How to rectify a notched joist?

JockWick

Hi there, the previous owner notched the floor joist of the 1st floor bathroom around 2cm deep to give way to lower shower floor. There are about 3 joists that are notched between 0.5cm - 2cm for around 1m length of the joist, The length of the joist is perhaps 5m length. So 1m of those 5m are notched 0.5cm, 1.2cm and 2cm.

Is there any steel plate I can use to strengthen the notched joist that is still in compliant with Australian standard?

MitchellMc

Welcome to the Bunnings Workshop community @JockWick. It's brilliant to have you join us, and many thanks for your question about repairing joists.

I’m not aware of any off-the-shelf steel plate systems that would suit this situation and still be considered compliant with Australian Standards for timber floor joists. In most residential settings, steel plates are not typically used to repair notched joists over that sort of length, and even if something suitable could be fabricated, it would almost certainly require engineering sign-off to confirm compliance.

A far more common and practical solution is to sister the joists. This involves fixing an additional length of timber alongside the existing joist so it spans beyond the notched section and into the unnotched timber. Doing this allows the load to be transferred away from the weakened area. In your case, a piece around 1.5 metres long or longer would be sensible, ensuring it extends well past the notching. Ideally, the sistered timber should be the same depth as the original joist and fixed using structural screws or bolts at appropriate spacing.

It’s also worth keeping the role of the timber in perspective. These are floor joists, not bearers. A bearer supporting walls or significant structural loads would be a very different story and would warrant engineering input straight away. A joist, on the other hand, is primarily supporting the floor and typical bathroom loads above it. Notching up to 20 mm out of a hardwood joist over a limited length is unlikely to be critical, particularly over a 5 metre span, assuming no unusually heavy loads are being added.

That said, sistering the joists is still a good idea. It will stiffen the floor, reduce any potential flex in the notched area, and provide extra reassurance going forward. If absolute compliance or certification is required, a structural engineer would need to assess the situation and specify a repair. For a practical and commonly accepted fix, laminating additional timber alongside the affected joists is usually the best path forward.

Please let me know if you have any questions.

Mitchell