Slab or Raised Floor for a Sloping Block: What Should You Choose?

Charles Christodoulou:  This is just my opinion, and you can check with the soil test. The people that do the soil test will probably be able to give you a better idea. But in my opinion, for what it’s worth — if you’ve got type H or worse soil conditions — even then, I would probably look at building on a concrete slab.

Where I live here in Melbourne, nearly all the houses used to be built on stumps, and the soil conditions were type H — highly reactive to water. A lot of those houses cracked. Doors wouldn’t open or close properly. It was a nightmare. Especially where my parents lived, houses had major cracking issues.

Now, in the western suburbs of Melbourne, no builder will build on stumps due to warranty issues. So you’d probably go for a slab. But when you build on a slope and use a concrete slab, you have to cut the soil away.

And where you cut the soil away, you’ve got to then fill the lower side to get the site level. But you’re incurring more cost, because in the filled area, you’ve got to dig through the fill and hit natural soil to embed the concrete beams. On the other side where you cut, you probably need a retaining wall — and they’re not cheap.

So you’ve got to weigh it up: do I spend more money and probably avoid cracks in the house, or do I spend less money and build on stumps, risking future cracks and repairs?

You’ve got different soil classifications. S is for stable soil condition — that’s good. Then M means moderately reactive. Actually, I think Type A is the best. I hope I’m right. I think it goes A, then S (for stable), then M (moderately reactive), then H (highly reactive), and E (extremely reactive).

Now, if you get M, I’d consider building on stumps, especially to avoid the high additional costs that come with cut and fill for a slab. This is what you’ve got to weigh up. There’s no quick fix. It really depends on how much slope you have.

Try and get an estimate from the concretor who’ll do the slab — ask them what the extra cost would be for cut and fill.

And then on top of that, speak to someone and get a rough quote for the retaining wall. If the wall is about a meter high, the cost can climb quickly.

I’ve got a rental property, and the retaining wall there is only about half a meter high. I used some 200x75mm treated pine sleepers — and they’ve held up for over 20 years. They’re still holding up.