I have been digging out the floor of a wooden outbuilding on my property, to prepare for laying a new concrete floor. The walls of the building have been built on top of old railway sleepers that were laid on compacted earth. Removing the floor has exposed the old sleepers, which are now rotten and no longer usable as supports. My question is what is the best option for ensuring the wooden walls are properly supported? The room is 5m by 3m and is joined to another building that sits on its own concrete floor. So jacking up the building to put new support in is not an option. My first thought was to build an underpinning foundation wall using concrete blocks. Location: Highlands of Scotland; the climate is cool and damp much of the year.
This is one of those "best" questions that's highly dependent on site conditions and your capabilities and prerogative. It also depends on how much footing you actually want (or need, if frost heave is a concern). I see two options:
Do as you suggest, first pouring footings and then setting block under the wall plates. This would require someone with some good block-laying skills. That sort of bottom-up work isn't easy. You'll need to fill in around your temporary legs, then remove those and close up the gaps.
Pour an integrated footing with the slab. Once you backfill and compact for the floor, shape adequate footings forms into the soil below the walls. When you pour, agitate the concrete well so that it flows under and against the wall plates. Ideally you'll do some of this from the outside to eliminate air cavities.