Designed to give you a better understanding of specifying metal web joists in your projects, this MiTek-sponsored CPD module will discuss the material’s properties, alongside possible applications, relevant regulations and various design considerations. Deadline for completion Friday 3 November 2023.
Popular among architects, specifiers, housebuilders and contractors, metal web joists (MWJ) have long been used in the UK construction industry. Prior to their introduction, floor joists were solid timber or I-joists.
I-joists are typically made from an OSB web with the top and bottom rails made of timber. As services typically now need to be run through underfloor spaces, contractors (including plumbers, electricians and MVHR installers) are required to drill holes and cut notches to access services.
When contractors arrive on site to fit their pipes, cables or ducting through I-joists, they have to refer to a technical guide detailing where they can and cannot drill holes; this is to ensure holes are not drilled too close together as that could weaken the I-joists.
MWJs are a combination of timber chords (a warren truss constructed from stress-graded timber) and pressed metal webs that join to form an “open-web” joist. Primarily used for floor and roof joists, the design of MWJs allows for services to be passed through the metal webs, enabling installation without the need to cut and notch.
The timber specified is often TR26 due to it being the same grade of timber used in the manufacturing of roof trusses – which most web joist manufacturers also make. This can save money and help streamline the project’s supply chain.
Learning objectives:
- Knowledge of design considerations and potential applications when specifying metal web joists
- Understanding of relevant guidance and building regulations that apply to metal web joists
- Awareness of the benefits and potential issues to be overcome when designing and installing metal web joists
Design and application
Suitable for timber frame or masonry walls, MWJs either depend on a hanger to support them or else are built directly into the wall elements.
For detached projects, the joists can be built into the inner leaf of the external wall and sealed with mastic or sat into end-caps (for airtightness). For terraced or semi-detached projects, hangers tend to be used.
Left image: MWJs are bottom chord supported, sitting on the blockwork. For airtightness, silicone sealant is applied. Or the joists can sit in end-caps. Middle image: MWJs are supported on hangers (specified by the manufacturer). This application could be used in a party wall scenario. Right image: MWJs are top chord supported, sitting on the rim beam of a timber frame. Only MWJs can be top-hung. I-joists and solid timber cannot.
For timber frames or brick and block construction, horns can be specified to allow for on-site tolerancing (shown in left image) and a trimmable end can be specified where up to 130mm can be trimmed off the end (shown in right image).
For timber frame structures, the most common MWJ detail is the top chord support end detail. The end of the web hooks over the wall plate, which increases the strength of the timber and allows for a faster floor installation as hangers are not required.
The metal webs themselves are hydraulically pressed into position in the MWJs using bespoke equipment. The webs are made from high-strength, pre-galvanised steel to ensure a good lifespan.
The metal webs are only sold to licensed manufacturers, and MWJs are covered by a European Technical Approval ETA and designed in accordance with EC5.
When a MWJ exceeds a span of 4m, a strong-back timber should be incorporated into the design to counter vibration – making it more rigid and less susceptible under footfall.
The strongback is nailed to each of the vertical posts and then restrained back to the block with an anchor strap, which is nailed to the side of the timber.
At this point, any soil pipes or solid services would be fed through the MWJs before the final set of joists are fitted. All the webs are lined up in order to facilitate flexible ducting and all without the need to drill a single hole.
Metal web rafters
A metal web rafter (MWR) – suitable for both pitched and flat roofs – is simply a MWJ being used as a rafter, for example sloping, and with different loadings.
MWRs are stronger than solid timber rafters, meaning they can span further and require less internal support, thanks to their lightweight properties.
A range of depths and end details provide flexibility in their application, and the need for expensive hangers and metalwork can be eliminated through clever detailing.
The open web construction also allows for services to be easily routed through the rafters with no need for cutting, drilling or battening out.
Calculations
MWJs can be designed in a range of depths. The greater the depth, the further it can span and the greater the load-bearing capacity.
- The 373mm (14in) and 421mm (16in) deep joists would most commonly be used in a heavy load situation or a non-domestic application but can be used in houses for their spanability.
- The 225mm (9in), 253mm (10in) and 304mm (12in) deep joists are the most popular sizes for domestic floors.
- The 202mm (8in) joists would probably be used to marry up with an existing 202mm (8in) solid timber joist system.
The load-bearing capacity also increases when a greater width of timber is used. When working with your supplier, digital software can be used to calculate the size of timber required. Even the minimum width would allow for a secure fixing on both the decking above and the plasterboard ceilings below.
As the timber used is 47mm thick, one can rest assured that in the unlikely event of a long nail being driven though the decking and through the timber cord, the services being carried within the MWJ will either be clear of the nail or be able to deflect away.
In a staircase situation, a two ply MWJ will support the load at either side with a trimmer at the stair head. For greater load carrying characteristics or to carry the load from a hoist track in a care home, a three ply MWJ may be required.
A range of tested fasteners are available to fasten the joists at specified test centres. However, the loading and design should be left to the timber engineering professionals and their design teams – they will calculate the optimum timber size and position of the metal webs to withstand the requirements of the application.
MVHR
The increasing specification of mechanical ventilation and heat recovery (MVHR) systems and smart home systems are only adding to the services occupying the floor zone, meaning MWJs look set to continue growing in popularity.
The push for airtightness has increased the requirement for MVH – and with suppliers having little to no inclination to design the duct runs, it is left to the contractor or installer to accommodate these within solid flooring systems.
Bringing benefits
The most obvious benefit to specifying MWJs is that it enables services – whether it be pipes, cables, wires or ducting – to be run through an underfloor space without the need to drill holes or cut notches. This saves a significant amount of time (and money) during installation while increasing architectural design freedom for internal room layouts.
MWJs also support the retrofit agenda by providing much better suitability for future building modifications. But the benefits begin long before the material arrives onsite.
MWJs and MWRs are designed and manufactured offsite – in a controlled factory environment – and tailored to the requirements of each individual project. A controlled manufacturing environment ensures quality control, meaning joists are made correctly the first time, diminishing the need for reworking or lead times being compromised.
Making materials to measure also minimises material wastage and prevents any unneeded materials from being delivered to the site – adding to a project’s green credentials.
Sustainability push
MWJs can be considered an environmentally conscious choice of building material, firstly because of the material’s composition but also because it caters for thicker insulation and more energy-efficient systems.
If the industry is going to meet the Future Homes Standard 2025 — requiring a 75% reduction in CO2 emissions when compared with the level set in current standards — then the use of ground-source and air-source heat pumps is required.
The incorporation of photovoltaic technology is another consideration, and MWJs provide a suitable option for facilitating the additional requirement of pipes, cables and ducts needing to pass through underfloor space.
Timber boasts obvious sustainability credentials, and stress-graded timber is typically sourced from managed forests with chain of custody certification. Environmental product declarations aim to increase transparency in the sector regarding each manufacturer’s green credentials, and declarations should provide both the product’s global warming potential and its ozone depletion potential.
During the manufacturing of MWJs, timbers can be spliced together to create longer lengths, thereby using timbers that might otherwise have been scrapped. Offcuts from the manufacture of the roof trusses can also be used as the vertical end blocks in the joists.
The fact that MWJs use less timber than, for example, a solid timber joist yet are able to span further is a clear win for the environment.
Health and safety
Offsite manufacturing usually leads to quicker (and safer) installation on site, with the need for fewer contractors working at height being beneficial for health and safety requirements.
Individual MWJs are also lighter in weight than most alternatives, thus avoiding potential over-exertion by the installation team, and with no cutting required, there is also no need for circular saws and additional sawdust.
A number of MWJ manufacturers also offer the option of having a pre-delivery site visit, to check spans and dimensions and prevent any reworking, scrapping or oversupplying of materials.
Acoustic considerations
The acoustic requirements specified by the National House Building Council (NHBC) for England, Wales and Northern Ireland is 40dB, with Scotland having a more stringent requirement of 43dB.
As you can see from the table below, the 202mm joists that were tested achieved a 40dB rating. As the joists get deeper, their acoustic performance improves. The sound characteristics improve with depth due to a bigger distance between the top and bottom flanges and the sound having difficulty crossing the void.
One of the key reasons for creaking problems in floors is deflection. When a floor above the joist is subjected to a load, the floor joist will flex or bend slightly, and a bounce will be experienced.
For the full potential of MWJs to be reached, as with any structural system or component, a high standard of design and installation is vital. This means ensuring that the maximum deflection limits and other performance criteria are met to avoid unwelcome creaking.
The NHBC stipulates that deflection or vibration should be addressed by designing the floor in accordance with BS EN 1995-1-1 and its UK National Annex – or design to deflection limits, based on total dead and imposed loads for combined bending and shear of 0.003x the span, with a maximum deflection of 14mm where strutting is provided, or 12mm where strutting is not provided.
MWJs have the ability to deliver improved acoustic performance in floors versus softwood but reducing the transmission of impact and airborne noise can only be achieved with the right approach.
Regulations
BS EN 1995-1-1 2005: Design of timber structures – Part 1 (Eurocode 5) outlines guidelines that should be followed when designing timber structures and elements.
When dividing timber into strength classes, an abbreviated strength class based on both the species and the grade of the timber is used. Identification markings that indicate the strength class and strength grade of timber should be stamped onto the timber in order to comply with regulations.
The following material information is required by BS EN 1995-1-1:
- Strength class
- Species/species combination of timber
- Strength grade
- Reference to the timber standard to which the timber was graded.
The moisture content of timber should not exceed 20% at the time of construction and while in service. Individual moisture contents are allowed to be up to 24% at the time of construction, but the average should be less than 20%.
Guidance for the specification of a timber joist in domestic dwellings stipulates that:
- Where a span is greater than 2.7m, bridging is required at 1.35m centres. The depth of the bridge should be at least ¾ the depth of the joist.
- In the case of herringbone strutting, 36mm should be the minimum target dimension; never use herringbone strutting in cases where the spacing of joists is greater than 3x the joist depth.
- Lateral stability of joists should be supplied at the ends and all along the length of the span. Three ways of providing lateral support are: building joists into masonry; bridging ends of joists with cuts of joists; or using joist hangers that provide lateral support.
- In cases where the joist hangers used do not provide adequate lateral support, use end bridging to support joists laterally. Seek the hanger manufacturer’s guidance if this is the case.
- At each point of support, provide a minimum bearing of 50mm and partition point loads.
Alternatively, an engineer can design joists and potentially calculate smaller section sizes which are more economical.
Fire
Approved Document B of Building Regulations 2010 sets out fire safety requirements, with section B3 outlining the requirements to mitigate fire spread in structures.
Fire resistance is not just joist-dependent but is dependent on the overall floor or wall construction. For example, floors in a dwelling of up to 5m above ground require 30 minutes of fire resistance (REI 30).
For inner-city timber frame construction projects and in situations where a timber frame is close to neighbouring properties, it is necessary to have a category C flooring system along with a category C wall system.
The metal webs themselves are classed as non-combustible in accordance with 96/603/EC: Commission Decision and fulfil the requirements of class A1 according to EN 13501-1 2007. The flanges are classed as D-s2, d0.
Successful testing has shown that timber pre-treated with a proprietary fire retardant prior to cutting and MWJs manufactured with supplementary nails through the webs in specific places provide a FireSafe accreditation.
To achieve FireSafe+ accreditation, a Rockwool batt is nailed down on one side of the MWJ, preventing any fire from propagating across the underfloor space.
Specification process
There is a UK-wide network of MWJ, MWR and roof truss manufacturers – from Inverness in the north to Redruth in the south, meaning design teams can choose a manufacturer close to the project site or close to their offices.
As Revit is the software of choice for most architects and designers, it is simply a case of conveying the design file to a MWJ designer so that the walls can be traced to design the roof or floor of the building within the software.
The 3D model will be an accurate replication of the building, with exact truss or floor configurations shown. This streamlines the process to ensure any design issues are picked up at an early stage.
3D software also enables the building designer to receive calculations for the products being supplied, enabling steels to be specified, as well as loadings on walls and foundations and so forth. Allowing contractors to establish or confirm where specific details are will help in confirming the route of services and is critical in helping surveyors to confirm the correct installation.
To create or detail using a MWJ, head to the NBS National BIM library and download the required MWJ Revit dynamic object. This will enable the user to create a realistic MWJ within the Revit model prior to it being designed by the manufacturer.
Please fill out the form below to complete the module and receive your certificate: