RSJ for Loft Conversion: Sizes, Supports, and UK Building Regulations Checklist
Loft conversions are one of the most cost-effective ways to add living space and property value. RSJ steel beams play a crucial role in most loft conversions, providing structural support for new floors, removing walls, and creating open spaces. This guide covers beam selection, installation locations, sizing requirements, and Building Regulations compliance for UK loft conversions in 2026.
Common RSJ Applications in Loft Conversions
1. Steel Ridge Beam
Purpose: Eliminates traditional collar ties, creating full-height usable space
Typical scenario:
- Existing roof has W-shaped trusses or collar ties limiting headroom
- Remove ties and install ridge beam to support rafters
- Dramatically improves usable volume
Beam location: At apex of roof, running full length of conversion
Typical sizes:
- Small loft (3-4m width): 203×133×25 RSJ
- Medium loft (4-5m width): 203×133×30 or 254×146×31 RSJ
- Large loft (5-6m width): 254×146×37 RSJ
Loads: Roof dead load (tiles, battens, insulation) + snow live load
Approximate load: 2.5-4.0 kN/m per side of roof
2. Floor Support Beams
Purpose: Strengthen existing ceiling joists to support floor loads
Two main configurations:
A) Steel beam supporting new floor joists:
- Beam spans across loft at mid-span of new joists
- Joists rest on beam via joist hangers
- Reduces individual joist spans
B) Steel beam replacing/supplementing ceiling joists:
- Beam acts as very strong joist
- Other joists hang from it or rest on it
- Common when original ceiling joists inadequate
Typical sizes:
- 3-4m span: 203×133×30 RSJ
- 4-5m span: 254×146×31 RSJ
- 5-6m span: 254×146×37 RSJ
Loads: Floor construction + bedroom live load (typically 8-12 kN/m total)
3. Stairwell Opening Header
Purpose: Frame opening where new stairs penetrate existing floor
Beam location: Supporting trimmed floor joists around stair opening
Typical size: 203×133×25 or 203×133×30 RSJ
Span: Usually 1.5-2.5m (width of stairwell)
4. Dormer Window Support
Purpose: Support roof structure where dormer interrupts main roof
Applications:
- Dormer cheeks (side walls)
- Dormer roof structure
- Front face beam
Typical sizes: Varies greatly, often smaller sections (152-203mm depth)
5. Party Wall Support
Purpose: Support party wall if removing lower-floor wall below loft
**Less common but sometimes required for open-plan living below
Typical sizes: Very similar heavy beams, often 254-305mm depth
Beam Sizing by Loft Configuration
Scenario 1: Basic Loft Conversion (Velux Windows)
Configuration:
- 4m wide semi-detached house
- Pitched roof with existing rafters
- No structural alterations to roof
- Velux windows only (no dormers)
- Strengthen floor only
RSJ requirements:
Floor support beam:
- Span: 6m (length of house)
- New floor joists span 2m each side to meet at beam
- Dead load: 0.6 kN/m² × 2m = 1.2 kN/m
- Live load: 1.5 kN/m² × 2m = 3.0 kN/m
- Total: ~4.2 kN/m
Beam selection: 203×133×30 RSJ (capacity ~6.0 kN/m at 6m span) ✓
Stairwell opening:
- Span: 2.0m
- Load: Similar to floor beam locally
- Beam: 203×133×25 RSJ
Total beam cost: £450-600
Scenario 2: Loft with Rear Dormer
Configuration:
- 5m wide terraced house
- Full-width rear dormer (5m × 2m projection)
- Ridge beam to remove collar ties
- Strengthened floor
RSJ requirements:
Ridge beam:
- Span: 7m (length of house)
- Roof load: 3.5 kN/m per side × 2 = 7.0 kN/m
- Beam: 254×146×37 RSJ (8m length total)
Floor beams (2× parallel):
- Span: 5m each
- Load: 5.5 kN/m
- Beams: 203×133×30 RSJ × 2
Dormer support:
- Several smaller beams for dormer structure
- Typically 152×127×37 or smaller
Total beam cost: £950-1,350
Scenario 3: Hip-to-Gable Loft Conversion
Configuration:
- 4.5m wide semi-detached
- Remove hipped end, build gable wall
- Full ridge beam
- Side dormer on gable end
- New floor structure
RSJ requirements:
Ridge beam:
- Span: 8m
- Load: 6.8 kN/m
- Beam: 254×146×43 RSJ
Floor beams:
- Main beam: 254×146×31 RSJ, 8m
- Secondary beams: 203×133×25 × 3
Gable end support:
- Steel goal posts or columns supporting new gable
- May use UC (column) sections
Total beam cost: £1,200-1,800
Building Regulations Checklist (UK 2026)
Planning Permission Requirements
Usually REQUIRED for:
- Loft conversion increasing roof height
- Dormer on front elevation
- Hip-to-gable conversion
- In conservation area
- Listed building
May NOT require planning if:
- Volume increase <40/50 m³ (terraced/detached)
- No increase in roof height
- Dormers on rear only
- Not in conservation area or listed
- Within permitted development rights
Always check with local planning authority!
Building Regulations Compliance
ALWAYS required - no exceptions
Key areas:
1. Structural Stability (Part A):
- Structural engineer calculations for all steel beams
- Foundation checks if adding significant load
- Proper connections and fixings
2. Fire Safety (Part B):
- 30-minute fire resistance for escape route
- Smoke alarms (interlinked)
- Fire doors to habitable rooms
- Protected stairway
- Emergency escape window in each habitable room
3. Sound Insulation (Part E):
- Floor construction achieving required acoustic performance
- Usually requires resilient bars under ceiling below
- Acoustic insulation between joists
4. Ventilation (Part F):
- Background ventilation (trickle vents)
- Purge ventilation (openable windows)
- Extract ventilation if bathroom/WC
5. Energy Efficiency (Part L):
- Insulation to roof: 270mm (loft-void standard) or 100mm between rafters
- U-values: 0.16-0.18 W/m²K
- Energy-efficient windows
6. Stairs (Part K):
- Pitch: Maximum 42°
- Headroom: Minimum 2m
- Handrails required
- Balustrading max 100mm gap
7. Glazing Safety (Part N):
- Safety glass where required
- Guards at windows/openings
8. Means of Escape:
- Escape window in each habitable room
- Min opening: 450mm high × 450mm wide
- Max sill height: 1100mm above floor
Building Control Process
Timeline and steps:
Week 1: Engage structural engineer
- Site survey
- Establish loads and requirements
- Initial beam sizing
Week 2-3: Structural design
- Engineer produces calculations
- Specifies all steel beams
- Details connections and supports
Week 4: Building Regulations application
- Submit engineer’s calculations
- Floor plans and sections
- Fire strategy
- Thermal calculations
- Fee: £400-650 typical
Week 5-7: Await approval
- Building Control reviews
- May request clarifications
- Approval granted
Week 8+: Construction with inspections
- Foundation inspection (if applicable)
- Floor joist/beam installation
- Staircase installation
- Fire protection inspection
- Final completion inspection
Critical: Don’t start work before approval, and notify Building Control 24-48 hours before each inspection
Installation Considerations
Access Challenges
Getting beams into loft:
Via internal stairs: Difficult/impossible for beams >4m
- May need to pass through windows
- Or hoist externally
Via scaffolding: Most common method
- Hoist beam up outside
- Pass through roof opening
- Cost of scaffolding: £800-1,500
Crane lift: For very heavy/long beams
- Lift directly to roof level
- Lower through prepared opening
- Cost: £300-600 for crane hire
Temporary Support
Critical during installation:
Support existing roof:
- Acrow props from floor below to rafters
- Before removing collar ties or structural elements
- Multiple props required (6-10 typical)
Support during beam installation:
- Props under beam as positioned
- Don’t remove until connections complete and new structure loaded
Connections
Ridge beam connections:
- Steel hangers from rafters to beam
- Typically manufactured items, specify in engineer’s drawings
- Bolted or nailed fixings
Floor beam connections:
- Joist hangers to steel beam flanges
- Bolt through flange with appropriate washers
- Ensure adequate spacing per joist spacing
End supports:
- Usually loadbearing walls
- Install padstones if bearing on masonry
- Minimum 100mm bearing, prefer 150mm
Cost Breakdown (2026 UK)
Basic Velux Loft Conversion
| Item | Cost |
|---|---|
| Structural engineer | £400-700 |
| Building Regulations fee | £400-600 |
| RSJ beams (floor + stairwell) | £450-600 |
| Padstones and fixings | £100-180 |
| Floor joists and boarding | £800-1,200 |
| Insulation (roof and floor) | £600-900 |
| Plasterboard and plaster | £700-1,100 |
| Staircase | £800-1,800 |
| 2× Velux windows | £800-1,400 |
| Electrics | £600-1,000 |
| Heating | £400-700 |
| Decoration | £500-900 |
| Total (DIY finish) | £6,550-10,080 |
| Professional installation | £15,000-22,000 |
Full Dormer Conversion
| Item | Cost |
|---|---|
| Structural engineer | £600-1,000 |
| Building Regulations | £500-750 |
| RSJ beams (ridge, floor, dormer) | £950-1,350 |
| Scaffolding | £1,200-1,800 |
| Dormer construction | £4,000-7,000 |
| Floor structure | £1,200-1,800 |
| Insulation | £900-1,400 |
| Plasterboard/plaster | £1,200-1,900 |
| Staircase | £1,000-2,200 |
| Windows/doors | £1,200-2,400 |
| Electrics | £800-1,400 |
| Bathroom (if included) | £2,500-5,000 |
| Heating | £600-1,000 |
| Decoration | £800-1,500 |
| Total (professional) | £25,000-45,000 |
Common Mistakes
1. Inadequate headroom:
- Not checking finished floor to ceiling height
- Building Regs minimum: 2.3m over 50% of floor area
- Solution: Lower ceiling below or raise roof (needs planning)
2. Undersized beams:
- Using rules of thumb instead of calculations
- Forgetting beam self-weight
- Solution: Always use structural engineer calculations
3. Poor fire protection:
- No fire doors
- Inadequate escape route
- Solution: Plan fire strategy early, include in Building Regs application
4. Insufficient insulation:
- Trying to save headroom by skimping on insulation
- Not meeting Part L requirements
- Solution: Use high-performance insulation, or build out further
5. Ignoring acoustics:
- Floor too noisy to rooms below
- Solution: Resilient bars, acoustic insulation, proper floor construction
Professional vs. DIY
Recommended approach:
Hire professionals for:
- Structural engineering design (essential, non-negotiable)
- Steel beam installation (safety-critical)
- Dormer construction (complex carpentry)
- Major structural works
- Fire protection installation
- Building Control liaison
Potential DIY elements:
- Interior finishing (plasterboard, decoration)
- Some insulation installation (supervised)
- Flooring laying
- Basic electrics (if qualified)
Realistic savings from DIY finishing: £3,000-6,000
DON’T attempt to DIY:
- Structural calculations
- Beam installation without experience
- Major roof alterations
- Gas or unqualified electrical work
Return on Investment
Typical value added:
- Basic conversion: £20,000-35,000 added value
- Full dormer conversion: £35,000-55,000 added value
- En-suite loft bedroom: £40,000-65,000 added value
Cost vs. value:
- Basic conversion cost: £15,000-22,000, value added: £25,000-35,000
- Net gain: £8,000-15,000
ROI: 120-160% typical
Better than extension: Loft conversions usually better ROI than ground floor extensions (cost less, add similar value)
Conclusion
RSJ steel beams are essential components of most loft conversions, providing structural support for floors, roofs, and new openings. Proper sizing requires structural engineering calculations, and all work must comply with Building Regulations including planning permission where applicable.
Key success factors:
- Engage structural engineer early - designs all beams correctly
- Obtain planning permission if required
- Building Regulations approval essential - no exceptions
- Professional beam installation - safety critical
- Plan access and lifting - beams are heavy and awkward
- Fire safety compliance - non-negotiable for habitable loft
- Adequate insulation and ventilation - for comfort and regulations
Budget guideline:
- Basic Velux conversion: £15,000-22,000 professional or £10,000-15,000 with DIY finishing
- Full dormer conversion: £25,000-45,000
The investment in proper structural design and RSJ beams ensures a safe, compliant, and valuable loft conversion that will last for decades.
Disclaimer: This guide provides general information only. All loft conversion work requires structural engineering design and Building Regulations approval. Never undertake structural alterations without professional involvement. Costs are indicative 2026 UK estimates.
