I'm looking for some advice around low profile decks - I'm tossing up doing a floating deck to extend my outdoor room to the fence to put a wider bbq station similar to the ideas below. This is so we can have more space under the outdoor room for lounge chairs etc.
The space in the red is 6.88m x 3.63m ~25sqm.
I only have around 100mm height to the outdoor room from the grass at the moment, and concerned on how I would be able to get the deck to breathe / airflow on the sides where the grass would go. My plan would be to dig out the grass and soil and use tuff blocks as the foundation system, up to and level with the outdoor room. I'd like to keep the grass butted up to the flashing of the deck, but this wouldn't leave any air I assume to travel under the deck.
What would be my options? Would something like reduce the size of the decks by 400mm either side (size of the patio columns) and put large decorative rocks down the sides as a gap to the grass allow enough air? Any other thoughts?
Regardless of how you approach the construction of this deck, it will be considered exceptionally low, and the lifespan of the timber will be impacted. I'd suggest at a minimum that you remove a layer of soil of approximately 150mm deep, so you end up with a total working height of 250mm. Lay 50mm of roadbase and compact it, then 50mm of drainage gravel. You'll then place pavers level with the drainage gravel at the locations for your foundations and the 90 x 45mm joists on top of them. The 50mm roadbase compacted to 40mm, 50mm drainage gravel, 50mm height of foundation, 90mm joist and 19mm decking should bring you very close to flush with the concrete.
I wouldn't suggest anything less than a 5mm spacing on your decking boards, as it will allow some ventilation. The biggest concern is water pooling under the deck, and high humidity combined will low airflow. To assist the drainage gravel with allowing water to not pool, you could consider adding agricultural drainage line under the deck to remove excess water.