All you need to know guide to Posi-Joist

by Ofentse Sefolo
All you need to know guide to Posi-Joist

Combining the lightness of timber and the strength of Posi-Strut steel web, Posi-Joists offer an innovative solution for the design and construction of flooring and roofing systems. The effective reduction of costs, alongside the maintenance of high standards of design and workmanship, is a key objective for architects, engineers, project managers and specifiers. Posi-Joist is one solution that can help you achieve these two goals.

Posi-Joists are parallel chord trusses consisting of plywood boards and MiTek’s patented Posi-Strut steel webbing. They are engineered joists that can serve as the basis for quality roof or floor structures with longer spans than alternative timber products. Each Posi-Joist is made to measure, as per the client’s specifications, resulting in a product that is custom made for each project, and providing flexibility that solid joist alternatives simply cannot. Let’s take a closer look at the benefits of Posi-Joist.

Benefits of Posi-Joist

Posi-Joist combines a range of benefits in the areas of cost-efficiency, flexibility and performance.

–         Cost-efficiency: The open-web design of Posi-Joists, together with their ability to span greater distances than standard joists, allows for several remarkable cost-saving benefits. The span capabilities  allow for increased joist centres when compared to other systems, reducing the amount of material needed for the job, and often reducing the need for load-bearing intermediate walls, which reduces overall costs substantially. Whether used in roofing or flooring systems, Posi-Joists enable much simpler installation and maintenance of services and utilities systems such as plumbing or HVAC ducting, reducing both labour costs and on-site build-up time.

–         Performance: Posi-Joists provide excellent flooring and roofing performance, providing a wide fixing area that makes the job of attaching floorboards and ceilings much easier. The unique open-web design creates a large service void in which plumbers, electricians, and HVAC contractors will find it easy to work. Posi-Joists are extremely lightweight, which means they are easy to handle, lessening the physical burden on construction teams.

–         Flexibility: Design flexibility is the main inspiration behind the concept of Posi-Joists. There is no limit to the variations of which the product is capable when it comes to length, width and depth measurements. Posi-Joists are custom built, according to your needs, so there is no limitation on the spaces you can design and build. It is also possible to create a variety of support conditions, such as trimmable ends, which incorporate solid timber blocks which can be engineered to provide 130mm allowance at each end, in case you need to trim and adjust on site. The wide range of end details available can eliminate the need for expensive hangers and metalwork. Most importantly, the Posi-Joist design works just as well for roofing rafters as it does for flooring joists.

Different types of Posi-Joist

As has already been discussed, the Posi-Joist concept is a versatile system that can work just as well for roofing as for flooring. In addition to the simple, straight, variable-length flooring joists, Posi-Rafters, a design variation suited specifically for roofing, have also been introduced. Posi-Rafters are suitable for both pitched roofs and flat roofs and are stronger than solid timber rafters, which allows them to span further with less internal support.

There is also Posi-Attic:  trusses which combine the solid feel of a Posi-Joist with the benefits of a conventional attic truss. These allow you to build attics with greater living or storage space, increasing the value you can deliver to your clients. What’s more, Posi-Joists can also be used in wall applications.

How to find the suitable Posi-Joist for your project

Aside from efficiency and cost-reduction, the primary attraction for Post-Joists and Posi-Rafters is their versatility. Since each Posi-Joist is made to measure, there is no need to seek out specialized joists and rafters for specific design needs; you can simply have them made to your requirements. The simple to answer to the question of how to source your idea Posi-Joists is that you don’t find them – you create them. There are different standard depths and widths of Posi-Rafters and Posi-Joists available. Which of these you choose will depend, not only on your required span, but also on other factors, including the kinds and number of supports you will have in place, the clearance you need between the flanges, the overall span of the area and the size of the load the rafters or floor joists will be expected to carry.

To help you create the perfect Posi-Joists or Posi-Rafters for your project, you can make use of MiTek’s industry-leading design and layout software which enables you to make project-specific designs. Detailed drawings can be provided for approval and manufacture and, with the use of the 3D viewer tools, you can share your designs and specs with whomever you choose. The viewing platform is fully interactive and can be viewed on any internet enabled device. In this way, designs can be shared with all essential project participants from the architect and project manager to the Posi-Joist manufacturing team. There are also charts and calculators available to help you work out the ideal span of your Posi-Rafters and Posi-Joists.

Ceiling joist span chart

You can use our calculator and joist span charts to help you select your Posi-Rafters. When you enter your chosen span in millimetres, a chart is generated showing you all the available options, including the types of steel web, and appropriate depths, widths and deflection to suit your chosen span.

What is the longest joist span?

The question of what the maximum joist span should be is not a simple one to answer, as it factors in a number of variables, including the width and depth of the joists, and the load they will be expected to carry. What is certain is that Posi-Joists and Posi-Rafters can span further than alternative solid timber products. If you wish to construct a floor or roof with a longer span, you can simply choose a Posi-Joist with a larger width to provide greater support across that span. There is an upper limit to the span that a single Posi-Joist can have, however. Our largest Posi-Joist, with a PS-16 Posi-Strut, a depth of 421mm, a space of 400mm between centres and a timber width of 147mm, can span a maximum of 8,3m. This is almost double the maximum span of most solid timber products.

What is the difference between Posi-Joist and Posi-Rafters

Posi-Joists and Posi-Rafters are very similar in design and construction but different in application. Posi-Joists are intended for use as floor joists, while Posi-Rafters are created for roofing structures. Both are parallel chord trusses that include the same lightweight structure and incorporate Posi-Strut steel webs.

Posi Joist – the economic choice

Posi-Joist will undoubtedly cut costs on any construction project. An initial linear metre cost comparison may suggest otherwise, since Posi-Joists cost more per unit than your average solid timber joist of roofing beams, but you will make up for this in overall savings on materials and labour.

Posi-Joist’s ability to span further than its competitors means that you can increase the joist centres and reduce the overall material content. Once the joists are installed, they make the job of service and utility installers much easier with their large, comfortable service voids. Thanks to the improved working space, plumbing and HVAC installers will spend fewer man-hours on the job.

Posi-Joists and Posi-Rafters are manufactured offsite and then delivered made-to-measure, making them easy to install. Also, because they are so lightweight, construction crews can carry, position and install much easier, which also cuts down on labour hours. Considerable material costs reductions are achieved because Posi-Joists eliminate the need for loadbearing intermediate walls. With fewer of these in your design, the overall estimate for your project can be reduced dramatically.

Installing the Posi-Joist floor

Installing Posi-Joists is extremely easy. Our installation guide, which you can download from our website, provides easy-to-follow pictorial guides to installing your Posi-Joists with masonry wall connections, soil pipe corners and internal loadbearing walls, as well as timber walls. Accessories such as packers, trimmers, noggins, restraint straps and strongbacks can be used to reinforce and brace the structure. The use of these is also dealt with in the guide. We also provide detailed instructions on the storage and handling of the Posi-Joists, which should not be kept in storage on site for extended periods.

Posi-Joists and Posi-Rafters are an excellent choice for construction projects where cost reduction and building speed are important factors. Their benefits will be particularly useful in affordable housing projects, and the construction of floors that will require the installation of HVAC and other services. They also provide a lightweight and affordable alternative to both concrete and conventional timber flooring.

MiTek is a global company that was founded in the United States in 1942. It now has branches in several countries, including South Africa. It is a comprehensive provider of integrated solutions for building, including the innovative flooring and roofing products Posi-Rafter and Posi-Joists. For more information on how these products can improve your construction projects, contact us.

 For more information on Posi visit: https://www.mitek.co.za/products/posi-joists/

MiTek Industries South Africa (Pty) Ltd

Office: +27 11 237 8744

Mobile: +27 82 463 9810

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