10 Common RSJ Installation Mistakes That Could Cause Structural Failure

RSJ beam installation errors can have catastrophic consequences, from progressive structural collapse to expensive remedial works and legal prosecution. This guide identifies the most common mistakes made during RSJ installation, explains why they’re dangerous, and shows how to avoid them for safe, compliant construction in 2026.

Mistake #1: Inadequate Temporary Support

The Error: Using too few acrow props or poor temporary works during installation

Why It’s Dangerous:

• Structure above can collapse during beam installation
• Risk of death or serious injury
• Progressive failure once movement starts

Real example: Victorian terrace knockthrough, only 2 props used for 4m opening. During brick removal, ceiling dropped 50mm. Building evacuated, emergency shoring required. Cost: £8,500 remedial + £3,000 structural engineer + project delay.

Correct Practice:

• Calculate temporary support loads properly
• Use 50-100% extra props beyond minimum
• Install strongbacks to distribute loads
• Load props progressively and evenly
• Never remove existing structure before temporary support fully loaded

Rule of thumb: Minimum 3-4 props for 4m opening, spaced evenly

Mistake #2: Insufficient Bearing Length

The Error: Beam sitting on <100mm of wall at each end

Why It’s Dangerous:

• Localized crushing of masonry
• Beam can punch through wall
• Progressive collapse as bearing fails

Real example: 254×146×31 RSJ with only 75mm bearing on brick wall. After 6 months, visible cracking in brickwork. Structural assessment revealed crushing. Beam had to be temporarily supported, wall rebuilt with proper padstones. Cost: £4,200.

Correct Practice:

• Minimum 100mm bearing each end (Building Regs)
• Prefer 150mm for safety margin
• Always use padstones on masonry
• Verify bearing capacity of supporting structure

Calculation: For 203×133×25 beam carrying 10 kN/m over 4m span:

• End reaction: ~20 kN each end
• On 150mm bearing: Pressure = 20,000N / (150×133mm) = 1.0 N/mm²
• Brickwork capacity: ~2.5 N/mm² typical ✓ Acceptable

With only 75mm bearing: Pressure = 2.0 N/mm² (marginal →risk of crushing)

Mistake #3: Oversized Beam Won’t Fit

The Error: Ordering beam before checking access/dimensions

Why It’s Problematic:

• Beam won’t fit through doors/stairs
• Can’t get to installation location
• Expensive emergency solutions needed

Real example: 305×165×40 UB ordered for loft conversion. Beam 8m long, won’t fit through any door or up stairs. Solution required crane lift over roof, tile removal, lowering through ceiling opening. Additional cost: £850 crane + £300 roof repairs.

Correct Practice:

• Plan delivery route BEFORE ordering
• Measure door widths, stair turns, ceiling heights
• Consider beam rigidity (long beams don’t bend around corners)
• Factor in beam depth + width when navigating
• Have contingency plan (window removal, crane lift)

Prevention: Include access assessment in structural engineer’s brief

Mistake #4: Wrong Beam Size/Grade

The Error: Using beam that looks similar but isn’t to specification

Why It’s Dangerous:

• Inadequate strength for loads
• Excessive deflection
• Potential failure under full loading

Real example: Engineer specified 203×133×30 S275. Builder used 203×133×25 S275 (“close enough, saves £80”). After 2 years, progressive sagging, cracked plaster. Building Control failure notice. Beam replaced at builder’s cost: £3,500.

Correct Practice:

• Follow engineer’s specification EXACTLY
• Verify beam markings on delivery
• Check mill certificates if provided
• NEVER substitute without engineer approval

Key point: Even one size smaller can reduce capacity by 15-20%

Mistake #5: No Padstones

The Error: Placing RSJ directly on brickwork without concrete padstones

Why It’s Dangerous:

• Point load crushing of brick
• Uneven bearing (rocks/wobbles)
• Long-term settlement

Real example: 254×146×37 RSJ installed directly on brick pier. After 18 months, visible sinking into brickwork, beam tilted 15mm. Load redistributed unevenly, cracking walls either side. Remedial work £2,800.

Correct Practice:

• ALWAYS use padstones for masonry bearing
• Padstone size per engineer’s specification
• Typically 440×215×100mm minimum
• Placed on 10mm mortar bed
• Must be level (both padstones at identical height)

Padstone purpose:

• Distributes load over larger area
• Provides hard, even bearing surface
• Prevents localized crushing

Mistake #6: Removing Temporary Support Too Soon

The Error: Props removed immediately after beam installation

Why It’s Dangerous:

• Masonry above not yet cured
• Sudden load transfer can cause cracking
• Settlement risk

Real example: Props removed same day as beam installed. Overnight, 5mm drop in ceiling, extensive cracking through house. Immediate re-propping required, masonry rebuild. Cost: £5,200 + temporary accommodation £1,400.

Correct Practice:

• Leave props for minimum 7 days (prefer 14 days)
• Ensure all masonry/concrete fully cured
• Remove progressively (quarter-turn at a time)
• Monitor for movement during removal
• Have engineer inspect before final prop removal

Curing times:

• Mortar: 7 days minimum
• Padstone concrete: 3-7 days
• Cold weather: Double these times
• Building Control should inspect before removal

Mistake #7: Poor Lateral Restraint

The Error: Top flange not adequately restrained against sideways buckling

Why It’s Dangerous:

• Lateral-torsional buckling failure
• Beam twists sideways under load
• Sudden catastrophic collapse possible

Real example: 6m ridge beam in loft conversion, rafters simply resting on top (not fixed). Beam gradually twisted 8° over 6 months. Collapse risk identified during survey for sale. Emergency restraint required: £6,800.

Correct Practice:

• Fix floor joists/rafters to beam top flange
• Use joist hangers bolted through flange
• Provide intermediate restraint at max 1.2m spacing
• For unrestrained applications, use wider (higher lateral stability) sections

Engineer should specify restraint requirements in calculations

Mistake #8: Wrong Beam Orientation

The Error: Installing beam sideways or upside-down

Why It’s Dangerous:

• Dramatically reduced bending capacity
• Unstable configuration
• Almost certain failure under full load

Real example: (Yes, this really happens!) 203×133×30 RSJ installed on its side (flanges vertical, web horizontal). Discovered during Building Control inspection. Immediate stop order. Beam removed and reinstalled correctly. Cost: £2,400 + 3-week delay.

Correct Practice:

• Beam must be oriented with flanges horizontal, web vertical
• “I” shape when viewed from end (never “H” or ”—”)
• Double-check before building masonry over
• Verify floor joists bear on TOP flange, not web

Obvious rule but mistakes happen in rushed/poorly supervised work!

Mistake #9: Cutting/Drilling Without Approval

The Error: Cutting holes or notches in beam for services without engineer approval

Why It’s Dangerous:

• Reduces section modulus
• Can reduce capacity by 30-50%+
• Stress concentrations at cuts

Real example: Builder drilled 80mm holes through web of 254×146×31 for plumbing pipes (three holes). Beam capacity reduced ~35%. Discovered during later renovation. Beam required reinforcement plates welding: £1,850.

Correct Practice:

• NEVER cut, drill, notch, or weld beam without engineer approval
• If services must pass through, engineer specifies:
  • Location (usually mid-depth of web, between 1/4 and 3/4 span points)
  • Maximum size
  • Reinforcement if required
• Plan service routes around structure, not through it

Even small holes can have big impact on capacity

Mistake #10: No Building Control Approval

The Error: Installing RSJ without Building Regulations approval

Why It’s Illegal:

• Building Act 1984 requires approval
• Invalidates insurance
• Can’t sell property
• Enforcement action possible

Real example: Knockthrough done without Building Control. 5 years later, selling house. Survey identifies unofficial work. Building Control retrospective application refused (can’t inspect concealed work). Options: Open up for inspection (£6k+) or remove beam, reinstate wall, hire engineer, redo properly (£15k+). Sale collapsed.

Correct Practice:

• Apply for Building Regs approval BEFORE work starts
• Fee: £200-450 typical
• Arrange all required inspections:
  • Temporary works (before removal)
  • After padstone installation
  • After beam installation (before making good)
  • Final completion
• Obtain completion certificate
• File certificate safely for future property sales

Legal requirement, not optional!

Additional Common Mistakes

Mistake #11: Ignoring DPC (Damp-Proof Course)

Error: No membrane between concrete padstone and steel beam

Result: Moisture wicks from concrete to steel, causing corrosion

Fix: Always place bitumen felt or plastic DPC layer

Mistake #12: Poor Quality Padstones

Error: Using weak concrete or damaged pre-cast padstones

Result: Crushing, uneven bearing, settlement

Fix: Minimum C20/25 grade concrete, check pre-cast for cracks

Mistake #13: Forgetting Fire Protection

Error: Exposed steel where fire protection required

Result: Building Control rejection, insurance issues

Fix: Check if beam creates escape route or other fire safety implications

Mistake #14: Not Accounting for Beam Weight

Error: Temporary support doesn’t include beam self-weight

Result: Overloaded props, instability

Fix: Add beam weight to load calculations (e.g., 203×133×30 = 30 kg/m = 0.3 kN/m)

Mistake #15: Mismatched Steel Grade

Error: Using S275 when S355 specified (or vice versa)

Result: Inadequate capacity or unnecessary cost

Fix: Verify grade markings on beam match engineer’s spec

Consequences of Installation Mistakes

Immediate Risks

• Structural collapse during installation
• Death or serious injury to workers/occupants
• Immediate Building Control stop order

Short-Term Consequences

• Progressive sagging (weeks to months)
• Cracking in plasterwork/finishes
• Doors/windows sticking due to movement

Long-Term Consequences

• Corrosion from inadequate protection
• Foundation settlement from overloading
• Property value impact when selling
• Insurance claims rejected due to non-compliance

Financial Impact

• Remedial works: £2,000-15,000+ typical
• Emergency shoring: £3,000-8,000
• Legal costs: if prosecution occurs
• Lost time: Project delays

Legal Consequences

• Building Control prosecution (fines up to £5,000)
• HSE prosecution if injuries result
• Civil liability for damage/injury
• Professional indemnity claims against builders

How to Avoid These Mistakes

1. Hire Qualified Professionals

Essential professionals:

• Chartered structural engineer for design
• Experienced builder/contractor for installation
• Building Control for regulatory oversight

Don’t skip professionals to save money - mistakes cost far more!

2. Follow Engineer’s Specifications Exactly

• Read and understand drawings/calculations
• Don’t substitutewithout approval
• Query anything unclear
• Keep site copies of specifications

3. Plan Thoroughly Before Starting

• Access routes
• Temporary support requirements
• Equipment needs (props, crane, scaffolding)
• Material procurement lead times
• Building Control inspection schedule

4. Use Adequate Temporary Support

• Calculate properly
• Over-specify (add 50-100% extra capacity)
• Install progressively
• Never skip this step!

5. Quality Control

• Inspect beam on delivery
• Verify dimensions and grade
• Check padstones
• Test temporary support before demolition
• Photograph each stage

6. Building Control Compliance

• Apply early
• Book inspections in advance
• Don’t conceal work before inspection
• Obtain completion certificate

7. Allow Adequate Time

• Don’t rush critical stages
• Respect curing times
• Plan for weather delays
• Buffer time for unexpected issues

Conclusion

RSJ installation mistakes range from annoying and expensive to potentially fatal. The ten most critical errors are:

1. Inadequate temporary support
2. Insufficient bearing length
3. Access/size planning failures
4. Wrong beam specification
5. No padstones
6. Premature prop removal
7. Poor lateral restraint
8. Incorrect orientation
9. Unauthorized modifications
10. No Building Control approval

Prevention is simple but essential:

• Hire qualified professionals (engineer + builder)
• Follow specifications exactly
• Don’t cut corners to save money
• Obtain Building Control approval
• Allow adequate time for each stage
• Inspect and verify throughout

The cost of doing it properly (£350-600 for engineer + £220-450 for Building Control) is trivial compared to remedial works (£2,000-15,000+) or the consequences of structural failure.

When in doubt, consult your structural engineer - they’re there to help ensure safety and compliance.

Disclaimer: This guide provides general information about common mistakes. Every project must be designed by a chartered structural engineer and approved by Building Control. Never undertake structural work without professional involvement.