I'd like to add a decking to my garden, dimensions are 5m width x 3m depth. Am planning to have 3m joists laid vertically (about 50cm apart) and 5m decking boards horizontally As the soil is not very stable I was wondering whether about 10 C16 joists (2 inches x 5 inches), with bearers on both ends (3m apart), would work, could anyone confirm?



2 Answers 2


Are you asking whether a 2x5" joist will span 15 feet (3m)? No. It'll be a trampoline. (Or yes, but it'll be a trampoline.) Here are your options as I see them:

  1. Go with your original plan and lay it on the soil. Then you'll have critical intermediate support even with loose soil. Worst-case scenario it settles into the soil and ends up lower then it started, or becomes uneven in the case of non-uniform soil.

  2. Lay a sleeper at the midpoint to cut the joist span in half. In this case I'd remove 4-6" of soil under each of the three sleepers and replace with crushed rock.

  3. (Preferred method): Remove 4-6" of soil and replace it with crushed rock laid over heavy landscape fabric. Then set your joists right on the rock.

  • 1
    Many thanks for reply, very useful, I hadn't realised the joists would be that flexible Now with regards to the crushed rock are we talking about gravel / pebbles or proper MOT type? Would I need concrete slabs as well to stabilise the joists? Thanks
    – RomRom
    Commented Jun 20, 2016 at 20:39
  • 1
    Crushed rock is more stable than slippery, rounded river rock. 3/4" to 2" screen would probably be suitable. I don't know what MOT is.
    – isherwood
    Commented Jun 20, 2016 at 20:42

it would be wise to pour either one contiguous slab of concrete or individual pads of concrete under the supports (leveled, of course). This will eliminate the worry of soil type

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