TECHNICAL FEASIBILITY

By Marcel Deravet (IFSB) and Ismaël Baraud (CSTB)

The technical feasibility of a rooftop greenhouse (RTG) project must consider all the usual procedures related to the construction sector and include the criteria related to the innovative nature of this typology/concept.

There is no supporting EU (European Union) guidance or standard regarding the integration of a greenhouse on a rooftop. Therefore, it is necessary to i) develop a technical design file explaining the chosen technical solutions, and ii) provide the justifications to demonstrate:

  • The strength and durability of the work

  • The feasibility of the construction and maintenance operations

  • The safety of users and stakeholders in the construction and operational phases

This technical guide will necessarily involve the various stakeholders of construction for them to describe their work while considering the specificities related to the construction of a greenhouse on a roof.


The purpose of this chapter is to present the points to be addressed to design a project and validate its feasibility.

NEW BUILDINGS

The focus is on evaluating the design project in relation to the building regulations currently applied in the country.

The designer should provide the various stakeholders with the assumptions to be considered when establishing technical feasibility.

Depending on its surface and its total or partial occupation of the roof, the RTG will be adapted to different roofing types:

  • multi-use roofs when the greenhouse is located nearby technical areas, areas accessible to pedestrians, inaccessible areas

  • greenhouse roofs when the greenhouse occupies the entire roof surface

In many cases, RTGs are located nearby equally innovative areas such as garden roofs, roofs equipped with solar panels, etc. In that case, it is necessary to demonstrate the overall feasibility of the RTG.

Designers assisted by the technical engineering office and structural engineers should justify structure calculations considering the permanent loads and the operation of the greenhouse, resistance to climate conditions (wind, snow) in order to demonstrate the solidity and durability of the structure.

The stakeholders are the building contractors and the greenhouse contractor. The assumptions of their calculations need to be worked on in common.


Designers should justify the choice of materials (insulation, waterproofing, coating) in terms of properties and technical performance to demonstrate the durability of the work. For example, they should justify the choice of a particular thermal insulation material (e.g. when it deviates from current construction rules or current thermal requirements).

Designers should justify where technical equipment should be installed and its connections with the greenhouse (plumbing, electricity, ventilation, etc.).

For example, the location of network crossings, ventilation exits, aeraulic systems, etc. will have to be justified.

EXISTING BUILDINGS

In this case, the focus is on diagnosing the roof and structure of the building to determine whether building a greenhouse on the roof is feasible and to implement the facilities in the building. The original structural calculations or a new structural report are needed.

The greenhouse designer and the renovation contractor will set up a technical file in which the calculation hypotheses have been collected, when they exist; otherwise, a new structural study of the building will have to be performed. This design/renovation file (Structural evaluation/structural report/building manual (UK)) must demonstrate that the building will be preserved, existing structures are solid, non-structural works are compatible with the construction of a greenhouse, and the safety of users and stakeholders will be ensured. If necessary, the technical file must detail the preparatory work required to strengthen the structure or modify it.

This chapter is organised in two parts:

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​Analysis of the overall aspect of the building

Structure: The primary criterion for determining whether a greenhouse can be built on the roof is the building's load capacity. The best information will come from the calculation note of the engineer who designed the structure, but we can also refer to Eurocode 1. The first analysis focuses on the structure and evidence justifying potential overload.

Description of the structure

  • owner must provide the building design teams with the plans, calculation assumptions and criteria taken into account for the construction of the original building

  • (structural builders) must provide the archives of the execution work, the beam plan, slabs, braces, service cavities, foundations, etc.

  • architect must provide the original architectural files if they still exist

If these data cannot be provided, a structural diagnostic must be carried out by a structural design office in accordance with local regulations. A topographic study and a plan showing the dimensions of the existing building are also necessary.

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Number and height of floors

The number of floors is relevant to determine the link between the greenhouse and the building. It is important to take this parameter into account. The floor below the roof should be well described to determine the possibilities of installing greenhouse facilities as close to the greenhouse as possible to reduce the length of the network (pipes, cables, etc.).

The basement floor should also be well described to identify the possibilities of installing a water tank, storing materials for the greenhouse, the primary heat pump...

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Accessibility

Accessibility to the roof is really relevant because if access is non-existent or difficult, major work should be done to create access while focusing on safety issues.

Accessibility depends on the number of floors. If the building has only one floor, accessibility may be limited to a simple staircase located indoors or outdoors. The staircase must be wide enough to be able to deliver the material to the greenhouse and ensure user safety.

If the building has several floors, accessibility will be made possible both via a staircase and via a lift to comply with fire safety regulations, to facilitate the movement of workers/visitors and the transport of loads, etc. In addition, dedicated access to the greenhouse may be necessary to take other activities in the building into account (everyday life in a residential building, working in an office building...).


Situation with respect to neighbouring buildings

The design and organisation of the RTG project will greatly differ depending on whether there is an interaction with neighbouring buildings or not. If the greenhouse is in contact with other buildings, this interaction must be evaluated and managed in terms of light, shading, noise, easements paths, etc.

The height of the building will be studied according to local urban planning regulations, modelling can predict the impact of the greenhouse on neighbouring buildings.


Detailed building analysis

Roof composition

The roof composition is directly linked to the energy chosen for the greenhouse project. It can lead to choices of different materials or equipment.

It should be well identified (e.g. a concrete slab, a steel structure, wood). The watertightness /roofing contractor will provide the maintenance contract, the roof layout, and the work record. The type of insulation and waterproofing technique will need to be identified in order to accurately study the load capacity and ensure the feasibility of the work.

Additional work may be needed to protect the roof. For example, adding heavy protection or a new mechanical high-strength sealing layer, suitable for pedestrian-accessible roofing.

Other work may be to be planned or avoided.

For example, when it comes to thermal insulation of the roof:

  • If the existing insulation is not mechanically resistant enough, it will have to be replaced. In this case, the designer, assisted by the thermal design office, will have to demonstrate that the choice of a new insulating material exclusively based on mechanical resistance, which often leads to lower thermal performance, is relevant. Therefore, they will have to argue in favour of an insulating material that does not reach regulatory thermal performances.

  • Similarly, if the existing thermal insulation is mechanically resistant enough but its thermal performance is low, the designer, assisted by the thermal design office, will have to demonstrate that keeping the existing insulation will be a source of savings (insulation + waterproofing will not be replaced) and that the energy from the greenhouse will participate in the recovery of the thermal loss of the existing roof.

  • For more information about insulation, see chapter



Roof plan (relief, slope, technical equipment, rainwater outlet, etc.)

The owner or watertightness/roofing contractor will provide the full list of roof installations and each maintenance contract (HVAC, antenna lightning rod, etc.).

The watertightness/roofing contractor will provide the layout of rainwater outlets. There may be an opportunity to determine how many rainwater outlets can be shut down and how many need to be kept collecting rainwater running down from the greenhouse cover.

If rainwater is to be collected, the transformation of rainwater inlets and downspouts and their connection to rainwater recovery tanks should be described.

In view of the experience the greenhouse structure must be integral with the building shell structure, therefore work should be done to create reinforced concrete or metal posts to anchor the greenhouse in the structure. Specific studies should be carried out to determine the wind resistance and stability of the greenhouse structure.


If technical equipment needs to be modified or moved, the transformation work will be described (elevation, displacement, connections, etc.).

If the roof slope needs to be corrected, the work will be described, and the chosen technical solutions will be detailed and justified.

The current trend of the construction market clearly favours the design of buildings with functional and accessible flat roofs. As a result, flat roofs and top floor floorings are increasingly designed to be compatible with pedestrian use. In the wake of this trend, reversing ongoing projects to propose that a greenhouse be installed is indeed possible. Modifications will be less than in the case of an inaccessible roof design. Architects are sensitive to the question of the multi-functionality of roofs and like to combine uses (greenhouses, entertainment area, technical area, etc.).


When existing technical equipment is preserved and is found inside the future greenhouse, it is necessary to determine how it will remain accessible for maintenance and how it will be secured (covering, barriers, etc.) to prevent greenhouse workers from interfering with this equipment.


Stairs and lifts

Stairs and lifts will be analysed and checked for their ability to bear the constraints related to a greenhouse. For example, a lift must have sufficient load capacity for the people and materials required daily for greenhouse activities. The sizing of the lift will also be in line with the number of products harvested in the greenhouse. Stairs will be located near the greenhouse to avoid crossing private spaces under the roof.

Otherwise, the design of new specific accesses (outdoor stairs, new lift sheath, foot bridges) and access paths to the greenhouse will be described from a technical point of view.

Basement

The basement of the building will be accurately described to determine its potential for transformation so that the facilities necessary for greenhouse activities can be installed (the surface area of the room will depend on the size of the greenhouse and the type of crop grown in it).


REFERENCES to go further in the roof design (please refer to your national institutions) :

- Building code and national construction rules

- Standards and Eurocode

- NF DTU (Norme Française  Document Technique  Unifié) series 43, 20-12 standards for constructions in France

- National technical approval, Technical Assessment of construction processes,

- Building Information Modelling (BIM) digital model