Extensive Green Roof on a Flat Deck
A green roof is not a garden put up high: it is a piece of drainage engineering that happens to be green. Its main job — holding water back and releasing it slowly — takes place under the substrate, in layers nobody will ever see again.
Overview
TAV. 00The extensive type is the essential version of the green roof: thin substrate, low-water vegetation, maintenance reduced to a couple of visits a year, and a weight that stays compatible with many existing decks. It has two functions that justify the investment on their own, and neither is aesthetic. The first is hydraulic: it retains a significant share of annual rainfall and, above all, delays the peak flow, relieving the urban network exactly when it is under stress. The second is protective: the waterproofing membrane, usually the first component to age because it sits under sun and thermal cycling, works in a stable environment once covered by the planting and lasts far longer. In exchange it demands discipline on three points — saturated weight, certified root resistance and accessible outlets.
The site score
TAV. HThe detail of the craftthe weight that changes with the rain
Checking the loads and the deck
Structural check on the saturated weight, forming or checking the fall, and laying the vapour barrier and insulation.
Ask for the saturated weight, not the dry one. They're two different numbers and one of them will make your check wrong.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The service you cannot see
What a green roof does to rainwater
The main benefit is not how much water it retains in total, but when it gives it back. On a waterproof roof rain reaches the sewer almost instantly, and every roof in a neighbourhood discharges at the same moment: it is that simultaneous peak that overwhelms the network. A green roof absorbs the first part of the event, slows the rest and spreads the discharge over hours instead of minutes. Total volume matters; the delay matters more.
A qualitative curve. Real performance is declared through the run-off coefficient of the specific system, a function of depth, storage capacity and fall: it is a product datum, not a rough estimate.
Two families, two different buildings
Extensive or intensive: the floor decides
The difference is not taste but weight, and everything else follows from it. The extensive type stays below a hundred and fifty kilos per square metre saturated and lives on sun and rain; the intensive type is a real garden, with deep substrates, shrubs, permanent irrigation and weights three or four times higher. On an existing building the choice is almost always already made by the floor: the mistake is starting from the image and discovering the constraint at the end.
Order-of-magnitude values to guide the choice. The saturated weight of the specific system, including every layer, is a figure the supplier declares and that must be requested before the structural check is set up.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QCan a green roof be built on an existing deck?
Often yes, and it is precisely the extensive type that makes it possible, but the sequence is fixed: first the structural check on the saturated weight, then everything else. Two aspects that get overlooked also have to be considered — the snow load, which does not slide off greenery and can add up, and the condition of the existing waterproofing, which almost always has to be renewed, because nobody will want to demolish the planting in five years' time for a membrane at the end of its life. Where the structural margin is tight, reduced-depth systems designed specifically for retrofit are available.
Is irrigation really necessary?
Once established, no, and this is the point that distinguishes extensive from intensive: the chosen species survive an Italian summer with no input. In the first season yes, always, because the roots have not yet reached the storage layer. The sensible answer is a simple emergency system, operated manually, needed in the first year and in exceptional summers. Providing it costs little at installation; adding it later means removing the substrate.
Is a green roof a fire problem?
Green, living vegetation does not propagate fire; dry vegetation does, which is why the inert margins and the maintenance are not aesthetic details. The gravel bands at the perimeter and around penetrations form a break in the fuel, and removing dry biomass is one of the items in the annual maintenance. In cases subject to fire assessment the requirements are demonstrated on the complete system, with tests on external fire performance: it is a figure to be requested from the supplier, not inferred from the choice of plants.
How do you find a leak under a green roof?
With difficulty, which is why testing before covering is non-negotiable. The techniques exist — electrical methods that locate the discontinuity without demolition — and they can be provided for during installation by adding a conductive layer under the membrane: it costs little then and is worth a great deal later. Without that provision you proceed by elimination, dividing the roof with the edge retaining profiles and flooding one sector at a time, which still means removing substrate. The practical lesson is always the same: on a green roof you spend on prevention, because diagnosis is the expensive chapter.