Ground-level hardwood decking can make your garden or swimming pool look exquisite. But for it to hold its shape and strength, it requires adequate ventilation. Poorly ventilated decks can lead to a buildup of humidity, and because the moisture has nowhere to go, it’s usually absorbed by the timber instead, leading to swelling, cracking, gaps around the boards edges, fungi infestations, and even snapping. When this happens, you may need to replace the entire deck, costing thousands of dollars.
These humidity issues are a risk for any deck that is close to the ground or enclosed in some way. Decks can be enclosed either by building materials like wood or plaster, by thick plants, or anything that drastically reduces the amount of airflow through the deck’s structure. The proximity to the ground means that even more moisture can get into the timber, especially if the ground has poor drainage. Rain exacerbates the problem too if it can reach the sub-deck space.
By ensuring good sub-floor ventilation, you’re helping to keep your timber strong, and maintaining a proper appearance. So how does it work?
How to create good deck ventilation
If your deck is enclosed or close to the ground, you’ll need to ensure a good cross-flow of air through the deck, to help reduce humidity levels. The amount of ventilation needed will depend on the moisture content of your soil too. The drier the soil is generally, the less ventilation you’ll need for the deck.
These are the key factors in achieving good ventilation for your deck:
• For decks close to the ground, the framing should be laid on stones or compacted gravel, which are themselves covered by a soil moisture barrier. This barrier is essential to keep the moisture below ground and can be achieved with a sheet of 200 µm black plastic.
• Ideally, the decking should be on a slight slope so that water can easily drain away. If this isn’t possible, you can use a “sock” covered perforated “ag pipe” to assist with drainage. This has shown to work well on flat ground.
• Board widths should not exceed 90mm. This allows more gap space for the entire deck and improves its overall ventilation. A wider gap between boards is also necessary for the same reason—at least 5mm.
• A species that has high water-resistant properties should be chosen. Your deck installer should be familiar with these.
If your deck installer is worth their salt, they should already know this information and will be able to install a well-ventilated deck for you. However, it’s worth being cautious and checking whether their work meets the above criteria, or asking them how they plan to achieve good ventilation before they start the work. The risk is high, after all.
A poorly ventilated deck may only last a few years before you start seeing severe damage, whereas a properly installed and maintained deck should have a lifespan of around 40 years. So be sure to confirm that your installer has a plan for achieving good deck ventilation.