How to stabilise concrete with steel rebar roof which is flaking?

Inspect the Roof:
Check the extent of the damage.
Identify areas with the most severe flaking and any exposed rebar.
Clean the Surface:
Remove Loose Material: Use a wire brush or a pressure washer to remove all loose, flaking concrete and any rust from exposed rebar.
Clean the Surface: Ensure the surface is free of debris, dust, and grease. This will help the repair materials adhere better.
Treat Exposed Rebar:
Rust Converter: Apply a rust converter to any exposed rebar. This will neutralize the rust and prevent further corrosion.
Rebar Primer: After the rust converter has dried, apply a primer specifically designed for rebar to protect it from moisture and further corrosion.
Apply a Bonding Agent:
Concrete Bonding Agent: Apply a concrete bonding agent to the cleaned surface. This helps new concrete or repair mortar adhere to the old concrete.
Patch the Concrete:
Repair Mortar: Use a high-quality concrete repair mortar to fill in areas where the concrete has flaked off. Ensure the mortar is well-suited for outdoor use and can bond to existing concrete.
Seal the Surface:
Concrete Sealer: Once the repair mortar has cured, apply a concrete sealer. Choose a sealer that is water-resistant and designed for exterior use. This will protect the concrete from moisture penetration and further damage.

Repair mortar by using a high quality concrete repair mortar and fill in areas where it’s flaked off make sure it’s suitable for outdoor use then when cured apply concrete sealer

Best option is to break it and fix new one because is very dangerous

Inspect the Roof:
Check the extent of the damage.
Identify areas with the most severe flaking and any exposed rebar.
Clean the Surface:
Remove Loose Material: Use a wire brush or a pressure washer to remove all loose, flaking concrete and any rust from exposed rebar.
Clean the Surface: Ensure the surface is free of debris, dust, and grease. This will help the repair materials adhere better.
Treat Exposed Rebar:
Rust Converter: Apply a rust converter to any exposed rebar. This will neutralize the rust and prevent further corrosion.
Rebar Primer: After the rust converter has dried, apply a primer specifically designed for rebar to protect it from moisture and further corrosion.
Apply a Bonding Agent:
Concrete Bonding Agent: Apply a concrete bonding agent to the cleaned surface. This helps new concrete or repair mortar adhere to the old concrete.
Patch the Concrete:
Repair Mortar: Use a high-quality concrete repair mortar to fill in areas where the concrete has flaked off. Ensure the mortar is well-suited for outdoor use and can bond to existing concrete.
Seal the Surface:
Concrete Sealer: Once the repair mortar has cured, apply a concrete sealer. Choose a sealer that is water-resistant and designed for exterior use. This will protect the concrete from moisture penetration and further damage

Repair Mortar: Use a high-quality concrete repair mortar to fill in areas where the concrete has flaked off. Ensure the mortar is well-suited for outdoor use and can bond to existing concrete. Seal the Surface: Concrete Sealer: Once the repair mortar has cured, apply a concrete sealer.

I suggest the best practice would be to install a waterproof membrane to protect this from flaking more I.e bitchumen etc to prevent the water ingress from further damage

What you’re seeing is classic rebar corrosion (often called concrete cancer or spalling). Water and air have reached the steel, the rust has expanded, and it’s pushing the cover concrete off. Painting over the flakes won’t stop it.
A proper holding repair until you can renew the roof:
1. Safety first?Falling concrete is a hazard. If large areas are hollow or the slab feels unsound, get it checked before you rely on it. Don’t walk on badly flaking areas.
2. Remove all loose material?Chip or scrape off every bit that is hollow or flaking until you reach sound concrete. Where bars are exposed, try to get behind them a little so the new mortar can wrap the steel.
3. Treat the rusted bars?Wire-brush back to bright metal. Coat with a zinc-rich rebar primer (e.g. Fosroc Nitoprime Zincrich or Sika Armatec / MonoTop rebar primer). Ordinary rust paint is not enough.
4. Rebuild the cover?Use a polymer-modified concrete repair mortar (Sika MonoTop, Fosroc Renderoc HB, or similar) — not ordinary sand-and-cement. Pack it tight around the bars and finish flush.
5. Seal the whole roof?Once cured, apply a waterproof / anti-carbonation coating so water and CO? can’t keep getting in. Options: Sikagard-type anti-carbonation paint, or a proper roof coating (EPDM liquid, acrylic roof coating, or similar) if you want a weatherproof finish.
That will slow the flaking and keep water out for a while. It is not a permanent structural repair — the steel will keep corroding in places you haven’t opened up. When you can, the long-term job is to strip back, treat all the steel, restore cover and put a proper waterproof system on (or replace the roof).
If a lot of bars are badly rusted or the slab is sagging, treat it as a structural issue, not just a coating job.

removing loose concrete, cleaning and treating exposed steel reinforcement, and then applying a suitable repair material

Using a good quality concrete repair compound.