What is a Posi-Joist?
Posi-Joists (or more generically called metal web joists or engineered joists) are a structural system combining timber and steel webbing, designed for use in both floor and roof applications. Each joist consists of a top and bottom chord, end columns and steel webs positioned in between. Their benefits over traditional floor joists include their reduced weight, increased stiffness, reduced vibration, improved acoustic properties and ability to route services without drilling, notching or modifying in any way. Posi-Joists are designed in specialist MiTek licensed software and manufactured using state-of-the-art machinery, ensuring millimetre perfect accuracy every time and rapid install times.
How Are Posi-Joists Made?
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Designing the Joists
First, we take your dimensions & drawings if you have them, and begin designing your floor or roof in our specialist software. We will attempt to find the most cost-effective method possible whilst maintaining the exceptional performance that Posi-Joists are known for. When the design is complete, you will be sent a quote, design and 3D model to view and approve. You can view one of our 3D models below.
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Cutting the Timber
Now that the design is complete, you've had a good look at it and are happy, a file is sent down to our factory that tells our Hundegger saws what needs cutting. These saws can cut jobs in a matter of minutes and will automatically combine jobs to reduce wastage, decreasing the cost for both you and us.
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Assembling the Joists
Once the timber components are cut, they're moved to one of our posi presses. Our skilled assemblers will start by setting out the timbers of your Posi-Joist on the press. The timbers are then clamped by the press to ensure they are perfectly straight and the correct depth is maintained. The steel webs are then accurately positioned in place and pressed, completing the posi-joist.
The Benefits of Posi-Joists
Free Design Service
Our Posi-Joists are designed in specialist MiTek PAMIR software that is only licensed to specific manufacturers. When we design your roof or floor, we take into account the type of loads you'll be subjecting it to, the potential deflection (or the 'flex') of the joists, what is most cost-effective and much more. Every joist is manufactured under ETA 20/1169 approval (EAD 130031-00-0304) and UKCA/CE certification. Full Eurocode calculations are included free with every order or quote, giving Building Control what they need and saving you the cost of a structural engineer in many cases.
High Quality Timber
We use the highest quality TR26 graded timber for all of our Posi-Joists. This is a grade specific to structural products such as Posi-Joists and Roof Trusses, and is stiffer, straighter and has fewer defects than timber you'd find at most builders merchants. It is also PEFC certified (PEFC ST 2002:2020), ensuring all timber is harvested from forests that are responsibly managed and fully traceable. Almost all of our timber is from a Swedish supplier called Vida—you can watch the video below to see a recent trip to their sawmills and forests.
Posi-Joist Span Table
All spans shown are maximum clear spans in millimetres. Deflections limited to 14mm. Domestic floor loads only. Posi-rafter spans based on 30° pitch. This table is a guide and the actual spans may differ slightly once a full design & quote has been carried out.
Floor Joist Configurations
| Depth (mm) | Joist Width (mm) | Centres (mm) | Clearance Between Flanges (mm) | Max Span (mm) |
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Flat Roof Joist Configurations
| Depth (mm) | Joist Width (mm) | Centres (mm) | Clearance Between Flanges (mm) | Max Span (mm) |
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Rafter Configurations (30° Pitch)
| Depth (mm) | Joist Width (mm) | Centres (mm) | Clearance Between Flanges (mm) | Max Span (mm) |
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What are the benefits of using Posi-Joists?
Posi-Joists offer a number of benefits over solid joists or even other engineered joists. Posi-Joists provide an almost infinite number of service routing opportunities thanks to their open web design, leading to a reduction in labour costs and more flexibility when installing HVAC or MVHR systems. Thanks to the void they create, they can also be insulated for improved energy efficiency and offer better acoustic and vibration properties than solid joists. To top it off, they'll span further whilst weighing less, making them more manoeuvrable on site. These benefits make them ideal for virtually every project, from someone constructing a flat roof over their new garage in the countryside, all the way to large renovations of historic buildings in London.
How do Posi-Joists differ from traditional floor joists?
Unlike a traditional solid floor joist, Posi-Joists feature metal webs connected to timber top and bottom chords, providing service gaps between the webs and reducing the overall weight. Thanks to this clever combination of materials, they will also span further than a traditional solid joist.
How far can Posi-Joists span?
Posi-Joists can span up to around 10 metres, dependant on the amount of supports, the loads subjected to it, etc. For a rough example of clear spans, you can use this span chart. For a more detailed answer, feel free to send us your plans for a custom design (free of charge).
How much do Posi-Joists cost?
Posi-Joists cost from around £25 ex VAT to over £200+ ex VAT per joist depending on their span, depth and overall specification.
How much do Posi-Joists weigh?
Posi-Joists available on our website weigh between approx 11kg and 48kg, although for bespoke designs they can be lighter or heavier.
What's included with a Posi-Joist online order?
When you order Posi-Joists from us, all strongback bracing timber is included, along with a comprehensive design that is sent shortly after purchase. When the joists are delivered, you'll also receive laminated layouts to help with installation.
Are Posi-Joists treated?
The only treatment available for Posi-Joists is a pesticide that protects against the House Longhorn Beetle. It is only necessary if you live within the areas marked on this map.
As standard, online orders are not treated. Please let us know if you require this and we can add it on.
Can you drill or notch Posi-Joists?
No—Posi-Joists, once designed and manufactured, generally can't be modified (with the exception of a trimmable end which is designed to be cut down by up to 130mm). If you need to modify a Posi-Joist we have already supplied, please contact us and we'll try our best to help.