03.01 Construction

Categories: Buildability

Introduction
Cladding is an all-encompassing term for the external skin of a building which keeps out the weather and provides the building’s aesthetic effect. In low-rise construction it may support its own weight but self-weight and wind loading are normally transferred to the structural building frame. It may form the full thickness of the vertical envelope of the building but can simply be the outer layer with additional layers providing insulation and the internal lining.
 

Principles of operation
Apart from providing the external appearance of the building, the main function of cladding is to protect the structure from the weather particularly rain. This may be achieved in one of three ways as follows:

• Porous materials. Porous materials such as brickwork absorb water during rain and subsequently dry out. If the wall is of sufficient thickness and the permeability is reasonably low water will not penetrate during a rainstorm. In modern construction a cavity is normally introduced into the wall to provide an additional barrier to the passage of water.
• Sealed construction. Impermeable cladding materials will only permit the passage of water at joints. Sealing the joints with gaskets or wet applied sealants provides a continuous impermeable layer.
• Rainscreen. As its name suggests, the purpose of the outer rainscreen panels is to shield the wall from direct rain. The joints between the panels may allow some water to penetrate but an air gap and airtight backing wall behind the panels combine to limit this penetration. This may be achieved by the drained and ventilated method in which the air gap is continuous and well ventilated to encourage drying out. Alternatively the pressure equalised system may be used in which the gap behind the panels is compartmentalised allowing the air to be pressurised by the wind. The reduced pressure difference across the panel joints limits water penetration. Rainscreen cladding is covered in greater detail in Package 02.

Methods of achieving weathertightness are described in greater detail in Package 01, Envelope sealing.
 

Description of main cladding types
There are many types of cladding available, which are described below, grouped according to type of construction. Some of the categories are clearly defined but others cover a range of options and some variations could be considered to fall in more than one category. The distinction between curtain walling and some other cladding types is particularly blurred.

In some cases weathertightness will always be achieved using the same method but in other cases apparently small changes to the design of the cladding will change the cladding from a sealed façade to a rainscreen. It is necessary to appreciate the effect of such decisions on the design of both the cladding and the supporting structure.
 

Profiled metal systems
Profiled metal sheeting has traditionally been considered as a relatively cheap form of cladding for agricultural and industrial buildings. More recently with a wider range of colours and profiles becoming available its use has been extended to include retail, office, leisure and education buildings. Guidance on the use of profiled metal is given in BS 5427.

Profiled sheets of aluminium or galvanised steel may be used in various ways as follows:

• The simplest form is a single uninsulated skin supported on cladding rails spanning between the main structural columns.
• For most structures it will be necessary to incorporate insulation and this can be accommodated by using two skins of metal sheeting separated by a spacer bar and with insulation in the resulting cavity, as shown in this image.
• The need for sheeting rails and spacer bars can be eliminated by using liner trays which span between columns providing both the internal lining and support for the outer sheets. The liner trays can also be filled with insulation.
• Composite panels can be formed from two metal skins separated by a layer of rigid insulation. Mineral wool can be bonded to the skins with adhesive while polyurethane (PUR) or polyisocyanurate (PIR) foams can either be bonded with adhesive or extruded between the skins, and autohesively bonded to them under expansion. The panels are supported on sheeting rails and fixed together at the edges only, relying on composite action between the skins and the core to prevent flexing of the panel between the fixing points. The edge joints may be formed by lap joints where the metal sheet overhangs the insulation along one edge or by tongue and groove joints as shown in this image.

Systems typically use profiled sheets having a cover width of between 600 and 1000mm, and a length of at least 2m.  The depth of the corrugations ranges from 7mm to 120mm, and the wavelength/pitch from 30 to 350mm.  The spacing of fixings depends upon the wind load and flexibility of the sheet (e.g. depth of profile) and pitch (of roof cladding), but fixings are typically made every 250-300mm along the spacers.  A maximum fixing spacing is suggested as being 450mm.  Aluminium has a much higher coefficient of thermal expansion than steel and the thermal movement of aluminium sheets must either be allowed for in the end lap joint design or controlled by limiting the length of sheets.

The simplest fixing technique is to use self-drilling, self-tapping screws with integral sealing washers through the valley of the profile and into the spacer. However, the fixings can also be made through the crown of the profile, in which case an additional spacer may be used to prevent the profile from being distorted and the need for an excessively long fixing.  To join and seal the sheets together, stitching fixings are used along the side and end overlaps, and these may again be at the peak or trough of the profile.

A more sophisticated technique is to use a form of hidden fixing.  This is common with standing seam systems, which also tend to have much shallower corrugations between the seams.  Standing seam sheets lock into fixing brackets which are fitted along the spacers, and overlap the neighbouring sheet.  Because these systems are designed to hide the fixings, the sheets will be narrower, and there will not be any intermediate fixings.

Secret fix cladding systems offer greater weathertightness reliability and final appearance due to the absence of fixings that penetrate through the outer sheet.
 

Small cladding panels
Cladding panels vary widely in size and materials used. This section describes cladding panels that are too small to span between the main structural framing members and are either supported by a backing wall or secondary framing members.

Small overlapping units such as tile hanging and weatherboarding have not been included in this category. Most of the cladding panels described in this section may be fixed with sealed joints but may also be used as rainscreen panels. The method of achieving weathertightness will affect the design of both the cladding and the supporting structure.

These materials are most likely to be used for commercial buildings although they may also be used for over-cladding existing structures including blocks of flats.

Various forms of cladding panel which can be supported on timber battens or metal rails are available. This method requires a backing wall to support the fixing rails and it will usually be necessary to incorporate insulation in the wall, generally in the cavity between the cladding panel and supporting wall. This form of cladding may be used for new construction but is particularly suitable for upgrading existing buildings.

For new construction the supporting rails can be made of heavier section so that they can span between floors. This allows the use of a lightweight internal lining.

Panels may be fixed to the supporting rails using screws, rivets, structural adhesive, a screw-fixed pressure plate, or the edges of the panels may be folded, punched and hung onto pins through the supporting rail.

A range of materials may be used for the cladding panels as follows

• Fibre cement sheets are manufactured in thicknesses between 5 and 10mm and in sizes up to 1220mmx3050mm. Both cellulose and glass fibres may be used and a wide range of finishes is available including untreated, various types of paint and resin bonded aggregate. Sheets are normally supplied to site ready cut to size and with predrilled fixing holes. Similar panels may be manufactured using fibre reinforced calcium silicate, resin laminate and glass reinforced polyester.
• Panels may be made from both aluminium and steel sheet and may be given increased stiffness by folding the edges or adding stiffeners either within or at the back of the panel.
• Thin composite metal panels may be formed from two layers of aluminium separated by a layer of polyethylene giving an overall panel thickness of 3 to 8 mm. The composite action of the layers gives a stronger panel than the aluminium alone. Panels can be used as flat sheets but can be bent to form curved panels or folded to form sharp corners if the inner layer of aluminium is first cut along the line of the fold.
• Thick composite panels may be formed from aluminium or steel strip separated by a core of insulation. These panels differ from those produced from profiled metal in that they are manufactured as rectangular panels and may have flat faces. The edges of the panels may incorporate grooves to facilitate fixings, which can then be hidden by gaskets. Alternatively a pressure plate fixing system can be used.

Stone has traditionally been used as masonry to form an external facing material for buildings but is now being increasingly used as a non loadbearing cladding as a result of developments in stone processing which allow stone to be cut into thin panels.

Stone types used for cladding are granites, marbles, hard limestones, slates, quartzites, limestones and sandstones that offer a range of colours and surface textures with good durability. Stone is covered in detail in Package 13.
 

Large cladding panels
Cladding panels with sufficient strength to span between discrete fixing points on the main building frame, often as storey height panels, may be manufactured from reinforced concrete or as pre-assembled curtain wall. Glass fibre reinforced polyester and glass fibre reinforced cement were introduced in the 1960s and 1970s respectively but have now largely fallen out of use. Some composite metal panel systems may be used to span horizontally between columns and strictly fall into the group but in other respects are as described above.

Precast concrete can be used to produce loadbearing cladding panels but they are normally non-load bearing. Guidance on their use is given in BS 8297.

Precast concrete cladding systems come in three forms:

• Small units supported on brackets and used to fill gaps between conventional glazing systems,
• Larger mullion and spandrel units which ‘cloak’ the structural frame members, often to form a window opening within each bay.  Units are normally supported on bearing pads on the concrete floor slab, with packing shims providing vertical adjustment.  Horizontal restraint and adjustment is provided by angle brackets and adjustable bolts,
• Full bay-width, storey-height panels with cast (‘punched’) window openings.  Panels are of a sufficient size and stiffness to be able to span horizontally or vertically between structural frame members without requiring any intermediate support.

Panel-to-panel joints are either weather sealed with single or double wet-applied seals or left open (but baffled to prevent direct water ingress).

Concrete panels/units can be produced with a variety of smooth and coarse finishes or faced with factory-set natural stone, clay brick or tiling systems. They can also be made from carefully selected materials to give the appearance of stone.
 

Fully supported metal sheeting
Copper and lead sheeting may be used for cladding but are expensive and hence only used to a limited extent where required for appearance on prestige buildings. Due to its weight and low strength lead must be fully supported, usually by plywood boards. Due to its cost, copper is used in thin sheets that also need continuous support. Guidance on the use of different metals is given in Section 12.04.
 

Curtain walling
Curtain walling is a form of vertical building enclosure which supports no load other than its own weight that of ancillary components and the environmental forces which act upon it. Although the term is sometimes restricted to metal framed curtain walls, the above definition embraces many different construction methods and materials including non-loadbearing precast concrete.
 

Description of curtain walling types
The classification of types of curtain walling varies but the following terms are commonly used:

• Stick
• Unitised
• Panellised
• Spandrel panel ribbon glazing
• Structural sealant glazing
• Structural glazing

Stick system curtain walling
The general arrangement of a stick system curtain wall is shown in this image.  Horizontal and vertical framing members (‘sticks’) are normally extruded aluminium protected by anodising or powder coating, but may be cold-rolled steel (for greater fire resistance) or aluminium clad with PVC-U.  Members are cut to length and machined in the factory prior to assembly on site as a kit of parts: vertical mullions, which are fixed to the floor slab, are erected first followed by horizontal transoms, which are fixed in-between mullions.  Mullions are typically spaced between 1.0 and 1.8m centres.  Into the framework are fitted infill units, which may comprise a mixture of fixed and opening glazing and insulated panels (which may have metal, glass or stone facings).  These units are typically sealed with gaskets and retained with a pressure plate, screw-fixed every 150-300 mm, although hammer-in structural gaskets are used for some stick systems.  The pressure plate is generally hidden with a snap-on cosmetic cover cap or overlapping gaskets.  The screw fixings can be exposed by removing the cover, which is typically produced in six metre lengths for vertical framing elements.  Fixings must be secured to the correct torque to retain the glazing/infill panels and to ensure proper compression of the gaskets for weathersealing.

Stick curtain walling is very common and versatile and can be used for anything from ‘glass towers’ tens of storeys high to single storey shop fronts. Because of the number of joints in stick curtain walling it is generally very good at accommodating variabilities and movement in the building frame. It is also suitable for irregular shaped buildings. Assembly is slow compared with pre-assembled systems and performance (e.g. weathertightness) is dependent on knowledgeable installers who are familiar with the assembly and sealing procedures for the particular system. Some pre-assembly of stick curtain wall frames is possible by the use of ‘ladder frames’.

Many manufacturers (systems suppliers) produce standard stick systems.  Insulated panels, usually designed for the project, may be faced with anything from aluminium or steel sheet, to glass or expensive stone composites.  Some companies produce project-specific bespoke systems - either designing frame profiles from scratch for each job, or using standard details for some parts of the frame and simply altering some small aspect to give the appropriate structural properties or appearance.  The type, complexity and budget of the project will normally determine whether a standard (i.e. ‘off-the-shelf’) or bespoke curtain wall is used.

Stick system curtain walling may be erected in one of three sequences:

• Stick system wall, method 1. image
• Stick system wall, method 2. image
• Stick system wall, method 3. image

Unitised curtain walling
Unitised systems comprise narrow, storey-height units of steel or aluminium framework, glazing and panels pre-assembled under controlled, factory conditions, image.  Mechanical handling is required to position, align and fix units onto pre-positioned brackets attached to the concrete floor slab or the structural frame.  Unitised systems are more complex in terms of framing system, have higher direct costs and are less common than stick systems.  The smaller number of site-sealed joints in unitised curtain walling simplifies and hastens enclosure of the building, requires fewer site staff and can make such systems cost effective.  If construction joints interlock consideration must be given to how damaged units could be removed and replaced.  The reduced number of site-made joints compared with stick systems, generally leads to a reduction in air and water leakage resulting from poor installation.
 

Panellised curtain walling
Panellised curtain walling comprises large prefabricated panels of bay width and storey height, which connect back to the primary structural columns or to the floor slabs close to the primary structure, image.  Fixing the panels close to the columns reduces problems due to deflection of the slab at mid span, which affect stick and unitised systems.

Panels may be of precast concrete or comprise a structural steel framework, which can be used to support most cladding materials (e.g. stone, metal and masonry).  Structural steel panellised walls are known as ‘truss walls’ in North America. Aluminium or galvanised steel skins are generally fixed to the frame with insulation in the cavity. The wall construction is then completed by a plasterboard lining and external cladding.

Joints may comprise gasketted interlocking extrusions, gaskets between separate extrusions or wet applied sealant.

The advantages of using panellised systems stem from the high utilisation of factory prefabrication, which allows better control of quality and rapid installation with the minimum number of site-sealed joints.  However to be cost effective a large number of identical panels is required.

Panellised systems are less common and more expensive than unitised construction.  The size and weight of panels is limited by the practicalities of manufacture, handling, storage, transport and erection.

Some authors do not differentiate between unitised and panellised systems, but panellised construction may have significant internal steel structure to support the extra weight, or may consist of precast concrete panels with openings for windows.
 

Spandrel panel ribbon glazing
Spandrel panel ribbon glazing is a long or continuous run of vision units fixed between spandrel panels supported by vertical columns or the floor slabs, image.

Glazed areas may comprise:

• Several standard windows fixed together on site by joining mullions,
• Pre-glazed, bay width, factory-assembled frames, or
• Individual framing sections and glass infill panels which are site assembled.

Ribbon glazing is often used in conjunction with spandrel panels, that is, horizontally spanning prefabricated or precast concrete units.  It may also be used with spandrels comprising upstand walls faced with rainscreen panels. Care needs to be taken when detailing interfaces with adjacent elements.

Ribbon glazing/spandrel panel construction generally results in buildings having a horizontal banded or strip appearance.
 

Structural sealant glazing
Structural sealant glazing is a form of glazing that can be applied to stick curtain wall systems and windows, particularly ribbon glazing. However it can also be used in unitised and panellised systems.  Instead of mechanical means (i.e. a pressure plate or structural gasket), the glass infill panels are attached with a factory-applied structural sealant (usually silicone) to metal carrier units which are then bolted into the framing grid on site.  External joints are weathersealed with a wet-applied sealant or a gasket, image.  Structural sealant glazing is described in greater detail in Section 10.03.

These walls are attractive to architects as they offer a smooth or semi-smooth facade.

Structural sealant glazing has been used in the USA for around 30 years where it was initially site applied direct to the framing.  However, this is no longer acceptable due to difficulties of application and replacement and all structural silicone joints are now made in a factory.

Glass replacement/resealing must be undertaken in a controlled environment using the correct materials.  All elements used in the construction must be compatible with the silicone sealant.

Structural sealant glazing systems can have sealant on two opposite sides or on all four-sides with or without the weight of glass supported mechanically.  Generally, the glass is mechanically supported to reduce the size of the sealant bead.

Structural sealant glazing can be used to create a building exterior that is free from protrusions, but the framing system will be visible at night when backlit.  Structural sealant glazing is more widely used on ‘prestige’ buildings and may be produced as a standard system, or on a project-by-project customised basis.  The framing members are often more widely spaced than for traditional stick systems.

Any of the previous types of curtain walling and ribbon glazing could incorporate structural silicone glazed elements.
 

Structural glazing - bolted assembly
Sheets of toughened glass are assembled with special bolts and brackets and supported by a secondary structure, image, to create a near transparent facade or roof with a flush external surface.

A multitude of discreet or prominent secondary structures can be designed (e.g. space frame, rigging or a series of mullions) which support the glazing through special brackets.  The joints between adjacent panes/glass units are weathersealed on site with wet-applied sealant.  Furter information about structural glazing is given in Section 10.04 (Bolted connections) and Section 10.05 (Glass assemblies)
 

Structural glazing - suspended assembly
Here the glass is fixed together with corner, rectangular, patch plates and the whole assembly is then either suspended from the top or stacked from the ground and wet-sealed on site, image.

Suspended glazing systems utilise the minimum amount of framing for a given glass area and are used as glazing features on prestige buildings, but also for prestige atria on otherwise simple buildings.

Glass fins may be used to brace the assembly.  In some designs a light truss stabilises the wall and transfers wind loading, while the weight of the glass is transferred through the corner plates and suspension system
 

Curtain walling applications
Stick curtain walls are used on larger office developments but may also be used on some low-cost office or industrial units, typically for one small part, such as an entrance.  Unitised or panellised curtain walling systems are generally adopted where the additional expense of factory assembly is compensated by faster installation. They are only economic where a large number of similar units or panels is required. The highest-cost bespoke curtain walling systems will generally only be used on prestige buildings, large or small.

Choice of curtain wall type is never straightforward.  Dominant factors are:

• Cost
• Appearance
• Timescale
• Access limitations

The lowest cost is often achieved with a standard aluminium-framed stick system.  Generally, costs increase with complexity, although factory assembly also increases costs.  Increasing the number of non-standard items will increase cost, not only due to the additional ‘material’ cost but also due to additional design work required to integrate the component(s) within the system and possibly because of the need for project testing.

The importance of appearance will depend upon the desired image that the building is to project.  A building situated in a highly visible or prestigious location may demand the use of more expensive materials, perhaps stone-faced insulated panels or a structural sealant or bolted glazing system with no external protrusions to interrupt the facade.  A building facade may be designed to compliment, or contrast with, the surrounding built environment.

Time-scale is important because there may be contractual limitations on the time available for assembling the facade.  A site-assembled stick system has the advantage that installation can start quickly, but it may then proceed more slowly than with factory-assembled units.  However, systems requiring factory pre-assembly must be carefully planned so that units are available when construction of the facade is planned to start, but the units must not be manufactured or delivered too soon or storage costs will be incurred.  Note that whilst the smaller number of site-made joints in pre-assembled systems simplifies installation and weatherproofing, far greater attention to the manufacturing and erection tolerances of both structure and cladding is required.
Other factors that are important include the ease of maintenance.  Replacement of a glazing unit in a ribbon glazing system might be undertaken by a local glazier, whilst in a structural sealant glazing system this might require a specialist contractor, maybe the original contractor.  Systems such as structural glazing must be designed so that breakage of a glass unit does not cause progressive failure of the facade. This may increase the cost of these systems.

The architect may select several different types of curtain wall for a building - for example ribbon glazing at the back of the building, a standard stick system for the front of the building, and a prestige suspended glazing for the atrium.  Efforts should be concentrated on the construction interfaces during the design development and testing phases to reduce the risk of subsequent buildability and performance problems.
 

Masonry
Masonry is the predominant form of wall construction for low rise housing and is widely used in all types of building although on large structures it is often used for small areas with less labour intensive cladding materials being used for large areas of façade. The wide range of materials available means that it can be suitable for both low-cost industrial buildings and prestige structures.

Masonry is a composite construction of individual brick or block units built up in horizontal, overlapping layers (courses) and bonded and sealed with mortar (sand, cement, and lime or plasticizer). Bricks may be manufactured from clay, calcium silicate or concrete and blocks are normally concrete or stone.

Cavity wall construction is used almost without exception for external walls because it provides an increased degree of thermal insulation and protection against water penetration compared with a solid wall of the same overall thickness. In modern construction the external leaf is normally a non load bearing cladding 100mm thick and the units are chosen primarily for their appearance, durability and cost. The load bearing structure may be a steel, concrete or timber frame or an inner leaf of load bearing masonry.

The inner leaf of a cavity wall may consist of concrete blocks, concrete or an insulated panel typically consisting of a timber frame with plywood or plasterboard sheathing. Where masonry is used for the inner leaf the requirements for the unit are normally low density (for insulation), adequate strength and low cost. An inner leaf is typically 100mm thick but this may be increased to improve insulation or strength.  Thermal insulation (typically mineral fibre quilt) is often required within the cavity to comply with Part L of the Building Regulations.

In load bearing masonry and non-loadbearing low rise construction the masonry will support its own weight but the external leaf needs to be tied to the inner leaf and structural frame to give it lateral stability. In non-structural, multi-storey applications the weight of the masonry should be transferred to the frame at each storey level. In the past this was often achieved by supporting the wall directly on the floor with brick slips on the edge of the floor. Current practice is generally to support the wall on metal angles. These should be adjustable, particularly in the lateral direction to ensure alignment and adequate bearing of each panel of masonry.
 

Other types of cladding
Weatherboarding and tile hanging are traditional forms of cladding which are generally confined to housing. PVC and fibre cement panels are now available as alternatives to timber for weatherboarding. Tile hanging may use traditional clay or concrete tiles, or slates of natural stone or fibre cement.

Rendering may be used as a decorative or weatherproofing finish on masonry walls but may also be used on a lightweight background. Traditionally this would be wooden lath but this has now been replaced by metal mesh that may either be expanded metal or a lightweight welded mesh. This form of cladding is not widely used and is generally restricted to housing.

The cladding types described in this Section are established methods. New systems or developments of existing systems using new materials are continually being produced, a recent example being the use of titanium and terracotta.