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.