Hello I'm hoping to lay a decking on a concrete slab with walls on four sides (3 walls and a railway sleeper wall). I could put ledger boards on all walls then joists between them with decking risers where necessary but I'm warey that thermal/moisture expansion could cause buckling at the centre as the ends are fixed. How many walls is it okay to fix to? Should I leave a gap between joists walls where it is not fixed? Also, any issues with laying it on a fairly old bit of concrete? Bit mouldy in places but was going to give it a clean, use some plastic decking risers to lift the joists away, and use 6mm decking board spacing to allow as much air in as possible. Thanks for any helpful advice.
could you not make decking a "raft" sitting on that concrete slab after you've checked slab to be 4" deep ( some may say 3" ). I'd be looking to lay strips of plastic DPC under the joists and not use too much fascia on the sides so there's good air flow. I occassionally lay decking, some regulars may come on later.
Thanks for the reply. By a "raft" do you mean not fix to any of the surrounding walls? Wasn't intending to have any fascia, just have the joists right up to the walls with decking boards on top though. Reckon I ought to leave a gap before the walls? Looks less good but better airflow ♂️
yes, make a decent frame sitting exclusively on the concrete slab. Even ledger boards are kept away from the walls by putting several washers on the masonry bolts to act as packers.The regular deckers always mention ventilation/airflow.
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