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What is Safe Working Load and why does it matter?

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If you’ve ever been on the hunt for wall fixings or hooks for a DIY project, chances are you’ve come across the term Safe Working Load. The same term is also sometimes used in engineering, for example in relation to cranes, hoists or winches, so you may have heard it in another context too. But what does it mean when it comes to picking wall fixings and why should you care? Let’s take a look. 

What does SWL mean and how is it calculated?

Safe Working Load is specified by the manufacturer and is generally defined as “the mass or force that a piece of lifting equipment, lifting device or accessory can safely use to lift, suspend, or lower a mass without fear of breaking.”  

It’s calculated by dividing the Minimum Breaking Strength (MBS) also known as Minimum Breaking Load (MBL) of the material by a safety factor, which is assigned depending on type and use of the equipment and tends to usually range from 4 to 6, except if failure would cause risk to someone’s life, in which case the safety factor is normally 10. For example, if the Minimum Breaking Strength of the material is 200kg and the safety factor is 5, it means the SWL is 40kg.  

The safety factor tends to vary between fixings, but for example for Corefix fixings the safety factor is 7 for pull out (tensile) force and 4 for shear force (more on these below!) 

Great, so what does all that mean? In a nutshell, it means that the Safe Working Load describes a force that is much less than the force needed to make the fixing break, but it’s the most weight it can hold safely.  

Safe Working Load = MBS/SF

Is it the same as maximum capacity?

Yes and no. It’s the maximum load the fixing can hold safely, but SWL isn’t actually used to mark the maximum capacity of anything, as it’s too vague and leaves a risk of legal issues.  

For example, Corefix’s Safe Working Load is 100kg on 4 fixings. The fixings have been tested to 250kg without failure, so it’s technically possible for them to hold more, but the installation would no longer be safe so we still recommend you stick to the Safe Working Loads! 

So I can just pick a fixing with a SWL greater than the weight of the item I’m hanging?

The short answer is yes, but it’s not always that simple, as fixings sometimes have different SWLs in different wall materials. For example, Fischer DuoPower can be used in multiple wall materials, and its SWL in 12mm plasterboard is up to 15kg, but in solid sand-lime brickwork it can hold up to 127kg! 

Even with plain plasterboard fixings, the SWL can vary depending on the thickness of your plasterboard. For example, Cobra WallGripper can hold up to 27kg in 10mm plasterboard and up to 38kg in 13mm plasterboard!  

Tensile vs Shear Load

In the case of wall fixings, the safe working load is usually referring to the force pulling straight down parallel to the wall, also called the shear force. And when you’re hanging something close to the wall, like a mirror or radiator, it’s really the only force you need to worry about. But what if you’re hanging something off the ceiling or installing a cantilever TV bracket (ie a TV that swings out from the wall)? That’s when it gets a bit more interesting as you’ll also need to consider the tensile force/pull force (pulling outwards/at a right angle to the fixing.)  

When you hang something from the ceiling, such as a plant pot, the main force at play isn’t the shear force, but the tensile/pull force, as the weight of the plant pot is pulling the fixings out of the ceiling rather than along it. 

The safety factor used to calculate a Safe Working Load is often higher for tensile strength compared to shear strength (like with Corefix mentioned above!) – this is because sufficient pull outwards would cause the fixing to fail suddenly, whereas in the case of increasing shear load it would bend but not necessarily break. 

The Safe Working Load marked on fixing packets usually means the shear force, but due to the higher safety factor the tensile SWL for the same fixing is usually much less, meaning that even if your hook can hold 20kg when attached to a wall, it won’t be able to hold the same in the ceiling! 

If you’re lucky, the ceiling SWL will also be displayed on the packaging, such as with Cobra Versahook, which is intended for both wall and ceiling use. Otherwise, you’ll need to look for the tensile SWL on product datasheets from the manufacturer to make sure your hanging plant pots don’t come crashing down! 

Similarly, if you’re installing a cantilever TV bracket, you’ll need to think about the tensile SWL. When the weight of the TV is close to the wall, the main force affecting the installation is the shear force, but as the weight of the TV moves further away from the wall, the tensile force increases and begins pulling the fixings out of the wall. So even if your fixings can hold a 20kg wall TV close to the wall, it doesn’t mean it’ll stay up further away from the wall!  

This is why you should always test the security of the fixing before mounting a heavy TV with a cantilever bracket, regardless of what your wall is made of and what fixings you’re using. You can also always get in touch with us to discuss – use our live chat or call us on 01248 295022! 

In a nutshell, you don’t want to ignore the Safe Working Load on the fixings you’re using for your project, but you’ll also need to consider your wall material and the shear and tensile forces affecting your installation. We hope this article has cleared away some of the mystique! 

If you still have questions or feel unsure, we’re here to help – call us on 01248 295022, get in touch over live chat (Mon-Fri 9am-4pm) or drop us an email on sales@wallwizard.co.uk!