Construction Systems
Green roof house
Green roofs have been used throughout history, even in very cold climates. In Nordic countries, for example, there is a traditional type of green-roof house made with straw that can last for several decades.
In our Mediterranean climate, there are some examples in vernacular architecture, such as the Galician hórreo or the Extremaduran chozo—both featuring steeply pitched roofs for small shelters.
But beyond traditional architecture, green roof houses are spreading today in eco-home construction thanks to the excellent thermal performance they offer.
It is worth remembering that the roof is the fifth façade of our home and the one that receives the most sun. For this reason, buffering all that heat with a layer of soil and plants is one of the best ways to regulate indoor temperature and prevent overheating.
In practice, many clients are still sceptical about using this type of system. In our climate, both pitched tiled roofs and inverted roofs (flat roofs) have been widely used. A green roof is simply a flat roof where, instead of protecting the surface with gravel or paving, it is covered with a layer of soil to which—together with the corresponding root-barrier membrane—plants are added.
In today’s article, we will aim to clarify what a green roof is, addressing any questions you may have about how it works and how it is installed. We will also discuss costs and maintenance—particularly important when it comes to an area of the home that is generally difficult to access.
Systems for a green roof
Before we begin, it is useful to understand what types or systems are currently available for building a green roof house.
The construction industry has become increasingly technical and stricter in terms of installation regulations, quality, and safety—above all to ensure the comfort of users in any building.
For this reason, a green roof house must meet strict standards in relation to quality and safety regulations.
Broadly speaking, we can group green roofs into two types:
Green roof on a flat roof.
It will always have a minimum slope (1% to 5%) for water drainage.
Green roof on a pitched roof.
There are limits to prevent soil from sliding or falling. In fact, anything above an 85% slope would be considered a steep roof and, as this is an extreme and very specific case in certain areas, I will not address it in this article.
Flat green roof house
A flat green roof house is considered to have a slope of between 1% and 5%.
On flat roofs, the green roof helps protect against harmful weather influences, thereby extending the service life of the roof.
To prevent the substrate from drying out—which would lead to a lack of vegetation—flat-roof greening systems include a special drainage layer that helps divert excess water and facilitate constant oxygen circulation in the soil.
A flat green roof house is built with different layers and components:
– Waterproofing membrane.
– Protective layer
– Drainage layer
– Filter layer
– Substrate (soil)
– Vegetation
Pitched green roof house
A pitched green roof house is distinguished by its slope:
– Slightly pitched roofs: between 5% and 36% slope.
– Steeply pitched roofs: between 36% and 85% slope.
– Very steep roofs, with a slope above 85%, which we will not cover due to the complexity of installation.
a. In the first case—slightly pitched roofs—we should note that the roof’s slope itself makes building a green roof house easier and more cost-effective.
We can consider the system as a single-layer roof system, meaning that no drainage layer or felt is needed to prevent water from pooling and damaging the roof.
The soil has the same capacity both to store and to drain excess water.
If coarse-grained particles (pumice, slag, expanded slate, or expanded clay) are mixed into the substrate, the system will perform better. They also help reduce the roof’s weight, increase its thermal insulation capacity, and allow the roots to breathe more easily.
b. In the second case—steeply pitched roofs—the system works in the same way as the previous one, but with the additional requirement of extra safety measures to prevent soil slippage.
These safety measures can range from techniques that use the vegetation itself to retain the substrate to construction measures. The choice depends on the roof slope, distance, substrate thickness, its composition and adhesion capacity, and finally the degree of rooting.
Crossbars beneath the waterproofing membrane are very effective for slip protection, although we must avoid damaging the waterproof layer by rounding the corners of the wood that is installed.
Another option is to use well-secured felts, meshes, or anti-slip mats fixed to the ridge, provided that the substrate is well compacted.
It is also possible to install the vegetation using turf rolls or vegetation mats, which can reduce the risk of substrate slippage.
What layers make up a green roof?
Now that we have reviewed the different roof types, let us look in detail at the layers that make up a green roof house.
Structure and thermal insulation
In principle, any structure—of any type—that can support the load of the soil thickness and the other layers installed above it is considered suitable.
Insulation plays an important role, as it can be found in two positions, leading us to classify the roof as: cold roof or warm roof.
The main difference between the two is that, in a cold roof, the thermal insulation is separated from the vegetation layer by an air cavity, and no vapour barrier system is installed.
In a warm roof, however, the air cavity is eliminated and a vapour barrier is placed beneath the thermal insulation layer to prevent water vapour from entering the insulation and condensing there—damaging everything and causing poor thermal performance in our green roof house.
Ultimately, if you are considering a green roof house, we recommend that you always use the warm roof system. It is the most cost-effective and efficient option for green roofs.
Roof membrane and protection against root penetration
The waterproof membrane is a synthetic textile product derived from petroleum; despite its ecological footprint, there are few real alternatives on the market.
It is a sheet that prevents water from entering the home and also protects the roof and structure from potential damage caused by plant roots.
Drainage Layer
The drainage layer is not only used to evacuate excess water to the outside; it also helps store the water needed for plant consumption.
We recommend using porous, lightweight mineral materials such as coarse-grained sands, expanded clay, expanded slate, expanded lava, pumice, and recycled slag and brick materials.
Substrate
The substrate is essentially the layer that supports the vegetation.
It serves as a nutrient medium and as water storage. It must have sufficient air volume in its pores to provide the roots with stable support and allow them to remain anchored.
The substrate layer mainly depends on the vegetation we are going to use. They must work in harmony with each other—in other words, in the end they must function as a single layer.
Which vegetation is most suitable for a green roof house?
To complete a green roof house, a vegetation layer is usually always installed. However, not all plants are suitable for this purpose.
When choosing plants, it is important to be clear about the following factors:
– Substrate thickness and its ability to store water.
– Roof slope.
– Wind exposure.
– Orientation, as roofs facing the sun (in our hemisphere, this would be south-facing) dry out faster.
– Shade.
– Local rainfall, which will determine whether the chosen vegetation needs to be more or less resistant to drought, frost, heavy rainfall…
– Vegetation should have a growth height between 10 and 20 cm, with flowering peaks no higher than 40 cm.
It is also important to bear in mind that vegetation plays a key role in a green roof house, as it ensures the thermal insulation effect, a cooler feel in summer, acoustic insulation, aesthetics, etc.
Types of vegetation
When deciding what vegetation to use on a green roof, we should always prioritise native species according to the microclimate.
We want the plants to stay alive with minimal or no maintenance—in other words, rainwater should be sufficient.
In some cases, it may be advisable to install an irrigation system for periods of severe drought. However, if you live in an overly dry area with low rainfall, a green roof may not be the best option.
We must bear in mind that a green roof performs optimally when the vegetation is thriving—alive, well rooted, and lush.
The most commonly used types of vegetation in the construction of a green roof house are: grass and wild herb seed, turf, and vegetation mats with mixed plants.
The choice of one type or another will depend on the parameters mentioned above, but above all, the most decisive factor will be the thickness of the substrate that supports it.
Let us look at the different substrate thicknesses according to the vegetation:
Grass and wild plant seed
This is a type of vegetation that requires a substrate thickness of 12 to 18 cm, generally with cellulose particles, clay, alginate, or other synthetic materials to prevent the injected seed from spreading.
Grass and herb seeds are difficult to germinate, but once they do, they are very resilient.
Cuttings (Sedum, sedum herb, and grass)
Sedum types are generally grown using pieces of shoots (cut parts of the plant).
They take root easily and can also withstand periods of drought.
For propagation by cuttings, around 40 shoots per m2 are needed.
The substrate thickness should be between 5 and 8 cm.
Pre-fabricated turf rolls
There are companies in many countries that produce turf in pre-fabricated rolls, although it should not be confused with artificial grass.
If you wish to install this system, you should always take the roll dimensions into account to accurately calculate how much you need to purchase.
In this case, the substrate thickness is usually around 2 cm.
Vegetation mats
Vegetation mats are also pre-fabricated systems assembled with felt, nets, or braided thread mesh.
They contain different mixes of moss, succulents, grasses, herbs, and bulb plants.
The substrate thickness ranges from 1.5 to 3.5 cm, and their maximum dimensions are 1 x 1 m.
Care, maintenance, and irrigation
In addition to all the thermal, functional, aesthetic, and ecological advantages of a green roof house, there is another aspect that supports choosing these systems to complete our home.
A green roof does not require extensive maintenance or specific care.
If the plants are well chosen, irrigation will generally occur naturally, and if a plant dies, it will regenerate thanks to the production and dispersal of seeds.
In any case, we advise that if you hire a company to build the green roof, you ensure whether or not special measures are required as part of the roof’s finishing care.
This could include initial watering, watering during seeding, and even additional seeding after installation.
If there are very long periods of drought—particularly on steep, sun-facing roofs or with substrates that store little water—artificial irrigation can be useful.
To do so, we will install hoses in the substrate, preferably in the upper area, to provide drip irrigation.
Other than that, a green roof house will not require any other type of maintenance, except for removing shrubs or tall weeds that may grow sporadically and pose a risk to the roof structure and layers, and to the home.
Costs and service life of a green roof
The market offering for green roof systems is so broad today that it is difficult to make precise statements about average costs.
In fact, the number of types available is surprising, as is the strong commitment by companies and contractors to these systems.
However, perhaps due to lack of awareness or budget constraints, it is a system that has so far not been used for either single-family or multi-family housing.
Achieving a cost-effective build starts with planning. For a green roof to be cost-effective, I recommend installing it on roofs with few breaks and with slopes that are not too steep.
We should also consider companies’ order backlogs, transport distance, site accessibility, and the time of year, as these factors will directly affect the product’s sale price.
Taking all this into account, a green roof house is currently slightly more expensive than a conventional-roof house, but this small difference is offset over the years through better building performance and maintenance savings.
Advantages of a green roof house
To conclude, let us review the advantages offered by a green roof house.
As mentioned in the previous section, the investment in this system will be reflected and offset by a range of very positive aspects—both for your finances and for your health and lifestyle.
A green roof house contributes to…
– Reducing paved surfaces.
– Oxygen production and carbon dioxide consumption.
– Cleaner, purer air.
– Regulating temperature and humidity inside and outside the home.
– Extending the service life of the roof’s waterproof membrane.
– Greater thermal insulation (summer and winter).
– Better acoustic insulation.
– Fire protection.
– Integration with the landscape and ecosystem (insects, aromas, aesthetic and psychological effects).
With this article, we have tried to select the most important aspects to understand how a green roof house works and what its advantages are.
As always, if you still have questions or new ones arise about these systems, please do not hesitate to contact us—a studio where a team of expert architects works, always ready to guide clients towards healthy, sustainable construction.