How far apart should decking joists be?

Decking can be a beautiful garden feature, as long as it’s installed properly. One term you’ll come across a lot is joist. But what is a joist, and how do you use them for decking?


Decking Joists

Simply put, a joist is a long piece of treated timber that forms part of the framework (or ‘subframe’) on which the decking boards are installed. The area your decking will cover will determine how many joists you need, but you should make sure they’re all spaced evenly. If the joists are too wide apart, the decking will warp because the timbers beneath it won’t be able to bear the load properly.

It's recommended to allow a 1% gradient that runs away from the building nearest your decking, and to put your decking boards lengthways atop the subframe, so that water dispersal is easier. This will help prevent water damage to your decking. You should also leave a 10mm gap between the first board/joist and the building to allow airflow.


Decking Joist Spacing

To ensure your decking holds up, the deck joist spacing should never exceed 350mm on centre. To get a more rigid feeling to the decking, space the joists out by 300mm or less on centre.

“On centre” means “between the centre point of the joists.” You can either measure each joist, mark its centrepoint, and measure between each one, or take your measurement from the outer edge of one joist to the inner edge of the next one. This should then achieve the correct spacing.

As you build, make sure you keep the boards even and in plane with each other.


Deck joist blocking

Once the decking joists are safely and evenly installed, it’s time for blocking. This involves putting smaller wooden blocks, or ‘noggins,’ in between each joist at a distance between 100 to 150mm of each other.

Deck joist blocking will help reduce movement and warping by the timbers, and twisting in the joists. Limiting the movement of the joists themselves will also reduce excessive gapping, and help towards a more uniform finished surface. Again, always ensure the joists, blocking, and any other parts of the framing are even and in plane with each other.

When it’s done right, all of this helps make your decking into the beautiful garden feature you deserve. For more information on proper decking installation, take a look at our installation resources.

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