I've built this internal step detail. It will eventually be 4 steps, 150mm rise, 280mm going, topped with Microcement.

The upper and lower adjacent floors will be C30 Cemfloor to 48-50mm depth. The steps need to be poured separately, ideally before the main screed. I have allowed for 50mm depth on the treads and 60mm on the risers. I'll include a movement joint where they meet the liquid screed. So, what mix should I use? Should I add mesh/fibers? I want to give the best odds of success even if it costs a little more.

These photos are before I put the insulation on. They are now insulated with 150mm PIR all round, bonded and mechanically fixed and the rest of the floor is also insulated.

This is the concept of the shuttering test fitted...

Some good into in here ref using SBR mixes. Using SBR seems to make for a really resinous, plastic cement that appears to have some flex. Some chicken wire maybe and fibres are dirt cheap.

With 50mm on the treads and 60mm on the risers you're really making thin toppings over PIR, so I'd treat it as a strong fine concrete rather than a screed. A plain 1:3 sand and cement mix tends to be weak at the nosings, so I'd go for roughly 1:1.5:3 cement, sharp sand and 6 or 10mm aggregate, mixed fairly stiff, or a bagged high strength fine concrete if you want consistency.

Polypropylene fibres are cheap and worth adding for shrinkage cracking, and a light mesh or galvanised chicken wire held mid depth on chairs helps keep any cracks closed, especially where the riser meets the tread. A dash of SBR in the gauging water improves toughness and bond, but don't overdo it.

The bigger risk is the PIR moving underneath. Make sure the boards are fully bedded with no voids or rocking, otherwise the nosings will crack under point loads however good the mix is.

Pour each tread and its riser in one go so there's no cold joint at the nosing, vibrate or rod it well against the shuttering, and keep it covered and damp for about a week before the Cemfloor goes in. Microcement wants a fully cured, dry base, so check the maker's moisture limit before coating.

13 minutes ago, JpManning said:

With 50mm on the treads and 60mm on the risers you're really making thin toppings over PIR, so I'd treat it as a strong fine concrete rather than a screed. A plain 1:3 sand and cement mix tends to be weak at the nosings, so I'd go for roughly 1:1.5:3 cement, sharp sand and 6 or 10mm aggregate, mixed fairly stiff, or a bagged high strength fine concrete if you want consistency.

Polypropylene fibres are cheap and worth adding for shrinkage cracking, and a light mesh or galvanised chicken wire held mid depth on chairs helps keep any cracks closed, especially where the riser meets the tread. A dash of SBR in the gauging water improves toughness and bond, but don't overdo it.

The bigger risk is the PIR moving underneath. Make sure the boards are fully bedded with no voids or rocking, otherwise the nosings will crack under point loads however good the mix is.

Pour each tread and its riser in one go so there's no cold joint at the nosing, vibrate or rod it well against the shuttering, and keep it covered and damp for about a week before the Cemfloor goes in. Microcement wants a fully cured, dry base, so check the maker's moisture limit before coating.

That's exactly what I need, thank you.

There is zero discernible movement in the PIR underfoot. I've been using the steps (with a protective layer on them) for a few weeks.

I built the shuttering to allow me to do the whole step arrangement in one hit. I plan to allow the final bottom riser to come down onto the PIR of the lower floor, then when the main screed goes in at that level, they can screed up to that vertical face, adding a 5mm expansion joint between. I haven't ruled out adding a 20mm high by 10mm deep shadow gap to lose the expansion joint in, but that's something I'll consider once I'm setting this all up.

At the top, I'll allow for the main screed to the upper floor to do the top tread (as that's part of the kitchen floor anyway) and the vertical riser between that and the third tread, allowing the Cemfloor to come down onto the PIR again and put a movement joint in there too. So essentially, I'll be laying 3 of the 4 steps and leaving the top step for the screeder.

Does that sound like a plan?

Presumably I'll need to add perimeter foam where the steps meet the walls at the sides?

That plan sounds solid.

Leaving the top tread and its riser for the Cemfloor pour keeps the kitchen floor continuous and puts the movement joint where the screeder expects it. Coming down onto the PIR at the bottom with a 5mm expansion joint against your riser face is fine; a shallow shadow gap is a neat way to hide that joint if you want it less visible.

Yes, fit perimeter foam where the steps meet the side walls, same as you would for a screed, so the pour can move a little without cracking. Keep the PIR fully supported under the nosings as before, and when the Cemfloor meets your concrete at those vertical faces make sure the joint filler is continuous and the surfaces are clean so you do not get a weak cold joint line telegraphing through the microcement.

If the microcement manufacturer wants a primer or bonded screed interface over mixed substrates, check that before you pour so you are not stuck with two different base types fighting each other under the finish.

The Cemfloor is pretty liquid and will find a way through the smallest gap. Also the installers like a thin plastic separation layer and a thin foam perimeter upstand. I am not sure it will work in a shuttered vertical application, but your installer should let you know.