Non raised decking - Advice on joist pitching etc

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I am about to start on a non raised decking, ie just laid on to existing paving slabs

I am seeking advice on the pitching of the timber joists. Some websites say 1.2m others 450mm.

Also advice on pitching of paving slabs under the joists. As I want to save as many slabs as I can, for another job.

Any advice please ??.

Also I intend to use 4 x 2 as opposed to 6 x 2, is there any reasons why I shouldn't ??

[The Medway Handyman]

1.2m will allow the boards to spring or bounce, far too wide. I'd go for 500mm or thereabouts with 28mm boards, 450mm with 22mm boards, assuming noggins are used.

None at all, as long as they don't have to span any great distance. A 4 x 2 joist will span around 1.8m without support, a 6 x 2 would span nearly 3m. Where the joist is fully supported on a flat concrete base I've used 3 x 2 and even 2 x 2 in the past.

might give some more info.