Planning information for roof structures
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.
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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]
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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
Fig. 4.1.3 Formation of roof openings by structural measures; Source:
[6]
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.
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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]
Fig. 4.1.5 Tubular feedthrough with system components through a roof structure
in sandwich construction; Source: [9]
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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]
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.
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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]
Fig. 4.1.8 Implementation example of concealed fastening of roof panels;
Source: [6]
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]
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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]
Fig. 4.1.11 Monopitch roof in sandwich construction (without roof overhand),
vertical section; Source: [9]
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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]
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.
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Fig. 4.1.13 Formation of a transverse joint in a roof structure in sandwich
construction; Source: [9]
Fig. 4.1.14 Formation of transverse joint in roof panels; Source: [9]
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.
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Fig. 4.1.15 Transverse joint and overlapping in roof panels; Source: [9]
Fig. 4.1.16 Eaves design for roof panels in sandwich construction;
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]
Fig. 4.1.18 Internal gutter between two roof surfaces in sandwich construction;
Source: [9]
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.
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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.
Rolf Koschade
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