- A Rainscreen system consists of an outer
panel, a ventilated cavity and an inner leaf
- In driving rain conditions
moisture forms a membrane across the baffled vertical and horizontal
joints
- The majority of water is deflected
off the outside face - any penetrating water is disposed of through
drainage
- Rainscreen systems differ from
brick wall sealed construction as the beneficial effects to air movement
are utilised
- A Rainscreen system is ‘pressure
equalised’ - the joints are open or lightly baffled, allowing pressure
equalisation in driving rain conditions to be instantaneous. Pressure
inside the cavity is equal to pressure outside - ie, precipitation has
no inclination to be driven into cavity
- A continuous vertical cavity –
Usually min 30 mm or for NHBC schemes 38 – 50 mm
|
| The
advantages |
- Installation is simple - allowing
external cladding and internal works to proceed speedily, early and
consecutively
- Problems of deterioration are
halted with minimal additional load being applied to the existing
structure (referred to as Overcladding)
- Rejuvenation of external appearance
- Use of dry trades not wet trades –
Modern Methods of Construction
- Energy saving - lower running costs due
to greatly improved thermal insulation
- Easily removed panels for monitoring of
structure
- Reduction of the risk of condensation
due to the elimination of cold bridges
|
| Combinations of
cladding are increasingly popular – This creates challenges when designing
the support system. The skin or façade of a building is one of its most
distinctive elements. Behind every beautiful façade is a support system
which has been engineered to suit that particular building’s conditions,
from its geographical position and its height to its structural material and
the weight and size of its cladding panels. |
| The basics |
| The weight of
cladding elements can vary enormously from a modest 5.6 kg per m2
for composite panels up to tile materials weighing in at c. 110kg per m2.
The building’s location is of key importance too due to wind loading; an
exposed coastal site will be very different to a low rise city centre
project. |
| These factors are
of key importance for determining the support system for the façade, but
then that can have a knock-on effect on the main structure of the building,
particularly with light weight steel framing which may be reduced – or
alternatively beefed up – as a result of the cladding and the way it is
attached. |
| A simple cladding
support system typically consists of ‘helping hand’ brackets, which are
fixed to the substrate at set vertical, and horizontal separations. Into
these attach profiles, usually vertical ones, which are often an ‘L’ or ‘T’
shape in section, although they can be more complex to accommodate different
cladding panels. |
| Cladding may be
visibly face fixed, normally riveted onto aluminium support profiles.
Architects are currently tending to prefer concealed fixing either
structurally bonding cladding elements onto the vertical support profile
[see diagram A] or if the specifier requires a mechanical concealed fix then
an additional horizontal grid and cleat system can be adopted |
| The vertical
profiles attach to the brackets by a combination of fixed and flexible
points to allow for dead loads and for dynamic load e.g. thermal movement.
Flexible points are vital due to the differing thermal performances of the
materials being combined. Take aluminium and high pressure laminate (HPL).
HPL panels are made from wood pulp and resin. In the winter the aluminium
will be at its minimum length, whilst the HPL will have taken on water and
will have expanded. In the summer, the reverse is true. Aluminium expands by
1.0 mm per linear m per 100 degrees C and in the UK temperature could vary
from -10 degrees C to 40 degrees C or possibly more. |
|
Specification |
| There are certain
key elements that a façade support system specialist needs in order to
design a system. First the type, weight per m2 and intended
layout (portrait or landscape) of the façade material. Second the preferred
fixing method: whether concealed or visible fix. Thirdly details about the
building, its height, window heights and its geographical location (in order
that wind loads can be determined). In addition it is essential to
understand the type and condition of the substrate to which the system is to
be fixed (steel or block / masonry), depth of insulation and the overall
cladding zone (including the correct ventilated cavity) |
| From this basic
information, a specialist façade engineer can calculate the type, ‘mix’, the
optimum horizontal and vertical spacing of brackets required for the bulk of
the building and also for more extreme conditions / special areas (such as
corners and under windows). Fixed point brackets must be able to withstand
the weight of the cladding panels and the wind suction, whereas flexible
point brackets have only the wind suction to withstand. The spacing of
brackets required will vary depending on the area of the building. Generally
more fixings are required at the edges and corners of buildings and around
window openings. |
| Often the
architect’s specification will consist only of the support system type and
some basic information about bracket spacing, or in some cases there will be
no mention of a support system. Some clients or contractors choose to employ
a specialist envelope designer to ‘fine tune’ the design and layout |
| Procurement |
| When assessing a
tender from a specialist contractor, package managers need to be aware of a
number of things. |
| Check that the
support system has been engineered to BS 6399-2 which covers wind loadings
of buildings. Make sure that the architect’s aesthetic intent can be
achieved, for example by concealing the fixings. |
| The major
practical consideration for contractors and installers is ‘buildability’ -
Will the size of the panel allow it to pass through the scaffolding? An
architect may envisage a large ‘monolithic’ panel - 3.6m by 1.2m. The panel
could weigh 130kg each. If you’re installing that on a windy day with a
tower crane, you will struggle so panels may need to be split down to assist
with installation |
| Control of heat
loss is of key importance, particularly since the introduction of the
Part L and the drive towards carbon neutral buildings. Procurers should
check that the detailed design allows the thermal requirements to be met.
For example check that the system limits thermal bridging by the inclusion
of insulation pads which sit between the bracket and the structure prevent.
(These pads also prevent a chemical reaction occurring between an aluminium
bracket and lime in a cement frame) |
| By the time a
cladding package is being procured, the type of cladding may have changed
from what the architect originally specified. In these situations it is
important to check that the calculations which determined the type and
design of support system still hold true. |
| The situation may
arise that a cheaper cladding material has been specified, but that it is
heavier than the original. In this case larger brackets or closer spacing
may be required. Alternatively, if the material is lighter, a lesser system
may be sufficient. This will lead to some savings in material and possible
savings in time if there are less fixings or a simpler support system. |
| Eurofox On
site |
| When it comes to
the installation stage, one of the biggest challenges faced by systems
suppliers such as Eurofox is short lead times. Specialist contractors are
unwilling to invest in materials until they are certain that their section
of works is about to begin. Preplanning which allows contractors to have a
certain start date and therefore give suppliers a reasonable lead-time helps
here. |
| Tolerances are
also an issue at this stage, since the cladding system must accommodate a
steel or concrete structure which is not bang on. Here a support system
which can cope with potentially large differences is very helpful. Eurofox
can accommodate cladding zones from 40mm to 380mm in 1mm increments. Systems
are designed with a factor of safety .If tolerances are far outside
specification, it may be necessary to check the calculations to ensure that
additional brackets are not required. Package managers should also check
that the correct materials and components are being used. For example,
stainless steel screws are required for aluminium because a galvanised steel
screw would cause the aluminium to corrode. It is also important to check
that inferior products have not been substituted, as these may not have the
traceability of the originally specified component. Aluminium systems should
be manufactured in accordance with |
| In
conclusion |
| There are no set
standards for procuring cladding support systems. Every application is
different and requires some engineering input. |
| Systems, which are
simple and logical to use, will lead to less errors and swifter
installation. Cladding is a serious business – Support systems need to be
‘fit for purpose’ |
| Eurofox can be
contacted on 01707333396 |
| see
www.eurofoxengineering.com |
