What is a joist?

A joist is a horizontal structural component used in framing to span an area. When used in a floor framing system, it provides security and support.

What material can I use for joists?

There are several materials that you can utilise to build the joists for your decking substructure. The first option would be timber. Timber joists must be treated if you are using them for decking projects. This is because they will be exposed to rain and moisture, which can lead to rotting over time.

Composite decking joists can also be utilised, as they are a more durable option to choose from. You can purchase 50mm x 50mm composite joists and use them for ground-level decking projects. You can also use 100mm x 50mm joists if you are building an elevated deck.

How do I keep my joists from rotting?

If you are installing composite decking, you are first going to need a strong substructure for the decking boards to be fitted onto. A frequently asked question that we hear on a weekly basis is, “Do I need composite joists for my composite decking?” Having composite posts and joists would definitely serve you with a more durable structure; however, they are roughly 4x the price of standard C16 joists. For this reason, many opt for timber frames.

A composite frame will not require any treatment over the years due to its weather-resistant properties; however, a timber frame will require some upkeep.

What can I do to treat my timber frame?

Given that the joists are the foundation and will be coming into direct contact with the ground, they are more susceptible to rain or water. Water/moisture are major causes of rotting on decks. In some cases, a timber frame can last years without any treatment at all. However, we would recommend you treat it at least once a year to minimise the chances of any structural defects in the deck.

One way you can stop the wood joist from rotting is by using pressure-treated wood as the substructure of your composite decking. Treated wood contains chemicals that prevent moisture absorption. Also, treated wood is firmer and stronger than untreated wood. Even when you use treated wood, you have to seal the lumber so that moisture will not enter it. Sealing your composite decking’s wooden joists will prevent them from rotting. 

One way you can treat the timber joists is by using a product such as Boron Ultra Gel and a piece of damp-proof material such as DPC plastic. By placing this under the joist, you will prevent the dampness from being transferred. 

This simple and quick practice will prevent the deterioration of the frame and ensure that you have a strong deck that will last for many years.

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