Concrete foundations on sandstone

[Bodgy]

Does anyone know if when digging trenches for foundations for a 2 storey construction, whether when you hit solid rock (sandstone and not a floater) you need to cut into this for the concrete trench that forms part of the bottom story's slab?

As I remember the trench needs to be 300mm deep. According to the regs would 150mm on top of the sandstone suffice?

The main house sits on this rock, the brick piers supporting the floor also sit on it. Just wonder if the regs have changed since house was built?

[silentC]

I've seen it done with some starter bars drilled into the rock! I also remember working on a house in Church Point where some of the posts were dynabolted to the rock. The builder I worked for reckoned solid rock was as good as concrete any day.

Not sure what the go is technically but probably the best would be to get the engineer to come out and do a site inspection. If he's happy, the council is happy, and if the council is happy, you're happy ;)

[elphingirl]

I'm not an engineer, but I know it's very important that there is adequate cover around your trench mesh (a minimum of 50mm I think). If any moisture reaches the steel (and the edge of the concrete will be moist) it will corrode along its length. Just my 2c.
Justine

[Exador]

In general, if you have hit rock in your excavation, you need to check with your engineer, as differential foundation movement could be a problem. What happens is that the end that is socketed on the rock doesn't move, whilst the end that is in soil is subject to the normal expansion-contraction of the soil with changing moisture.

I have seen some really terrible cracking as a result of inadequate footings in a socketed situation. If your structure is 2 storeys, I urge you strongly to go see an engineer before doing anything.

BTW, paradoxically, if your footing is socketed at one end, you will very probably need more substantial footings in the free end than you would otherwise.