Ground deck: What to consider when building a low deck

Are you building a ground deck that is 400mm or lower to the ground? One thing to be sure of is that a great deal of planning and risk mitigation is needed to ensure your low deck stands the test of time. The space beneath the floating deck will need adequate ventilation, the surface will require sufficient drainage as well as plans made for future maintenance and termite control access. Moisture is the enemy here.

Assessing the ground deck's location

The key factor about the deck's location is ensuring minimal moisture/water in the area. The deck will need to be sloped away from the attached structure to prevent water pooling. In some cases, you may need to install drainage pipes to ensure efficient removal of water from beneath the deck. A plastic membrane installed under the deck, topped with gravel or sand to secure it, effectively shields the timber from soil moisture and inhibits vegetation growth.

Choosing the right timber

For lower decks or those directly on the ground, the joists, posts and bearers should be of Durability Class 1 and H4 treated or better.

Suitable timber species can include;

• Spotted gum
• Ironbark
• White Mahogany
• Tallowwood
• Blackbutt
• Merbau (kwila)
• Other preservative treated Pine of H5 or higher

Before the joists, posts, bearers or boards are placed, it's important to seal all cut ends with a preservative depending on the final coating you plan to apply.

Decking board sizes

To prevent cupping and warping, decking boards sizes such as 90x19 or 86x19 are best. If you choose to have wider deck boards like 135x19, you need to ensure the width of the decking board does not surpass four times its thickness to avoid potential warping issues due to moisture changes. A narrow board ranging from 19-22mm thickness is the exception to this 4:1 rule as its narrow width mitigates the risk of water pooling on top of the decking board, causing cupping. We recommend a minimum spacing of 5mm between boards to allow for airflow.

Hardware considerations

For your decking hardware, galvanized or stainless-steel are best. The following steps are crucial to ensure the structural integrity of the timber:

• Secure wider decking boards with two nails per joist, 12mm away from the edges.
• Predrill to prevent splitting of the boards.
• Screws offer a more secure grip than nails and it is recommended to only use screws

We recommend using deformed ring shank nails or screws for treated pine joists.

Low deck construction stage

If the floating deck is extremely close to the ground, you may be able to make a "bearer-less construction" work - where the joists sit on a nailing plate on concrete beam walls. This method is suitable if the concrete doesn't block drainage. Alternatively, timber bearers could rest directly on well-drained gravel/sand or a concrete base, provided they're of at least Durability Class 1 or treated with H5 preservative. Again, ensuring water can drain is crucial.

Termite management and maintenance

A ground deck should only use timber that's resistant to termites. However, it is also crucial to safeguard the connected dwelling. Creating a suitable gap for regular inspection or installing a removable panel are some suitable options to consider when planning the deck build to mitigate the effort of maintenance and inspections down the line.

Before recoating, make sure the deck is thoroughly cleaned, especially clearing debris between boards and over joists. Avoid over-watering potted plants and use drip trays with small cleats to elevate the pots. Whenever possible, clean decks with a broom or blower instead of hosing them down.

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