By Nicolas Ancion (ULg), David Volk (EBF), Ismaël Baraud (CSTB), Laurent Reynier (CSTB), Bernard de Gouvello (CSTB), Marcel Deravet (IFSB), Nicolas Brulard (Gally)


Is building an RTG allowed?

Any such construction requires a building permit issued by the municipal administration.  Each municipality has its own regulations, and this is therefore the first step to be taken: make an appointment with the technical service of the municipality in which you want to build the RTG.

Building a greenhouse on a roof is something new, and it is an innovative solution. It is like adding a new level on the top of a building. As far as the different kinds of works on a roof are concerned, modifying a building or adding a new level is allowed, especially on a flat roof, if the structure can bear the load of the greenhouse and related activities. A building permit is indispensable!

Could the greenhouse be used as a public space?

It depends on the classification of the flat roof and on the greenhouse target (production, social role, educational role, etc). If the flat roof is classified as “accessible”, people can have access to it. The sub-classification is private or public. Depending on it, several criteria must be complied with concerning public access (fire safety, security access, capacity, etc.). The RTG will be built according to this choice. Furthermore, calculations and safety standards are different for an RTG open to the public.
The sanitary aspect of production must also be considered.  In a production greenhouse, the public is generally not welcome inside the greenhouse.

How does one determine the load-bearing capacity of the roof?

The load-bearing capacity is calculated by a structural engineer and the design office according to a European standard method (Eurocodes). The calculation hypothesis must take safety, the exploitation load, and the accidental load into account.

NB: for fish tanks or for specific heavy loads, a specific calculation must be done for the roof area concerned by this overload. It might be more interesting to install them on the ground below, which also needs to be validated for these loads.

What should the minimum load capacity be?

The minimum load capacity is 400 kg/m² or 500 kg/m² for all kinds of accessible roofs. If the roof was designed for garden use, a greater load has to be considered for the calculation.  If the roof is also used for vehicles or parking, the minimum load capacity is calculated based on the type of vehicles to be parked. In the case of public reception, a load capacity in the order of 400 to 500 kg / m2 is used.
For greenhouses that are not open to the public, a load of 150 kg/m2 is required, to which the weight of the growing elements and the weight of the greenhouse itself must be added.

If the load-bearing capacity is OK, how much does it cost to install a “techno-structure”? And how does it work?

A new techno-structure intended for building a greenhouse on a flat roof is generally made of a steel frame anchored in the concrete sub-structure and superimposed, with or without a new floor. These kinds of techno-structures are nearly the same as for every kind of technical equipment or super-imposition on a flat roof. The cost depends on the configuration of the roof, the size of the greenhouse and local constraints to build it.

Which type of access needs to be planned?

Production in the greenhouse requires human presence daily, and a daily flow of material, delivery, and storage. Access must be comfortable, safe, and sustainable. Several kinds of access can be planned, i.e. a lift for people and materials, combined with stairs for people visiting the site, a footpath around the greenhouse for circulation. Access is the same as it is for all kinds of accessible flat roofs.

How to waterproof a roof after building an RTG?

The waterproofing of the flat roof is important to protect the structure and the building below because the greenhouse is not considered as the roof of the building. The greenhouse is not waterproof, and the production activity uses water. Special attention is required to avoid disputes or trouble in the building below. Like any kind of flat roof, waterproofing must be properly designed and installed, using high-performance products. In case of a dispute, investigating the causes of a leak or an infiltration can be quite expensive.

What features should the building structure present to have a greenhouse rehabilitated or integrated on its roof?

The building structure should be resistant at all levels because the overload from the greenhouse will be borne by the whole structure, not just the roof. The structure will have the right bearing capacity at the level of its foundations and at all other levels.

Advantages & disadvantages of a metal structure on the roof; how much could it cost?

A metal structure is the first technical solution identified for building a greenhouse because conventional greenhouses are made with it. The design is well known, calculation methods are well known, installation of the steel frame is well known.
The average cost is 175 €/m². Nevertheless, an average cost is difficult to claim, because it is linked to other parameters such as the existing structure.
Advantages: the new structure does not need to cover the whole roof, and the structure can be built in several phases (to limit investment)
If the building is not too high, the new access should not be costly.
Disadvantages: still costly. A new access needs to be built; waterproofing and insulation still need to be re-examined.

How should I install and manage water storage?

Each water storage equipment requires an analysis of the local volume of rainwater per year and per month to determine the ratio of its use for the greenhouse. Water tanks are considered as a heavy overload, and their layout depends on calculations linked to the structure. If possible, the water tank will be located on the ground or belowground, and the harvested water will be pumped back to the rooftop to be used. If the storage tank is in the greenhouse itself, specific precautions have to be taken (see Chapter II.7).
The European standard EN 16941-1 details all the requirements for a rainwater harvesting system.

What are the different possibilities for the greenhouse envelope? Lifetime (easy to change?) + price

Possibilities for RTG envelopes are the same as for all conventional greenhouses. Considering the localisation on a roof, safety glass or polycarbonate might be more adapted for RTGs.
The lifetime and the durability/resistance of the material is a very important criterion because each replacement of the envelope is expensive due to the stoppage of the production activity.
Replacements are also costly because the work must be done at +/- great height. Lifting devices are therefore necessary.