A floor slab may look like one line item in a structural concept, but it influences almost every part of the building: from ceiling heights and mechanical coordination to embodied carbon and long-term performance.
In Vancouver, the typical floor system is a conventional 8-inch reinforced concrete slab. This system has become the industry standard as it provides an effective balance between structural performance and architectural flexibility. The span capabilities of an 8-inch slab allow for efficient column spacing, which results in larger floor areas and greater flexibility for residential unit planning.
Embedding HVAC ducts within the slab depth minimizes the need for bulkheads and soffits, creating flatter ceilings and more efficient interior spaces. This approach can reduce floor-to-floor heights, increase ceiling heights within the same overall building height, and in some cases even allow additional storeys within municipal height restrictions.
These benefits are particularly attractive in high-rise residential, hotel, and office developments where maximizing usable area within strict zoning height limits is a key project objective. With fewer bulkheads and dropped ceilings, spaces feel larger and more open, architects gain greater freedom with interior layouts, and window head heights can be increased to improve daylight and views.
Reducing floor-to-floor heights can mean less façade area, less cladding, shorter elevator and stair shaft heights, and a smaller overall building volume—ultimately resulting in material and cost savings. Sometimes the standard isn’t the best solution.
Comparing Typical Slab Systems
Each system brings a different balance of span, thickness, service integration and construction effort. The right choice depends on what the project is trying to achieve.
| Slab Option | In-Slab Ducts Allowed | Transfer Slab/Beams Required | Key Advantages | Key Drawbacks | Schedule / Performance Impact |
| 8″ Conventional Reinforced (CR) | Yes | Typically required | Good fit for residential layouts and services coordination | Tower columns do not align with parking grids | Baseline |
| 7″ Conventional Reinforced (CR) | No | Typically required | Significant material savings | Highest column count and limited-service integration | No schedule/performance impact compared with baseline |
| 7″–7.5″ PT Slab | No | Typically required | Reduced slab thickness for baseline spans | PT installation may add 1–2 days per floor cycle | Reduced deflection, creep, and long-term movement |
| 8″ PT Slab | Yes | Often eliminated | Potentially fewer columns and significant structural efficiencies | PT coordination required with in-slab ducts; may add 1–2 days per floor cycle | Reduced deflection and creep; may add 1–2 days per floor cycle |
What about embodied carbon?
The selection of a floor slab system at first glance may not seem like a major driver of embodied carbon. However, due to floor assemblies being repeated on each level of a building, even small reductions in slab thickness or material quantities can translate into significant carbon savings. This chart shows a comparison of carbon emission index.
Case Study – A Height Problem Without an Easy Trade-Off
In one of our recent projects, due to shadow impacts on a neighbouring school, the development required a reduction in overall building height. At the same time, the project team wanted to preserve the project’s pro forma, including unit count, rentable area, and overall project viability.
Removing a floor would have solved the height issue, but at a significant cost to the project. Rather than eliminating storeys, we looked at the building as a complete system and asked a better question: could the same development objectives be achieved in less vertical space? One solution to this question was to eliminate the in-slab ducts which allowed the slab thickness to be reduced to the minimum required for efficient structural performance. Instead of providing bulkheads for services under the slab, vertical fan coil units system was adopted which allowed to embed the services in partition walls.
Although the thinner slab required additional columns to achieve the span limitations, the overall reduction in structural weight provided advantages throughout the building. Lower gravity loads translated into material savings in the transfer slabs, foundations, and shear walls, while the reduced floor-to-floor height lowered façade requirements and overall building volume.
Rather than pursuing the reduced number of storeys, we evaluated which combination of structural and mechanical systems produced the greatest overall benefit for the project that allowed for the same number of units as proposed originally.
By rethinking both the structural and mechanical strategy, our team reduced the typical floor-to-floor height from 2900 mm to 2770 mm, a savings of 130 mm per storey. Across the height of the building, this reduction significantly mitigated shadow impacts on the neighbouring school grounds while maintaining the project’s development objectives.
What One Inch Saved: A Small Change with a Significant Carbon Benefit
The most compelling part of the solution is its scale. The typical slab was reduced by only one inch. And, despite its simplicity, the reduction generated meaningful embodied carbon savings across multiple structural components.
In a preliminary project analysis, we achieved the following reductions:
- 18% reduction in the embodied carbon of the typical and podium slabs
- 7% reduction in the embodied carbon of the shear walls
- 13% reduction in the embodied carbon of the transfer slabs and foundations
- Approximately 5% increase in the embodied carbon of the columns due to additional column locations
Overall, the total estimated embodied carbon associated with the structural system decreased from 14,789 tCO₂e to 13,900 tCO₂e, representing an overall reduction of approximately 6%.
For a design change as simple as reducing slab thickness by one inch, the carbon savings are significant and demonstrate how early structural decisions can influence both project performance and sustainability outcomes.
Mechanical Strategy Comparison
A few considerations while deciding whether to adopt fan coil units vertically or embed the ducts in floor slab are summarized below.
| Aspect | In-Slab Ducts | Vertical Fan Coil Units |
| Structural Complexity | High | Low |
| Reinforcement Congestion | High | Low |
| Mechanical Shafts Required | Lower | Higher |
| Future Flexibility | Limited | High |
| Maintenance Access | Difficult | Easier |
| Ceiling Layout | Cleaner | May require service closets |
| Construction Coordination | Complex | Simpler |
Lessons Learned
The project demonstrates the value of challenging conventional design assumptions early in the design process. While the traditional 8-inch slab with in-slab ducts remains an effective solution for many projects, it may not necessarily be the optimal choice.
By bringing the structural, mechanical, and architectural teams together to evaluate alternative strategies, we were able to achieve multiple objectives simultaneously:
- Reduced overall building height
- Minimized shadow impacts on neighbouring properties
- Maintained project density and rentable area
- Reduced material consumption
- Lowered embodied carbon
- Simplified structural and mechanical coordination
For this project, a one-inch reduction in slab thickness created measurable benefits across the entire building. It helped reduce building height, preserve development value, lower material consumption and decrease embodied carbon.
This project serves as a reminder that some of the most effective design moves are not dramatic. They come from asking the right questions early, challenging the default, and all project disciplines coming together to understand how one seemingly small decision can influence the whole building. Moving beyond conventional practice doesn’t always mean introducing something new. Sometimes it means looking at familiar systems differently and understanding how small decisions can unlock benefits across the entire building.
Written by Aniket Tolani, M Sc, P Eng – Project Engineer
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