Deck <50mm above grade

[silly_savage]

Looking for some advice on planning a small deck, probably less than 3x3m. The garden is currently excavated about 250mm below where I want the surface to be. I'd like for beds to be within 50mm of this level and (stone slab) paths to and from flush with it. All the info I've seen on ground level decking actually seems to place them slightly above the grade but about 100mm or more, presumably to allow for ventilation through the structure.

No drawings yet but bare with me, I'll try and describe my plan. 200x100mm concrete blocks will form the perimeter and retaining wall holding back the beds, top those with some kind of paver, I'm assuming 50mm thick, then construct the frame within the interior, 'floating' on some kind of dwarf wall/footing with DPC. I considered using joist hangers but can't imagine a way to stop moisture entering the wooden structure from the retaining wall (or how to sufficiently damp proof the retaining wall).

This should give space for 150mm joists with a small ventilation gap to the ground and obviously avoid soil from the beds encroaching under the deck over time.

Does this sound like a sensible plan? Has anyone done similar? Key is in trying to maintain a step-free transition from (stone paving) paths to the deck and not much drop to the surrounding flower beds.

I'll try and update with some drawings soon.

 

[silly_savage]

Just realised I can use spacers to fix a ledger to the retaining wall without risk of it getting damp. One issue is the bolt for it would be perilously close to the top of the block, possibly fixed by using a shallower cladding for the top of the wall such as brick slips.

 

[big-all]

General comments
are you forming a sort of sump where surface water or even groundwater will collect if it can't drain or if above the water table will fill up from below
is there any fall and exit to allow the water to drain away on the land

 

[silly_savage]

Yeah I'll incorporate a fall to the deck, at whatever gradient is recommended (2%?)

I was thinking of installing a soakaway under the deck, although probably not immediately and technically it would be against building regs. The lower path will drain down to an ako.

 

[Mr Chibs]

Good diagrams

I’m not sure I’d be happy with having the bolt that high up the block, over time/use guess it may dislodge the blocks.

Think I’d be either looking to build some honeycomb sleeper walls or concrete pads with embedded posts/spurs (wood/concrete/metal).
There are a few summerhouse base YT videos where there concrete-in threaded rods to sit a timber plate on, bit of a faff but looks very solid and easy to get level.
You could still build your retaining wall as your diagram.

 

[silly_savage]

Good diagrams

I’m not sure I’d be happy with having the bolt that high up the block, over time/use guess it may dislodge the blocks.

Think I’d be either looking to build some honeycomb sleeper walls or concrete pads with embedded posts/spurs (wood/concrete/metal).
There are a few summerhouse base YT videos where there concrete-in threaded rods to sit a timber plate on, bit of a faff but looks very solid and easy to get level.
You could still build your retaining wall as your diagram.

Thanks for your reply. I think you're right, supporting the frame on piers is going to be the way forward.