I want to address an issue we are seeing in the design and construction of decks and outdoor wood projects to help assist future buyers, whether that is the homeowner, builder, architect, landscape architect, or friendly neighbor/relative helping to build the deck. The issue is the importance of airflow and its impact on decking materials. In our Install Guidelines, we address this issue as follows:
Do not construct your deck without ground side ventilation. Proper air circulation beneath the deck is critical to dimensional stability. A minimum of two feet (24 inches) of unobstructed airspace under the deck is essential. Boards requiring straightening should be clamped in the desired position until properly fastened. A bow wrench will work well. Tongue & Groove decking should only be installed in a screened in or covered environment with no exposure to moisture to the wood.
To provide some context here as to why wood needs air circulation and can have issues being built on-grade, we need to look into the physics of wood. Wood is a porous material consisting of a complex network of tiny cells and fibers, which make up its structure. These channels allow wood to absorb and release moisture. The porosity of wood is essential for its natural functions, including the transport of water and nutrients within a tree. However, this porosity also makes wood susceptible to changes in moisture content, can lead to expansion and contraction as well as potential issues like warping or cracking.
The porosity of wood varies by wood species and is a key characteristic that makes certain species good for construction materials inside or outside. For decking in particular, if the wood decking is subjected to constant moisture under it, this will cause the wood to expand and contract, twist, crack, and even cause it to rot prematurely. This is why air circulation is paramount for a quality design and construction for decking, so ensuring the 24” of airflow without moisture under the deck. (note even if you have a 24” gap between the ground and deck board we do not recommend “boxing in” the under side of the deck and If you do clad underneath the deck, leave plenty of gap spacing between the cladding, at least 2”)
We are seeing the lack of airflow and moisture mistakes more and more in the design and construction. Once the deck is installed it looks great, but after a cool heavy rain on a Tuesday and then a sunny hot and dry Wednesday, the wood will expand and contract. The wood is experiencing a humidity variant, where the bottom of the board is cool and humid, and the top of the board is dry and warm. This friction is relieved by the board cupping.
This is why we suggest 24” of airflow under the deck, this helps keep the bottom of the deck board dry. It is also a good practice to do any grading under a deck before deck construction to ensure that moisture, rain, morning dew, snow, etc… will not puddle under the decking. We see some customers doing an under deck decking system with a bladder like system under the deck where the deck is off the ground and the customer wants another outdoor space under the deck. If installed properly these can perform very well. However, we have seen instances where they are not installed properly or the bladder fills with trash or leaves, causing the water to dam up. This has the same effect as not having the 24” of airflow and having the moisture under the deck.
And this is just not specific to our species of wood, Black Locust and Sassafras. This is for soft wood decking species like pine, cedar, or redwood; tropical hardwoods; thermally modified woods; and composite/plastic decking. Most all manufacturers of any decking product suggest 24” of airflow. In fact, Black Locust and Sassafras will experience less movement in service than most any wood or decking material. Composite/plastic decking moves much more than Black Locust or Sassafras in service, sideways and lengthwise.
However, given that we suggest 24” of airflow under the decking, there are certain cases where the terrain at the project site will not allow for 24”. You can still use our decking, but to ensure you have a quality design and build we suggest following these detailed guidelines. We have dozens of success stories here, one was only 4” off the ground using our 4” x 4” Black Locust posts laid in the dirt to form the joist structure (click here for pictures of the Black Locust Boardwalk -Bridgehampton, NY). Here are the guidelines for installs without the 24” of airflow:
- Do as much prep work as possible with grading to maximize airflow and ensure that moisture will flow out from under the wood. This could include a combination of grading and drains. Avoid “boxing in” the deck with cladding, this will prevent airflow.
- Use a narrower decking board. The 5/4 x 4 decking instead of the 5/4 x 6, or 2 x 4 instead of 2 x 6. The narrower board is more stable and will not cup.
- Pre-finish all 6 sides of the board before install. By finishing the underside of the board, this will prevent moisture from coming into the cell structure of the wood.
- Use more joists, consider going 12” on center, this will provide more connections for the wood.
- Face screw the wood. This will provide a better connection to the joists than hidden fasteners or other install methods.
- Ensure ¼” gap spacing side to side (butt ends can still be flush). The gap spacing will allow moisture to vent from under the deck.
By following these guidelines you can have a great and long lasting deck and these guidelines can apply to any decking material, not just our products/species. So even if you don’t choose Robi Decking I hope you find this useful. Also these same guideline principles apply to all outdoor wood projects such as benches.
If you have any questions, we are here to help via email at sales@robidecking.com or via phone/text at 336.365.2850, feel free to contact us anytime, we are here to help.