ᐅ Forming and Pouring Ceilings Yourself – Any Experiences?

Hello everyone,
this is not directly related to house construction but seems to have a similar structural approach.

I am building a pool in the garden and have two utility shafts here that I want to cover with a concrete slab, including an opening to later climb down with a ladder.
The walls are made from 24cm (9.5 inches) concrete formwork blocks.

Dimensions of shaft 1 (pool equipment)
1.5m x 2m (5 feet x 6.5 feet)

Dimensions of shaft 2 (water tank)
2m x 2m (6.5 feet x 6.5 feet)

For the access opening, I was thinking of making it the size of a manhole cover, so I can later use a suitable cover for it.

Now the question is, what is the best way to create a concrete slab on top? I have already searched online but only found information about how slabs for houses must be built or references to precast elements.

On the slab of the first shaft, a terrace made of wooden decking will be built. On the slab of the second shaft, soil will be placed later to fully plant the surface.

My specific questions are:

- How should the formwork for this be constructed?
- What kind of reinforcement is required? (For example, is a simple steel reinforcement mesh sufficient?)
- What is the minimum thickness for the slab?

I look forward to your help and advice.

Thank you.

Hello abc12345,

abc12345 schrieb:

My questions specifically are:

- How should I make the formwork for this?

The same way as done in a precast concrete factory: a wooden base and four side panels. The wooden base will remain as permanent formwork underneath.

abc12345 schrieb:

- What should the reinforcement look like for this? (Is a simple steel reinforcement mesh sufficient?)

A single reinforcement mesh alone will definitely not be enough here. You’ll most likely need additional reinforcement bars arranged in a ring around the opening. Your structural engineer will calculate this for you.

Keep in mind that you need to somehow support the ring around the manhole cover within the slab. I assume the access opening is planned to be in the center?

abc12345 schrieb:

- What is the minimum required thickness of the slab?

This will also be calculated by your structural engineer. I would never attempt to cast a slab with a circular opening of about ø60-80cm (ø24-31 inches) myself. Especially since you mentioned additional loads on the slab. For example, on a terrace, easily 100-150kg/m² (20-30 lb/ft²) for one person plus a wooden structure—your engineer will probably calculate higher loads—and that is a considerable amount.

I would recommend ordering this as precast concrete elements. But keep in mind that you will need a crane both to unload them from the truck and to place them onto your walls. Assuming a 15cm (6 inches) thick slab, the small slab alone will weigh around 1.1 tons! I would strongly advise against attempting this on your own.

Thank you for your response. I don’t want to pay a structural engineer just for a small “ceiling” like this. I was hoping there would be a simple solution for such short spans.
Considering the cost of a structural engineer, it might be easier in practice to massively overbuild and, like with our floor slabs in the house, install steel beams.
However, since I don’t want to use a sledgehammer to crack a nut, I was hoping, as mentioned, to rely on practical experience.

A prefab building is not an option here. This is “only” a cover for a water tank and a technical area.

The dimensions 1.5x2 and 2x2 are not just covers with a large hole in them.

If you are going to have soil and a terrace on top anyway, just go to a metalworker and have a suitable cover plate made. Include a handle to lift it. Done. Otherwise, I agree with @Simon-189. Consult a structural engineer.

I'm already looking at a four-figure amount just for a cover... It would be much cheaper for me to simply install three steel beams across the width, panel them from below with boards and support them, then pour concrete on top. If that works for houses, it should definitely hold as a ceiling as well.

abc12345 schrieb:

I can definitely save money by simply installing three steel beams across the width,

Are you doing that considering the steel consumption? I estimate a reasonable HEA 120 I-beam costs around €150 per 2,000mm (6.6 feet). That alone will run into four figures without concrete. Also, your support must be properly reinforced (rebar), otherwise the edge will break off immediately when subjected to significant load. Concrete is very brittle without reinforcement.