Hollow core slabs rarely make it into the renders. They are covered by screed, hidden above ceilings, and nobody photographs them. They are also, on a lot of projects, the single precast decision that returns the most money.

What they are

A hollow core slab is a pre-stressed concrete floor unit cast with continuous voids running along its length. The voids remove concrete from the middle of the section — where, in bending, it contributes least — while the pre-stressing strands sit near the bottom where the tension is.

The result is a floor element that is substantially lighter than a solid slab of equivalent depth, but spans considerably further.

Where the money actually comes from

Most people assume the saving is in the floor itself. Usually it is not. The saving cascades downward through the structure.

1. Fewer columns and beams

Longer clear spans mean fewer intermediate supports. Every column you delete is a foundation you do not pour, a connection you do not detail, and an obstruction the tenant does not have to design around. In car parks, warehousing and retail, column-free area is directly commercially valuable.

2. Lighter structure below

Reduced dead load propagates. Lighter floors mean smaller beams, smaller columns and smaller foundations. On poor ground — where foundation cost is disproportionate — this can be the dominant saving on the whole frame.

3. Programme

Hollow core units arrive cured and are erected directly. No formwork, no propping, no waiting for a floor to gain strength before the trade above can start. A floor plate can go from bare frame to working platform in a day, which pulls every following trade forward.

4. Services integration

The voids are usable. Small services can be routed through the cores rather than beneath the soffit, which can save real ceiling void depth. Across several floors, that can translate into a whole additional storey within the same height envelope.

Practical span guidance

Actual capacity depends on depth, strand configuration, topping and loading, and every project must be designed on its own numbers. As a rough orientation for early planning:

• Shallower units suit residential floor loads over moderate spans — typically apartment and villa floor plates
• Mid-depth units cover most office and light commercial loading
• Deeper units handle car parking, storage and heavier commercial loads, or long clear spans

The useful design habit is to work backwards. Start from the clear span the architecture actually needs, then select depth — rather than picking a depth and discovering the grid must change.

Where hollow core is the wrong answer

• Heavily irregular floor plates. Hollow core is a one-way spanning product cut from a long bed. Curves, frequent large penetrations and highly irregular edges generate offcuts and site work that erode the saving.
• Very large openings. A stair or lift core is fine and routine. A floor plate perforated by many big penetrations may be better as an in-situ or composite solution.
• Cantilevers. Pre-stressed units are designed for simple spans. Cantilevered balconies usually need a different element.
• No crane access. Units are heavy and long. If they cannot be lifted into position, the discussion ends there.

Details that decide whether it works

Three items cause most of the problems we see when hollow core underperforms expectations:

• Bearing. Adequate bearing length at supports is not negotiable, and it must be coordinated with the supporting beam or wall early — not discovered on site.
• Structural topping. Where diaphragm action is required, the topping and its connection detailing do that work. It is not simply a levelling screed.
• Penetration planning. Openings coordinated before casting are trivial. Openings discovered after delivery mean cutting units in the field, which is slow, messy and structurally sensitive.

The short version

Hollow core rewards early decisions and punishes late ones. Get the grid, the spans and the penetrations agreed before production starts and it is one of the most cost-effective elements available. Leave those decisions until units are on the ground and the advantage disappears into site cutting and rework.