Building stone steps

[Sarah C]

We want to build a set of 3 steps between 2 levels in our garden. Each step 800mm wide, 400mm tread, 150mm riser, giving an overall 450mm rise over 1200mm distance. The 800mm x 1200mm area for the steps is currently all at the lower level, which is clay based subsoil. We plan to build them from pieces of spare cornish slate walling stone, mortared together.
What would be a suitable base? We were thinking of a concrete slab (800mm x 1200mm), say 100mm thick. Also would low density blocks be OK as filling under the higher steps?

[lutonlagerlout]

a concrete base is a must, and you need concrete blocks for outdoors,lo density are not suitable
or alternatively class b engineering bricks
LLL

[Sarah C]

Thanks LLL.
We'll use concrete blocks. Would 100mm be thick enough for the slab?

[lutonlagerlout]

depends what your subgrade is
if its chalk or rock yes
if its clay then really 225mm of concrete
its hard to say without looking sarah
cheers LLL

[Sarah C]

Thanks for the advice LLL.
Sarah