How to Avoid Expanding Decking Boards

Q: I installed a deck and had to return soon after to fix it as the decking boards had expanded. The decking is 135×19 Spotted Gum and I believe should not have moved as much as it has. The decking is an extension to the rear of an older home that covered a lawn area. B Nelson, Hampton.

A: After viewing the many photos, I can tell you it’s not the timbers fault. There is no ventilation opportunity in this deck and this is the disastrous result. All the sides of the decking have been enclosed with more decking as baseboards with similar narrow gaps.

The decking is also very low to the ground, approx. 200mm from the underside of the decking. There is expansion of the decking boards, tenting of boards, broken fixings, cupping and mould growth on the underside.

Rainy days quickly increase the moisture content in this limited subdeck. Restricted ventilation prevents the subdeck space from drying, leading to decking expansion as it
absorbs moisture. When ventilation is restricted the subdeck space cannot dry and leads to decking expansion as it takes up moisture. Subdeck ventilation is having a good cross flow of air beneath the decking that runs from one side to the other always offering fresh air.

A 3mm gap spacing for 135×19 mm decking is insufficient. The gaps between decking boards are there to allow for the natural expansion of the timber during rainy periods. Our Decking Guidelines call for wide decking boards up to 140mm to have a gap spacing of a minimum of 6mm.

Decking up to 90mm wide requires a minimum 4mm gap spacing. If 135×19 decking is low to the ground or faces poor ventilation, increase the gap spacing between boards to 8mm.

These decking boards have expanded an additional 2-3mm each. This rate of expansion indicates that the moisture content of the decking is extremely high. It’s not uncommon to find moisture content readings of over 20% in these situations.

The problem worsens when the closed gaps cause the decking to hold water on the surface. Even though the surface is able to dry, the underside will remain wet for longer from trapped moisture. This is what causes cupping in decking.

Removing soil beneath the decking does create more space but not ventilation. It also can create a low spot for water to pool in. Install drainage to direct water away from beneath the decking.

Moulds commonly start to grow when water is trapped under the deck. If the deck is low to the ground and has limited ventilation, then I would suggest the following to help limit any future problems.

• Reduce the decking boards to a narrower 70mm or 90mm width to lower the expansion rate
• Use a durable and stable decking species like Merbau or consider Composite decking
• Pre-coat the decking on all sides, including the bottom, before installing
• Increase the gap spacing
• Use a 200um black plastic to cover the ground beneath the decking and prevent water pooling. The ground must slope away from the home
• Use a waterproof membrane between the joist and the decking
• Use minimum 10g x 65mm decking screws.
• Maintain finishing protective coatings as required.

If you have a question or need expert advice, please email below

[email protected]

Additional Information on Decking Spacing

Proper decking board spacing is crucial for the longevity and performance of your deck. Adequate gaps between boards prevent moisture buildup, reducing the risk of expansion and structural issues.

Importance of Proper Decking Board Spacing

Ensuring the right gap between decking boards is vital for ventilation, moisture control, and overall deck health. Proper spacing allows air to circulate, helping to dry out any moisture and prevent the wood from swelling, warping, or developing mould. Additionally, it accommodates the natural expansion and contraction of wood, ensuring the deck’s structural integrity and aesthetic appeal over time.

Recommended Spacing Guidelines

Optimal spacing varies by wood type and environment. For most hardwoods, a 3-5mm gap is ideal, while humid climates may require 5-7mm for better airflow. Narrower boards typically need smaller gaps. Always consult manufacturer guidelines for specific recommendations based on wood species and local climate.

Common Issues Caused by Improper Spacing

Improper spacing can cause moisture to trap between boards, leading to expanding decking boards (like the question asked here), cupping, and mould growth, or cause boards to shift if gaps are too wide. Proper spacing prevents these issues by allowing the deck to breathe and expand naturally, maintaining its stability and appearance.

Best Practices for Maintaining Decking Board Spacing

Use spacers during installation for consistent gaps. Regularly inspect for shifting or warping and adjust as needed. Clean between boards to prevent debris buildup and apply sealant or protective coating to minimise moisture absorption. These practices ensure optimal spacing and long-term deck durability.