High performing, cost-effective floors can be easily achieved by using correctly designed and installed metal web joists. But what is a metal web joist? This article provides all the information you need to understand the form and function of these extremely useful timber products. 
They consist of two parallel chords, made from strength-graded timber, joined together with engineered steel webs. Popular with architects, specifiers, homebuilders and contractors, factory-made metal web joists are lightweight, strong and versatile.
Advantages of metal web joists
They can bridge greater clear spans than competing products of the same depth giving the building designer a large flexibility for room layout. Frequently used in the construction of floors – but also used in roofs – metal web joists are faster to install and more cost-effective than other flooring solutions. However, the most important benefit of a metal web joist is the easy access they provide for services, with no need to cut or drill holes in the joist. Pipework, ducting and cabling for heating, ventilation or electrics can easily be routed through the joist space long after the metal web joists have been installed.
Where and how to use them
Metal web joists and beams can create a variety of floor, roof and wall constructions. They can be combined with solid and engineered wood products or structural steel, either on site – to provide complete floor and roof elements – or via offsite manufacturing, to deliver fully assembled floor or roof cassettes.
Metal web joists are suitable for both flat and pitched roofs where, due to their span capability, they can provide an economical alternative to steel, concrete and other materials. While routinely used in housebuilding, metal web joists can also be used in commercial and industrial buildings constructed from timber frame, masonry, steel or concrete.
Sizes
Metal web joists are available in a wide range of sizes to suit almost any project. Actual dimensions are dependent on loads, spans and specified centres. While the technology exists to make extra-long joists, in practice length is limited by the practicality of handling and transportation. Metal web joists are manufactured offsite to your exact requirements to eliminate on-site wastage of both time and materials. However, if required they can be supplied with a solid trimmable end to accommodate any minor on-site variations.
Flexibility in terms of chord width, plus a variety of different depths allows metal web joists to meet customer needs in the most efficient way possible. Chord widths typically range between 72mm and 147mm; while joist depths range from 200mm up to 425mm. For more detailed information about sizes, see the Buyer’s Guide to Metal Web, free to download from the Trussed Rafter Association website.
Fire and acoustics
Metal web joists fully comply with all relevant standards in relation to fire and acoustics. Standard generic details have been developed to address the most common requirements of a design. These are available through the Robust Details’ handbook in terms of acoustics, and from your chosen metal web joist supplier in terms of defined 30- and 60-minute tested fire solutions.
Working with a metal web joist designer
We would recommend that anyone considering metal web joists should speak to a Trussed Rafter Association member to discuss specific needs as early as possible in the project. They can advise on the best option for your project, in terms of metal web joist design, manufacture and installation:
- Building and supply
The designer supplies detailed project information directly to the metal web designer, who then designs the metal web joists using specialist software, and who may take on some or all aspects of the overall floor design. Once approved, the components will be manufactured and supplied to site. - Design, supply and fit
Where required, in addition to design, manufacture and supply of metal web joists, the provider can also install them. - Design, construct (as pre-assembled cassettes) and supply
Metal web joists are constructed offsite, in the factory and delivered to site as pre-assembled floor units, termed cassettes. Being offsite manufactured, floor cassettes help reduce construction time and labour required on-site – improving build schedules and potentially enhancing build quality. A cassette floor can, for example, provide a floor to work from while working on the next floor up.
Communication is key
The building designer, typically the architect or structural engineer, has overall responsibility for the design and structural integrity of the building – they should provide the metal web joist manufacturer with all relevant information. They should ensure that the floor design is compatible with the supporting structure and that the floor is adequately connected to the walls.
Information sharing between the building designer and the metal web designer is key to the process. Essentially, the building designer’s main responsibility is to share dimensional load information related to the lateral restraint of walls via the floor. The metal web designer’s main responsibilities can be summarised as:
- Ensuring the safe support of all floor design loads
- Ensuring that loads are adequately transferred to the building structure
- The design of fixings between metal web joists and timber beams within the floor zone.
Defining correct loadings
The most critical aspect of any floor design is to ensure that the correct loadings are defined at the design stage. This will ensure a safe, effective, and cost-efficient floor design for the client. Design software allows the designer to accommodate both dead and imposed loads applicable to the use and occupancy of the building. The floor designer should be advised of any unusual loads which may be applied to the floor system, e.g. spa baths, snooker tables, hoists etc. Metal web joists can be designed to be supported on either the bottom chord or the top chord and can be built into supporting elements or connected via joist hangers.
Lateral restraint
Lateral restraint is essential to connect walls and floor elements. A metal web joist pulls the walls together, providing restraint. Specification of the correct arrangement for restraint is the responsibility of the building designer, but metal web joist designers can assist in identification and supply of both perpendicular and parallel lateral restraint metalwork.
Size and location of required floor openings
The location, size and type of stair or other openings within the floor area is one of the most critical aspects of design in order to transfer the loads and deal with the practicalities of creating the opening through the use of trimmers and girders or multiple metal web joists.
Floor vibration
Specific customer requirements regarding floor vibration or deflection limits should always be discussed with the floor designer. Herringbone strutting is not required with metal web joists. Where the span exceeds four metres a strongback is normally used to dampen vibration by increasing the stiffness of the floor and reduce deflection by load sharing. Trussed Rafter Association members can provide guidance on installation.
A metal web joist glossary
Chord – the top or bottom structural timber element of a metal web joist.
Clear span – the distance between bearing points at each end of the metal web joist.
Connector plate – the metal plate having integral teeth punched from the plate material. It is used for joining timber in one plane with no overlap. It will have an accreditation certificate and will be usually manufactured from galvanised steel.
Metal web joist – a parallel chord truss made from strength-graded timber, joined with engineered steel webs.
Metal webs – V-shaped metal plates having integral teeth punched to a patent design from the plate material. They are applied to both sides of a metal web joist to separate and support the top and bottom chords. Each web will have an accreditation certificate and will be manufactured, usually, from galvanised steel.
Nogging – Timber pieces fitted at right angles between the metal web joists to form fixing points.
Restraint strap – Metal component designed to tie joist and walls for restraint purposes.
Strongback – Used to dampen vibration by increasing the stiffness of the floor and reduce deflection by load sharing.
Trimmer – Additional metal web joists or pieces of timber used to frame around openings.
For detailed further information, see the Trussed Rafter Association’s Buyer’s Guide to Metal Web
