Avoiding Timber Moisture & High Humidity Problems

How Ventilation, Drainage and Coatings Keep Timber Durable

What must be remembered when it comes to achieving board stability is that wood is a hygroscopic material, meaning it naturally absorbs and releases moisture from the surrounding environment. This constant movement can affect the stability, strength and long-term performance of timber products. Understanding how timber responds to changing moisture conditions is the first step in preventing issues such as warping, cupping and splitting, and ensures your project maintains its durability and structural integrity.

Timber Flooring & Decking

When it comes to timber flooring and timber decking, the sub-floor space must be well-ventilated. To achieve good ventilation, a cross-flow of air is required. If this is not achieved, it could likely mean a buildup of humidity in the sub-floor causing an increase in moisture content, as well as board size, with costly consequences.

To avoid the above we suggest the following:

• Proper drainage is installed
• In the case of timber flooring, the regulations call for air vents drawing 6,000 squared millimeters of air spaced at one metre intervals around the external wall perimeter
• In the case of a timber deck, baseboards are to be gapped so that plenty of air can enter and exit the sub-deck space. I often see the baseboards completely closed off
• Install a waterproof membrane between the decking and floor joist, otherwise the joist can absorb moisture, particularly with treated pine, allowing the fixing pins to move too easily  
• In both cases, a good cross-flow of ventilation is the desirable outcome
• If cross-flow is not possible, such as a deck around a concrete slab, explore the use of mechanical sub-deck fans, one drawing in and another opposite drawing air out.
• I am of the firm belief that decking boards should be oil-coated all round before installation, with special attention to the trimmed end grain and a final coat after installation
• Consider coating maintenance once moisture ceases to bead on the surface.

In-Ground Timber Posts

In the case of in-ground timber posts, such as treated pine posts, cypress posts or merbau posts, whatever the size, we suggest:

• Give the part that is in the ground a good coating of bitumen paint
• If the soil is of a heavy nature or the post is likely to experience excessive moisture, provide the post with another coating of bitumen paint
• Paint the top of the post with a waterproofing agent to keep moisture from penetrating the end grain
• For Cypress Pine posts, select the greater heartwood section for in-ground use.

Exposed External Timbers

In the case of exposed external timbers, we recommend the following, even for durable species:

• Apply an oil-based coating to the exposed faces, paying attention to the top edge of the beam. The aim here is to avoid cracking where moisture can settle and cause further deterioration of the beam
• Again, apply an oil-based coating where there are joins or metal joiners. Bowens has a pre-coated timber service that can help alleviate some of the issues, saving time and costs onsite.

There are some common, yet potentially damaging issues that may arise if external timbers are not correctly coated:

• If wood is exposed to the weather without any protective coating to inhibit water penetration, the natural tannins will be leached gradually from its surface cells and a greyish appearance will develop
• Subsequent wetting and drying will cause checking (small splits) and raised grain. Even if initially the surface was smooth, it will become roughened
• On the other hand, wood surfaces that are protected from the weather will largely maintain their natural colour, particularly if a UV stabiliser is added to a natural clear oil coating.

Feel free to contact Jeff on 0412 550 740 for any further customer advice you may require.