Depends on the size design and load of the deck but use treated sawn timber at least 2 x 6 c16 grade.
The big sheds generally stock what you will need but select the timber yourself and examine for twists, splitting curing etc. If you order online say from Wickes you are at the mercy of whoever does the picking. It’s even worth hiring a van to carry the long pieces if you’re carrying out a big project to ensure control over selection of timber
I used 4x4 every 1.5m, to 70cm depth (Decking raised by about 60cm at the lowest ground level point)
Then 6x2 for ledgers and joists, at 50cm centres. Noggins were made from offcuts of joists. All C24 grade timber. M10 coach bolts hold the whole construction together, with some joist hangers for the intermediate joists. No bounce at all!
Maybe just a Jersey issue, but B&Q (The only shed) has a terrible selection of timber and its expensive. Got a builders merchant to supply it all, which also meant getting 5.4m lengths which saved joins....
Is your decking really close to the ground? I was able to fit a 6x2 width-ways under the joists, so that I only needed a ground post every 1.5m in both directions.
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