I planned to build a low-level decking on top of my concrete floor with a span around 1.8m across the grassy part to the right of the photo (below). I planned to use 70 x 35mm for the joist as it has enough height to be just level with the floor of the house after decking is installed.
The concrete slab is 145mm lower than the house floor. The soil is around 80mm lower than the top of the concrete slab. The total area is 7.34 x 5.20m. The area in concrete is 7.34 x 3.2m and the area in soil is 7.34 x 2m.
The length is 7.2m and used around 19 joists across.
My father and I completed the deck in about three weekends.