The joists were laid at 600mm (2 ft) spacing and wedge supports placed under any hollows between the levelled joists and the concrete below. Where the joists we not individually long enough for the span, they were lapped and screwed through with additional supports under the lap joint.
A 2 metre spirit level was used. If this is not your normal bit of kit, then use a 2metre straightedge and lay the spirit level on top. It is important to get the joists perfectly level with each other - even if sloping overall, to ensure that when the deckboards are screwed into the joists, there is no lifting of any joist timbers.
This would cause rattle on a free standing deck. Normally the weight of the combined deck joists and deckboards would be enough to ensure that the deck sits on the base without any undue movement or rattle.
To enable a good level surface of the joists, it is a good idea to lay a spare board across the joists - a joist or sturdy deckboard will do - and screw through to the joistst. This will help to bring them all to a determined level and allow wedging/padding of any hollows.
The inspection cover of the drain had a rectangular insert joist frame which formed a removable lid cover once the deck was laid. It is important to always make provision for any of the service inspection chambers below any form of decking.
With lap joints of the joists, allow at least 12 in overlap. As the joist are not actually 'spanning' a gap, but simply laid on the ground. hefty screws are adequate for the join. Added to that, the robust decking boards which will be screwed down will help keep everything in place.
Once the joists are firmly laid down, decking started. Screws were the choice with this free standing deck, to ensure a 'solid' platform.