Wanted: Concrete Roofing Expert!

[andreascarlsson]

- I live in a semi-detached house built in 1982.
- I am going to lower the ceiling and install spotlights and speakers.
- A good ceiling speaker measures 10 cm in depth/height.
- The concrete elements separating floors 1 and 2 in the semi-detached house are 25 cm high. The concrete is of the lighter type, which you can easily drive a nail into, but there is at least one layer of rebar as well.
- To lower the ceiling as little as possible, I thought of chiseling out space for the speaker magnet and coil. This involves a diameter of about 15 cm, 6 cm deep. Possibly drilling until you hit the rebar?
- In total there will be six such holes on the ground floor of the house (a floor that is about 60 m² in itself).

My question is: Does the load-bearing capacity of the concrete ceiling deteriorate because of these holes?

I assume it’s not a problem, but I thought I’d check if anyone actually knows...!?

 

[Mainstream]

I won't comment on the concrete roof, but you can't build in the speaker like that. It needs air around it to perform as intended.

 

[andreascarlsson]

I won't comment on the concrete ceiling, but you can't build in the speaker like that. It needs air around it to perform as intended.

It has plenty of air in the rest of the suspended ceiling...?

Otherwise, one could just chisel out a cylinder as big as the speaker.

 

[Ingenjören]

Are you sure it's a cast-in-place concrete slab and not prestressed concrete elements in the ceiling?

 

[myrstack]

If you know the manufacturer of the slab, it might be possible to find out the placement of the reinforcement. Notching 5cm deep on the underside can be far from trivial if you encounter reinforcement, as the bottom reinforcement is meant to handle tensile forces on the underside of the slab. If you avoid this, it should be feasible.

 

[andreascarlsson]

Thanks for the answers!

Are you sure it's a cast-in-place concrete slab and not prestressed concrete elements in the ceiling?

Unfortunately, I have no idea, because I don't know what you mean... They are prefabricated concrete elements, so they are not cast on site, if that helps?

If you know which manufacturer the slab comes from, it might be possible to find out how the reinforcement is laid. Cutting 5cm deep at the bottom can be far from trivial if you encounter reinforcement, as the bottom reinforcement is supposed to take tension forces on the underside of the slab. If you avoid it, it should be feasible.

My thought is to stop when I hit the reinforcement.

The more I think about it, all the concrete below the lower reinforcement (because there should be two reinforcement bars in a 25 cm thick element, right?) should purely be cosmetic. It shouldn't create any load-bearing capacity at all, right?

 

[Villa vista]

If you can nail into the floor structure, it might be a lightweight concrete floor. If the speakers are placed near load-bearing walls, the hole drilling shouldn't be problematic. Regarding reinforcement: it should be deeper than 5cm.

 

[myrstack]

The more I think about it, all the concrete under the lower reinforcement (since there should be two reinforcement bars in a 25 cm thick element?) should be purely cosmetic. It shouldn't create any load-bearing capacity at all, right?

Cosmetic is a bit of a stretch. The concrete/light concrete around the lower reinforcement in a slab is meant to adhere to the reinforcement and distribute the tensile stresses that are transferred to it. Removing the concrete for a short distance might have less significance from that perspective, but at the same time, with slabs, one should be cautious.

 

[myrstack]

If you can nail into the beam structure, it could be a lightweight concrete beam structure. If the speakers are placed near load-bearing walls, drilling holes shouldn't be problematic. Regarding reinforcement: it should be deeper than 5cm.

With a 25 cm thick beam structure, it is more likely that the reinforcement is shallower than 5 cm at the bottom rather than deeper inside the concrete. Probably both for economic reasons, as thinner reinforcement can be used, but also to allow drilling in the middle.

 

[Villa vista]

True about the reinforcement. But it's just about taking a sample hole. Drilling is usually done at the edge of the prefabricated slab. And not in the middle.

 

[roli]

I would guess that reinforcement bars appear about 2-3 cm into the concrete slab. The deeper the bars are placed, the thinner the slab can be made. And thinner elements save material and transportation costs. Of course, there is a minimum cover requirement that must be met. Unfortunately, to determine what that is, you will likely need a manufacturing drawing or calculation from the manufacturer who made the element. The element is now indoors, which means the cover requirement is relatively small. So, 6 centimeters cannot be chiseled or drilled away without damaging the reinforcement; you might possibly fit between the bars, but without a drawing or calculation, you can't know for sure how closely the bars are placed and where.

 

[Ingenjören]

If it is a floor slab element made of lightweight concrete, it may be reinforced with prestressed piano wire, and one should be careful with these as they are easier to damage than a regular reinforcing bar. Checking whether it is concrete or lightweight concrete is easy if you chip away a fragment from a corner. However, I agree with previous writers that the cover layer, regardless of material, is probably only about 20mm.

 

[Mainstream]

It has loads of air in the rest of the ceiling...?

Otherwise, you might as well chisel out a cylinder that is the same size as the element.

That's not enough unfortunately. If it's going to perform as intended, you need more space for it. A cylinder the same size is OK, the depth should be at least about 10 cm behind the magnet. If you can't meet this requirement, you might as well choose some smaller and cheaper elements because it will sound dull anyway.

 

[andreascarlsson]

Thanks again for all the answers!

I will drill this weekend to see what happens. I will get back with some form of photographic evidence!