Planning criteria at a glance

Sandwich construction utilises clearly defined, large surface components and a joining system prefabricated in the factory. The elements have small dimensional tolerances and are easy to handle. They do not present significant difficulties, either in planning or in implementation. In order to fully exploit the design possibilities and the economic
efficiency of sandwich construction, it is helpful to observe the following planning criteria:

• Early planning coordination
• Large surface area planning
• Detailed system planning
• Tight connections
• Rational methods for roof openings

Early planning coordination

An important criterion is early coordination between supporting structure planning, structural engineers, specialist engineers, sandwich manufacturers and architects. Due to the high degree of pre-fabrication, selection of the sandwich elements and the structural accessories should take place as early as possible in the design phase. Through early system selection and coordination of those involved in the construction work, the main architect can ensure an optimum number of possibilities for combining his design requirements with the economic advantages of sandwich construction.

Large surface area planning

With their overall widths (from 800 mm to 1,250 mm) and supply lengths of sometimes up to 24,000 mm, sandwich elements are large-surface components. The better the plan uses this area, the more economically the substructure and assembly of the sandwich elements can be carried out. Due to the large supply lengths, the roof surfaces in many buildings can be formed without transverse joints. With appropriate pre-planning, the sandwich elements can be manufactured in exactly the right length.

Detailed system planning

Detailed system planning requires thorough consideration of the system options, including the structural accessories available. The associate companies of GALILEO - Creative sandwich construction offer a wide range of standard solutions for detailed planning, in the form of design drawings and CAD files. These standard details are intended to be used by the architect as a stimulus for his own detailed solutions, under consideration of the respective system conditions. Viewing of the manufacturers' component catalogues for systemcompliant structural accessories is recommended.
As a rule, sandwich manufacturers coordinate their system accessories for material and colour selection as well as for fastening techniques with their sandwich systems.

Tight connection structures

Sandwich construction is characterised by outstanding heat insulation capacity and by a particularly tight joint formation, sealed at the factory. Therefore, it must be ensured when planning all connection details that this constructive advantage is not diminished by thermal bridges or by unretentive connections. So, for example, with overhanging sandwich elements, a so-called "temperature separating section" should be provided, which prevents thermal conduction by the inner shell.

Naturally, the load-bearing capacity must still be guaranteed for this detail. Wherever warm air can escape outside through structural joints, suitable sealing measures must be professionally carried out. For on-site seals in the area of connections, as well as for longitudinal and transverse joints of the sandwich elements, examples of how this can
best be achieved can be taken from the illustrations in this information leaflet.

 

Fig. 4.1.1 Gable roof in sandwich construction; Source: [9]

  top

Roof openings

Basically, there are two options for planning roof openings:

• Subsequent cutting out of the opening with contemporary tools
• Formation of the opening by structural measures

 
 

Fig. 4.1.2 Formation of roof openings by cutting out with panel cutter - see also Info Sheet 5.3; Source: Factory photo TP 150-0 V150, Trumpf GmbH Deutschland

  top

Fig. 4.1.3 Formation of roof openings by structural measures; Source: [6]

  top

Selection of the method depends on the respective size of the opening and on the position, i.e. on the grid dimensions of the sandwich elements. For example, when fitting large-surface rows of windows, it might as a rule be best to form these roof openings
by cutting the core of the system in the appropriate positions, whereas if the roof openings deviate from the system grid dimension and are not too large, cutting out these openings can be more advantageous.

Lighting elements - system-compatible roof accessories

As a rule, manufacturers can supply an extensive and practically proven range of accessories for roof components and appropriate structural accessories, such as lighting elements, breather tubes, attic components or gutters, for example. To facilitate planning and design, detailed consideration of the component catalogues is recommended.

 

Fig. 4.1.4 Connection of upper rows of windows in the ridge area - left-hand drawing: vertical, right-hand drawing: parallel to the profile of the roof elements; Source: [9]

  top

Fig. 4.1.5 Tubular feedthrough with system components through a roof structure in sandwich construction; Source: [9]

  top

Fastening the roofing panels

For fastening sandwich elements, a basic distinction can be made between systems with concealed fastening and systems with visible fastening. While the type with concealed fastening depends on the respective sandwich construction of the manufacturer, the principle of visible fastening presents only slight differences. The screw type, the number and dimensions of the required fixing screws depend on a number of factors:

• Type of load-bearing structure
• Approval of the fasteners
• Design loads in accordance with DIN 1055
• Wind pull loading

 

Fig. 4.1.6 Fastening roofing panels to steel and wood profiles with self-tapping steel screws; Source: [15]

  top

Fasteners

Only building inspection-approved screws may be used: in this system, for example, rust-free screws with plain washers and vulcanised seals. The required number of screws for surface fastening, as well as for the edge and corner areas, is defined
by the structural engineering. Self-tapping fastening screws with supporting thread are also used.

Element fastening

In the case of roof panels, screwing down takes place both through the horizontal beading and through the vertical beading. Where extreme static conditions are present, e.g. in the case of very high wind pull forces and/or in the edge and corner areas, additional screw fittings may also be necessary.

 

Fig. 4.1.7 Implementation example for the visible fastening of roof panels; Source: [9]

  top

Fig. 4.1.8 Implementation example of concealed fastening of roof panels; Source: [6]

  top

Connection on the longitudinal joint

The overlapped longitudinal joint is screwed down on steel facings at a distance of a £ 600 mm, and on aluminium and copper facings at a distance of a £ 400 mm.

 

Fig. 4.1.9 Overlapped longitudinal joint; Source: [9]

  top

Roof types

In the case of roof panels for sandwich construction, pitched roofs with any roof shape, e.g. ridge roof, monopitch roof or shed roof can be manufactured.

 

Fig. 4.1.10 Ridge roof in sandwich construction - vertical section of a ridge design; Source: [9]

  top

Fig. 4.1.11 Monopitch roof in sandwich construction (without roof overhand), vertical section; Source: [9]

  top

Minimum roof pitch

Roof panels have a special profile formation with sealing tape integrated at the factory. The tightness of such roof panels guarantees the diversion of water by a minimum roof pitch and where the relevant installation and assembly regulations are observed. For roofs without transverse joint and without roof openings, the minimum roof pitch is
normally 3° = 5.2%. For roof structures with a transverse joint, a minimum roof pitch of 5° = 8.6% must be observed.

 

Fig. 4.1.12 Minimum roof pitch for sandwich roofs with and without transverse joint; Source: [9]

  top

Transverse joint in the roof area

Even though there are proven solutions for the professional formation of transverse joints in sandwich construction, roof structures without transverse joints are preferable for fundamental structural engineering reasons. Due to the long supply lengths, many roof surfaces can be formed without a transverse joint. If the provision of a transverse
joint is necessary, correct arrangement of the sandwich elements and careful sealing must be ensured. Fig. 4.1.13 shows arrangement, installation direction and assembly sequence of the sandwich elements for a transverse joint. Fig. 4.1.14 shows an implementation example in which the sealing of the transverse joint is visible.

 
 

Fig. 4.1.13 Formation of a transverse joint in a roof structure in sandwich construction; Source: [9]

  top

Fig. 4.1.14 Formation of transverse joint in roof panels; Source: [9]

  top

Cut edges

It must be ensured that the cut edges of sandwich elements are air-flushed.

Gutters

Water can be discharged from roof surfaces in sandwich construction by means of both external and internal gutters. In the case of internal gutters, appropriate emergency overflows must be provided. The gutters should be wide enough to allow access for cleaning and maintenance work.

Gutter heating

The roof structure must be designed so that no standing water can form over the entire roofing. In the event of snow and ice in the eaves area or in the collecting area of a snow guard, this could result in obstruction of the melt water. In order to avoid water building up, the installation of gutter heating is recommended, particularly in regions
prone to snow.

 

Fig. 4.1.15 Transverse joint and overlapping in roof panels; Source: [9]

  top

Fig. 4.1.16 Eaves design for roof panels in sandwich construction;

  top

Roof projections

In the case of roof projections from heated or air-conditioned rooms, a thermal section on the room-facing metal cover skin of the sandwich elements may be necessary
to interrupt the heat conduction. In this case, special attention must be paid to the load bearing capacity on this detail.

 

Fig. 4.1.17 Verge flashing with roof projection for roof and wall panels in sandwich construction; Source: [9]

  top

Fig. 4.1.18 Internal gutter between two roof surfaces in sandwich construction; Source: [9]

  top

Snow guard

The smooth metal surfaces of the sandwich elements favour self cleaning of the roof surface by rainwater, but unfortunately also slipping off of snow. In order to prevent obstruction of the eaves, the provision of a snow guard is a good idea in snow-prone areas, even with a relatively small roof pitch. Standard commercial snow guards can be
used for this. These are placed on the horizontal beadings and screwed down through the sandwich elements into the purlins. In addition, appropriately canted and perforated
metal sheet profiles or special steel pipes can also be used. In any event, it is recommended that the design of the snow guard, particularly the type of fastening onto the beadings, is agreed with the manufacturer of the sandwich system.

Lightning protection

If a room is enclosed on all sides by electro conductive surfaces, then the interior of this room will be shielded from electromagnetic and electrostatic fields ("Faraday cage"). Sandwich constructions, in which the external component surfaces consist of electrically conducting materials, form a "Faraday cage" under certain conditions. For this purpose,
wall and roof panels must be electro conductively connected together and must also have sufficient earthing. Rain gutters and attic components, as well as roofing with air insulation and wall components in galvanised steel sheets can also contribute to shielding against electric fields. (cf. also [1]).

Effect of the roof on the building environment

The heat insulation of the roof can be adapted to the respective requirements by selecting the insulation thickness. Particular importance is attributed to the juxtaposition to the roof. Implementation examples for different detail connections in the roof area are provided in the illustrations in this information leaflet. The relevant standards must be complied with for corrosion protection, sound insulation and structural fire protection of the roofing. The specific planning criteria for the use of sandwich elements for roofs are system-dependent. Therefore, the manufacturer's current planning documents are always mandatory.

Roof renovations

Sandwich elements are particularly suitable for roof renovations, as generally no change in the substructure is required, due to their light weight.