What is Deflection & Why is it Important?

What is deflection?

Deflection refers to the amount a structural element bends or sags when subjected to a load—for example, when weight is applied to a floor joist. Imagine a wooden beam spanning between two supports; when you put weight on it, the beam will bend slightly. This bending is deflection, and is to be expected with all beams/joists, however, it's important to control the amount of it to ensure your building is safe.

Why is it important?

Excessive deflection, particularly within a floor or roof joist, can cause permanent damage or even failure. This could be as little as a small crack, all the way to a complete failure, leading to the floor collapsing in on itself (although you'd have to try pretty hard for this to happen). 

Too much deflection can also be visible in a room, leading to the floor sagging in the middle like a big trampoline. Speaking of trampolines...

It can also cause your floors to feel bouncy. You've most likely walked around an old home and noticed the floors vibrate as you stomp around. We can probably forgive this on joists that are 500 years old, but you don't want it in your new home. 

How is it controlled?

When we design your roof or floor, we do so in specialist software that takes into account the amount of joists you need, what they're resting on, what you plan to put on them (i.e. a water tank in a loft will require additional loads), what type of roof you have, and so on. Once we've designed your project, the software will provide us with a list of calculations for you to submit to building control, including the deflection. We limit our deflections to around 14mm, ensuring your floor is safe and fit for purpose, however, this is fully adaptable and can be made even stiffer if required. We can do this by increasing the depth of the joist, placing them closer together, increasing the timber size, running the joists a different way, and so on.