priced before the plants
A green roof's load is not the plants, it is the growing medium when it has just rained, and the difference between dry and saturated is what an engineer works to. On a new garden room or extension the structure is designed for it from the start and the cost is small. On an existing garage or flat roof the answer is a structural check first, and it comes back one of three ways: it carries an extensive build up as it stands, it carries one after the joists are doubled or the deck is replaced, or it does not carry one at all. Get that answer before pricing anything above the membrane, because the third answer turns a nice idea into a rebuild.
Green roof with solar panels, or a biosolar roof, and why the two suit each other
Putting an array over a green roof is a better combination than it sounds. The panels are ballasted by the substrate and the frames sit on it rather than being bolted through the membrane, which removes the commonest cause of a leak on a flat roof array. The planting keeps the surface cooler than a bare membrane or a gravel ballast does, and a cooler panel is a slightly more productive one. The two conditions are that the structure has to carry both loads together, which is the engineer's question rather than the installer's, and that somebody has to be able to reach the plants around and under the panels, so the array is laid out with maintenance access rather than packed to the edges.
What goes wrong on a green roof, and what the maintenance is
Drainage outlets and the gravel margins around them are what fail, because a blocked outlet on a roof holding a saturated substrate is a serious load as well as a leak. The maintenance is a couple of visits a year: clear the outlets, pull anything self seeded that will root deeply, and top up the substrate where it has thinned. A new sedum roof usually needs watering through its first dry summer and rarely after that. The membrane below it is the thing you cannot inspect, which is the honest argument against a green roof on a deck whose waterproofing is already old.
When a green roof is the right answer, and when it is not
It is the right answer on a garden room, a single storey extension or a garage roof that is overlooked from above, where the roof is a view rather than a surface, and where a planning condition asks for a biodiversity or a drainage benefit and this delivers one. It is the wrong answer where the structure would need rebuilding to carry it, where nobody will maintain it, and where the roof is steeply pitched. And it is a poor substitute for insulation: the substrate does very little thermally, so the insulation still has to be in the build up underneath, at the same thickness it would need without any plants on top.
Questions people ask about green roof installers cost
Does a green roof last longer than a normal flat roof?
The membrane under it is protected from sunlight and from the daily temperature swing that ages a bare roof, so in principle yes. Against that, you cannot see it, and a slow leak under a metre of planting is found late. The trade off is real rather than one sided, and it argues for a new membrane rather than an old one under any green roof.
Do I need planning permission for a green roof?
Replacing a roof covering on a house usually does not need permission by itself, and a planning condition sometimes requires a green roof rather than forbidding one. The permission question is about the building work underneath it, so if the roof is part of a new extension or garden room, the answer is the answer for that structure.
Can any flat roofer fit one?
Any competent flat roofer can build the waterproofing, and that is the half that matters most. The layers above are a specialist system with a manufacturer's details to follow, so ask what systems the firm has installed, how many, and whether the warranty covers the whole build up or only the membrane.
Is a sedum roof cheaper than a green roof?
A sedum roof is a green roof, the extensive kind, and it is the cheaper end of the same family. Where a quote is much cheaper than another for the same area, the difference is usually the substrate depth, whether a proper drainage layer is included, and whether the membrane below it is new.