Span
The unobstructed horizontal distance between supports of a beam, slab, or truss. Span is the primary driver of structural member size and cost in residential construction.
What is a span and how is it measured?
A span is the clear horizontal distance between structural supports, typically measured between the face-to-face distance of columns or from a bearing wall to a column. Measured in metres, span is fundamental to residential design because it governs how wide a floor can stretch, how far a roof can cantilever, and determines how much structural material is needed to keep it safe and serviceable over time. Span is the single most direct driver of construction cost in residential design and cascades into every structural decision that follows.
How does span affect the size of structural members?
Bending moment, the force that causes members to bend, increases dramatically with span. A beam spanning twice as far experiences four times the bending moment. In reinforced concrete, this means a monolithic slab spanning 4 metres might be 180–200 mm thick, but the same slab spanning 7 metres requires 280–320 mm thickness. For beams, a 5 m span might need 400 mm depth while 7 m demands 600–700 mm. That extra depth directly translates to more concrete, more reinforcement steel, and higher cost per square metre.
Why does larger span increase costs so dramatically for open-plan homes?
When clients request open-plan living, kitchen, dining, and lounge flowing together without columns, they are asking the structure to span further. This costs money in multiple ways: larger members consume more material, deeper slabs require more labour and formwork, heavier loads demand deeper or more expensive foundations, and larger concrete beams create larger thermal bridges, increasing heat loss if exposed on the interior or exterior.
In typical Slovak residential passive-house projects, cost per square metre rises sharply for spans beyond 7–8 metres. At 9 metres and above, expense becomes difficult to justify without specific reasons such as large glazed facades or commercial programmes that amortize cost across many units. The cost per square metre can triple for spans beyond 10 metres compared to a 5-metre span.
What are typical span ranges for different structural systems?
Residential structural systems have different natural span ranges, each with economic and practical limits.
| Structural System | Economic Span (m) | Practical Max (m) | Notes |
|---|---|---|---|
| Masonry load-bearing walls (timber joists) | 3–4 | 4–5 | Limited flexibility; few open plans possible. |
| Reinforced concrete monolithic slab | 5–7 | 8–9 | Standard for Central European residential. Cost rises sharply above 7 m. |
| Post-tensioned concrete slab | 6–8 | 10–12 | Handles longer spans with less depth; 10–15% cost premium often recovers through material savings. |
| Reinforced concrete frame | 6–8 | 10–12 | Greater flexibility; intermediate columns reduce secondary-member span. |
| Roof truss (timber or steel) | 6–15 | 20+ | Efficient over long spans; residential cost rises steeply above 12–15 m. |
How can architects reduce cost without visible columns?
Strategic column placement breaks long spans into shorter ones. A 9-metre room can be divided with an interior column or stiffening wall, using two 4–5 metre spans instead of one 9-metre span, dramatically reducing slab depth and cost. Partial open-plan design works equally well: removing walls between kitchen and dining (2–3 m span) while separating the lounge with a stiffening wall or column line (creating 5–6 m span) satisfies the desire for openness while controlling cost.
Post-tensioned concrete allows 1–2 metres greater span at the same depth, or the same span at notably less depth, by compressing the slab and reducing tensile stress. Hybrid systems, such as a skeleton frame structure with lighter floor plates, distribute span across multiple members.
How does span affect floor height and deflection?
Structural depth increases with span, so total floor-to-floor height must increase. In residential construction, typical floor-to-floor is 3.3–3.5 metres. A monolithic slab for 5-metre span plus floor buildup (screed, insulation, finish) consumes roughly 350 mm. For 7 metres, the slab alone might be 300 mm plus 150–200 mm for buildup, leaving minimal structural height available. Deflection, how much the slab bends under load, also increases sharply with span and must be controlled through thickness. Excessive deflection can jam doors, crack partitions, and damage finishes.
How does span affect thermal performance?
Longer spans require thicker structural members, which can create significant thermal bridges if not carefully detailed. A deep concrete beam exposed on the interior face is a large mass with minimal insulation; in winter, its surface temperature drops significantly below room temperature, creating discomfort and potential condensation. A thermal bridge through a 600 mm deep beam is far more problematic than one through a 400 mm beam. Long-span design therefore requires careful attention to thermal-break details and avoiding cantilevers that penetrate the insulation layer.
How should span be planned in the design process?
Span should be a primary design decision made during concept design, not an afterthought. Early coordination with a structural engineer identifies which spans are economical for the chosen system (monolithic concrete, reinforced-concrete frame, or roof truss) and which require expensive solutions like post-tensioning. The best residential designs in Slovakia typically use modest spans of 5–7 metres for floors, paired with intelligent column placement, stiffening walls, or visual tricks to preserve openness while controlling cost and performance risk.
| Project Type | Typical Span | Reasoning |
|---|---|---|
| Urban renovation (tight budget) | 4–5 m | Minimal footprint; columns integrated into plan. |
| Suburban passive-house family home | 5–7 m | Balance between open-plan and construction cost. |
| Large rural plot (premium budget) | 6–9 m | Longer spans justified by larger budget and desire for views. |
Frequently asked questions
- Why does span matter to a house owner?
- Span directly controls the depth and cost of your floor structure. A 6 m open-plan living room requires a thicker reinforced slab or deeper beams than a 4 m space. Larger spans mean heavier structural members, more concrete, more steel, and significantly higher construction costs.
- How much thicker does a slab need to be for a larger span?
- Slab thickness typically increases non-linearly with span. A monolithic slab spanning 4 m might be 200 mm thick, but a 7 m span may require 300-350 mm. Doubling the span does not double the depth, but the increase is substantial and directly adds cost.
- Can you use columns to make larger open-plan rooms without a thicker slab?
- Yes. Adding intermediate columns breaks long spans into shorter ones, allowing thinner slabs and smaller beams. This is the structural strategy behind most residential open-plans, but columns occupy floor space, which many clients want to avoid.
- What is the economic span limit for residential concrete construction?
- In typical Slovak passive-house construction, economic span ranges are 5–7 m for monolithic slabs and 7–9 m for post-tensioned slabs. Beyond 8–9 m, the cost per square meter rises sharply as slab depth increases dramatically and reinforcement becomes heavy.
- Does span affect passive-house performance?
- Indirectly. Larger spans require thicker structural members, which can create larger thermal bridges if not carefully detailed. A deep concrete beam exposed on the interior or exterior can be a significant source of unwanted heat loss.
- Why can't beams just be longer without getting deeper?
- Beams bend under load, and the amount of bending increases with span squared. Doubling the span increases deflection fourfold. To keep deflection within acceptable limits and control cracking, the beam must deepen significantly.