I'm planning on building a timber walkway around the house, 60cm wide under the eves and the inside joists bolted to the concrete house foundation. The house was built less than 2 years ago with no landscaping done, and so there is a 50cm thick layer of compacted metal wide enough to build the walkway on. The walkway is only around the back of the house for access to my shed, not for general use.
For the middle and outside joists, do I still need to drill holes and concrete in posts or will it be fine laying the joists with packers on top of the metal? Or use old stepping stones as footings?
Sorry, should have said 90cm which is 40cm between joists (45cm centres). That is all I need as there is little space anyway. For moving large objects I've got 3m wide access up the other side of the house.
I'd probably try it with old paving slabs and make sure I could remove the bolts so if it wasn't stable I could concrete in posts later on. I'm guessing you're not building this very high? If it was something that could be fallen off I'd definitely concrete it in but for a duck walk I wouldn't bother, especially if I had to dig through 50cm of compacted metals :-(
If you are keeping it relatively low then there is no need for posts and bearers. I would not fix to the slab either but leave the entire thing floating (raft). If you use 40 mm thick decking (rather than 25 mm) you would not need a centre joist as they can span 600 cc (400 mm for 25 mm thick). Do a cost exercise of 50% more joists (include 50% more screws, connectors and extra labour to install) vs thicker decking. You could do the same for 100 mm vs 150 mm wide decking, but 100 mm is the standard. Think about whether or not you need griptread (which is a PITA to clean the grooves).
My suggestion: Two 100x50 H4 treated joists (or sized to whatever height you want the deck to be), running parallel each side of the deck and laid directly on the ground. You may need to flatten and dig the joists in slightly so they are level. Then screw fix your decking (2 fixings each side) on to the joists.
H4 is the minimum treatment for timber in contact with soil, H5 is for timber buried in soil. H3.2 (traditional tanalised) is the minimum treatment for any other exposed timber (decking and deck joists and bearers normally).
You could build sections that can be lifted and moved whatever length you like. These can be tied together or it can built as one piece. You will need to think about joist/joist connections either way so the sections don't pull apart with movement. Strap nails or knuckle plate for permanent connection (e.g. pryda or lumberlok) or straps screw fixed for removable rafts. Your fixings and straps will rust faster that close to the ground so I suggest using well greased stainless steel if you want this to last more than a few years.