"Just use the next size up to be safe." Every builder has said it. It sounds prudent. It's actually wasteful, and it can create problems. Here's why proper structural design saves you money.
The Myth of "Better Safe Than Sorry"
Structural engineers design beams with safety factors already built in. When we calculate that a 203×133×25 UKB will work, it's not a marginal decision—it's designed to carry significantly more load than it will ever see in normal use.
UK design codes (Eurocodes) require partial safety factors of 1.35 to 1.5 on permanent loads and 1.5 on variable loads. This means the beam is designed to carry at least 35-50% more load than it actually experiences. The safety margin is already there.
Specifying a 254×146 UKB when calculations show a 203×133 UKB is adequate doesn't make it "safer"—it just makes it more expensive and harder to install.
The Direct Costs
1. Steel Material Cost
Steel is priced per kilogram. A heavier section costs proportionally more:
- 203×133×25 UKB: £25 per meter (approx.)
- 254×146×37 UKB: £40 per meter (approx.)
For a 4m span, that's £100 vs. £160—an extra £60 in steel alone. Multiply this across multiple beams in an extension, and you're adding hundreds of pounds unnecessarily.
2. Larger Padstones
Heavier beams impose higher bearing loads, which require larger padstones (the concrete blocks that support the beam ends). A 203×133 UKB might need 215×215×100mm padstones; a 254×146 UKB needs 215×215×140mm or larger.
More concrete, more time to install, potentially deeper pockets into the masonry = higher labor and material costs.
3. Installation Difficulty
Weight matters on site. A 25 kg/m beam (4m span = 100kg total) can be manhandled by two people. A 37 kg/m beam (4m span = 148kg) needs three people or lifting equipment.
If your builder needs to hire a mini crane or telehandler because the beam is too heavy to lift manually, that's an extra £200-£400 hire cost.
The Hidden Costs
1. Deflection Mismatch
Oversized beams are stiffer than necessary. This sounds good—less deflection, right? But in refurbishment work, mismatched stiffness can cause problems.
Example: You install a very stiff 305×165 UKB in an opening within an older, flexible structure. The surrounding brickwork settles slightly over time (normal movement). The stiff beam doesn't move. Result: cracking at the interface.
Engineers design deflection limits (typically span/360 for domestic work) to ensure the beam moves compatiblywith the rest of the structure. Going significantly stiffer than calculated can create problems, not prevent them.
2. Thermal Movement and Expansion
Steel expands and contracts with temperature. Larger sections have greater mass and surface area, meaning larger absolute movement. In most domestic situations this is negligible, but in poorly detailed installations (beam tightly wedged into pockets without provision for movement), it can create cracking.
3. Fire Protection Costs
If fire protection is required (common in flats and some terraced houses), larger sections need thicker fire protection due to their section factor (surface area to volume ratio). Intumescent paint or board cladding costs scale with surface area and thickness required.
When "Bigger" Actually Causes Problems
Headroom Loss
A 203mm deep beam vs. a 254mm deep beam = 51mm extra depth. In a loft conversion where headroom is already tight, this could mean failing to meet the 2m minimum headroom requirement.
Going wider instead (e.g., using a 254×254 UC instead of a deeper UKB) solves depth but creates a bulky, expensive beam and requires even larger padstones.
Connection Details
If the beam connects to other steelwork (common in extensions), oversized sections complicate connection design. The connecting plates, bolts, and welds all scale with the section size. More fabrication = higher cost.
How Pragmatic Engineering Saves Money
Good structural engineers don't just design safe structures—they optimize for cost, buildability, and practicality. This means:
- Right-sizing sections: Specifying the lightest, most economical section that meets code requirements. Not the smallest possible (which risks deflection issues), but not wastefully large either.
- Considering buildability: If two sections both work structurally, we choose the one that's easier to install, more readily available, or creates fewer knock-on costs (smaller padstones, less fire protection, etc.).
- Avoiding over-complication: Sometimes a slightly larger but simpler design is more economical overall than a highly optimized complex design. We balance technical efficiency with real-world practicality.
- Material alternatives: For short spans or light loading, engineered timber (glulam, LVL) can be more cost-effective than steel. We recommend timber where appropriate.
What "Good Enough" Actually Means
In engineering, "good enough" doesn't mean cutting corners—it means meeting the requirement without wasteful over-provision.
A properly designed 203×133×25 UKB that meets deflection limits, satisfies Building Regulations, and has adequate safety factors is not "just good enough"—it's optimal. Specifying a 254×146 UKB for the same application isn't "better," it's wasteful.
When Bigger Is Actually Justified
There are legitimate reasons to go larger than the minimum:
- Future-proofing: If you plan to add a storey above later, designing for that now is sensible.
- Deflection-sensitive finishes: If you're installing large-format tiles or stone cladding below the beam, we might specify a stiffer section to minimize deflection cracking.
- Vibration concerns: Long-span timber floors can feel "bouncy" if beams are too flexible. Slightly stiffer sections reduce vibration.
These are engineering decisions, considered in context. Not a blanket "bigger is safer" approach.
Real-World Example
Scenario:
Wall removal in a two-storey terrace. 3.8m span, cavity wall + pitched roof + first floor above.
Calculated requirement:
203×133×30 UKB in S275 steel.
Builder's suggestion:
"Use a 254×146×37 to be safe."
Cost impact:
- Steel: £40 extra
- Larger padstones: £30 extra materials + 1 hour extra labor (£50)
- Heavier lifting: Mini crane hire (£250)
- Total unnecessary cost: £370
The 203×133×30 UKB was designed to carry the load with appropriate safety factors. The 254×146×37 UKB added zero additional safety but cost £370 more.
Trust Your Engineer's Calculations
If your structural engineer specifies a certain section size, it's not a guess or a minimum—it's the result of detailed calculations balancing strength, stiffness, economy, and buildability.
If your builder suggests going bigger "to be safe," ask: safe against what? The calculated section already includes safety factors for worst-case loading, material variability, and construction tolerances.
Over-specification isn't prudent—it's wasteful. FM Structural designs to optimize cost and performance, not to over-engineer because it's easier than thinking carefully about the actual requirement.
How We Approach This
At FM Structural, we don't specify larger sections than necessary. Our approach:
- Calculate the actual loads using BS EN 1991 (Eurocode 1: Actions on Structures)
- Design to BS EN 1993 (Eurocode 3: Design of Steel Structures) with code-mandated safety factors
- Check deflection against serviceable limits (typically span/360 for domestic work)
- Specify the lightest section that meets all requirements
- Provide buildability notes to help your builder install efficiently
This saves you money without compromising safety. It's how engineering should be done.
