Decking design advice needed – L-shaped, enclosed, fall & layout (DIY Project)

[Luiz]

 

Hi all,

I’m rebuilding an L-shaped deck and would really appreciate some experienced input on the best structural layout and fall direction before I commit.

 

Key details:
    •    The deck wraps around the corner of the house (L-shape).
    •    Approx sizes:
    •    Section A: ~150 cm deep (from house) × ~380 cm long
    •    Section B: ~370 cm × ~454 cm
    •    The deck is enclosed by walls on all sides (house walls + boundary walls)
    •    Height is ~300 mm above ground.
    •    I’m rebuilding the entire substructure (posts, beams, joists), so layout can be changed.
    •    Sub-soil is soft.
 

What I’m unsure about:
    1.    Board direction
    •    Should boards run in the same direction across the whole deck, or is it better to change direction between the two legs of the L (e.g. parallel to the house on the larger section and perpendicular on the narrower one, with picture framing)?

    2.    Fall / drainage
    •    Given the deck is enclosed by walls on multiple sides, what is the correct fall strategy?
    •    Is it better to:
    •    fall toward one outer/open edge only?
    •    fall diagonally?
    •    accept two planes with a subtle transition?

    3.    Substructure approach
    •    Best practice for posts on soft ground (concrete pads vs posts in concrete vs adjustable supports).
    •    Typical post spacing and beam layout for a deck of this size.

My priority is:
    •    a buildable, professional solution
    •    no twisted or planed joists
    •    boards sitting fully flat on joists
    •    avoiding water being trapped against walls

I’m less concerned about “perfect symmetry” and more about what experienced deck builders would actually do in this situation.
I have attached a drawing of what's currently in place...please ignore the drainage strip as it is doesn't actually exist.
 

Thanks in advance — any advice appreciated.

 

Edited 10 hours ago by Luiz
Missing text

[JohnMo]

Definitely in all directions fall away from house, not falling towards house anywhere.

 

What is the wall? Someone elses house or a boundary wall

 

Plank direction is personal choice, but will affect sub structure design

 

If you are using composit boards you will be at 300-400mm centres on the sub structure.

 

I used flat plates 12mm thick and about 200mm wide galvanised steel, sandwiched between timber and then post created in place. The structure needs to be robust, mine was on a new build and designed by structural engineer, so most the wood is 10x2, light duty stuff is 6x2.  The perimeter is doubled up 10x2.

 

I

 

[Gus Potter]

Have a read though this old manual. 

TRADA Timber decking The professionals manual 3rd edition.pdf