I’m trying to get my head around building a low-level timber deck over a concrete slab with minimal clearance.
The timber board would be Fijian Mahogany, 140x21mm.
The clearance between concrete slab and top of the door threshold is 95mm.
The clearance between concrete slab and bottom of the door threshold is only 65mm.
I’m thinking the following;
I cut into the door threshold and use 70x45mm joists (70mm joist + 21mm board = 91mm).
The finished floor level would then be only 4mm below the top of the threshold. I’m guessing this might be a terrible idea with a door facing west…
I cut into the door threshold just enough to fit a 90x45mm joist on its face (45mm joist + 21mm board = 66mm).
Would this be enough space for ventilation between slab and the timber board to last, let’s say 10+ years?
What would you do?
Perhaps composite decking would be more suitable but I would prefer timber both because of look and to keep the cost down.
Any advice would be much appreciated!
Whichever way you look at it, you'll be creating an ultra-low deck, and the longevity of it will be compromised to some extent due to the lack of airflow through the structure. Whether there is no space under the joists or 30mm, we are really out of the realm of circulative airflow. It's hard to determine the length of time the deck will last; I suspect you'll get at least ten years. One of the deciding factors is how fast decay and rot can set in. If the deck is exposed to the weather and every time it rains the water ends up pooling under the deck on the concrete, it will create a moist environment that promotes decay. Given how low this deck is, you might like to consider an oversized spacing of the gap between decking boards. I'd recommend a gap of at least 6mm, as this will allow the deck to breathe and allow any trapped water to evaporate out of the structure.
I'd likely go for the 90 x 45mm on its face so it's elevated above the concrete on spacers. That will at least get it out of any pooling water. You can also paint the underside of your joists with bitumen paint to help prevent moisture uptake.
Please let me know if you have any questions.