Building a Garage with RSJ Beams: Full Plans, Costs, and Load Calculations 2026
Building a garage is a significant home improvement project that adds value and functionality to your property. Using RSJ steel beams for critical structural elements provides strength, durability, and design flexibility. This comprehensive guide covers planning, design, beam selection, costs, and regulations for garage construction using steel beams in 2026.
Why Use RSJ Beams in Garage Construction?
Key Structural Elements Using Steel
1. Header beam over garage door:
- Spans 2.4m (single) to 6m (double) garage doors
- Supports wall, floor, or roof above
- Steel allows thinner profile than alternative materials
2. Ridge beam (if applicable):
- Spans length of garage for pitched roof
- Supports roof rafters
- Eliminates need for internal supports
3. Purlins and supports:
- Secondary structural members
- Distribute loads to main beams
4. Mezzanine/storage floor beams:
- Create storage space above garage
- Support significant loads safely
Advantages of Steel in Garage Construction
Strength: Span wider openings without intermediate posts - maximize vehicle access and internal space
Durability: Resistant to rot, insects, warping - ideal for potentially damp garage environment
Fire resistance: Better performance than timber when properly protected
Insurance: Some insurers prefer steel-frame construction for reduced fire risk
Future-proofing: Easy to add mezzanine, lift equipment, heavy storage later
Typical Garage Configurations
Single Garage (2.4-3.0m door)
Dimensions:
- Width: 3.0-3.6m internal
- Depth: 5.0-6.0m
- Height: 2.4-2.8m to eaves
Key RSJ requirement:
- Header beam over door: 203×133×25 or 203×133×30
- Typical span: 3.2-3.5m (door + bearing)
Application scenarios:
- Garage only (no room above): Lighter beam sufficient
- Storage above: Medium beam
- Bedroom/room above: Heavier beam per structural engineer
Double Garage (4.5-6.0m door)
Dimensions:
- Width: 5.0-6.5m internal
- Depth: 5.5-6.5m
- Height: 2.4-2.8m to eaves
Key RSJ requirements:
- Header beam: 254×146×37 to 305×165×46 UB depending on load above
- Typical span: 5.2-6.3m
Optional RSJ elements:
- Ridge beam if pitched roof
- Mezzanine floor beams
- Side wall support beams
Garage with Room Above
Additional considerations:
- Much heavier loads on header beam
- Floor joists spanning onto header or separate beams
- Typically requires 254mm or 305mm beams
- Professional structural engineering essential
Beam Sizing for Common Scenarios
Scenario 1: Single Garage, No Load Above
Configuration:
- 3.0m wide up-and-over door
- Brick/block walls
- Pitched timber roof with tiles
- No room or storage above
Loads on header beam:
- Masonry above door (triangular load pattern, ~2m high): 4.5 kN/m
- Roof load (limited by arch action): 2.0 kN/m
- Total UDL: ~6.5 kN/m
Beam required:
- Span: 3.3m (3.0m door + 150mm bearing each end)
- Load: 6.5 kN/m
Selection from span tables: 203×133×25 RSJ at 3.3m, capacity ~10 kN/m ✓ Adequate with safety margin
Cost: £260-330 for beam
Scenario 2: Double Garage, Storage Above
Configuration:
- 5.0m wide sectional door
- Timber mezzanine floor above (half of garage depth)
- Floor load: 2.5 kN/m² (storage use)
- Floor depth: 2.5m
Loads on header beam:
- Masonry: 5.5 kN/m
- Mezzanine floor dead load: 1.0 kN/m² × 2.5m = 2.5 kN/m
- Mezzanine floor live load: 2.5 kN/m² × 2.5m = 6.25 kN/m
- Total UDL: ~14.3 kN/m
Beam required:
- Span: 5.3m
- Load: 14.3 kN/m
Selection: 305×165×40 UB at 5.3m span - check tables: capacity ~15 kN/m ✓
Cost: £420-540 for beam
Scenario 3: Single Garage with Bedroom Above
Configuration:
- 2.7m wide roller door
- Full first-floor bedroom above garage
- Timber floor construction
- Floor joists 4.5m span rest mid-point on header beam
Loads on header beam:
- Masonry (full height to roof): 9.0 kN/m
- Floor dead load: 0.6 kN/m² × 2.25m = 1.35 kN/m
- Floor live load: 1.5 kN/m² × 2.25m = 3.38 kN/m
- Roof (fraction): 2.5 kN/m
- Total UDL: ~16.2 kN/m
Beam required:
- Span: 3.0m
- Load: 16.2 kN/m
Selection: 254×146×31 RSJ - capacity at 3.0m ~18 kN/m ✓ Or 254×146×37 RSJ for better deflection control
Cost: £235-310 for beam
Scenario 4: Double Garage with Concrete Floor Above
Configuration:
- 5.2m wide bi-fold doors
- Reinforced concrete floor slab above (potential vehicle storage/driveway)
- Slab: 150mm thick concrete
- Live load: 5.0 kN/m² (vehicle)
Loads on header beam:
- Masonry to first floor: 7.5 kN/m
- Concrete slab dead load: 3.75 kN/m² × 5.0m width = 18.75 kN/m
- Live load: 5.0 kN/m² × 5.0m = 25 kN/m
- Total UDL: ~51.3 kN/m (VERY HEAVY!)
Beam required: This exceeds typical residential beam capacities!
Solutions:
-
Use very large beam: 406×178×67 UB or larger
- Cost: £750-1,000+ for beam alone
- Difficult to install without crane
-
Add intermediate support: Column mid-span
- Two 3m spans instead of one 6m span
- Each beam: 254×146×43 or similar
- Column on foundation required
- More economical
Most likely specification: 254×146×43 RSJ ×2 with central column Cost: £550-700 for beams + £800-1,200 for column and foundation
Critical: This scenario REQUIRES professional structural engineering
Ridge Beam Sizing (Pitched Roof)
If garage has pitched roof without collar ties:
Typical requirements:
- Garage width: 5.5m
- Ridge beam spans length of garage: 6.0m
- Roof: Tiled, 35° pitch
- Rafters at 400mm centers
Loads on ridge beam:
- Dead load (tiles, battens, rafters): ~0.8 kN/m per side of roof
- Live load (snow): ~0.75 kN/m per side
- Total: ~3.1 kN/m per side × 2 sides = 6.2 kN/m
Beam required (6m span, 6.2 kN/m): 305×165×40 UB - check capacity ✓
Cost: £440-570
Alternative: Use traditional collar-tie roof structure eliminates need for ridge beam, uses timber throughout - cheaper but less headroom inside
Building Regulations Compliance (UK 2026)
Planning Permission
Usually NOT required if:
- Garage <30 m² floor area
- Height <4m (pitched roof) or <3m (flat roof)
- Not forward of principal elevation
- Within boundary limits (usually 50% of plot)
MAY be required if:
- Attached to listed building
- In conservation area
- Exceeds size limits
- Close to boundaries -Forward of building line
Check with local planning authority before proceeding!
Building Regulations Approval
ALWAYS required for:
- New garage construction
- Structural alterations to existing garage4
- Addition of room above garage
Application process:
- Engage structural engineer (£400-800 for garage project)
- Engineer designs structure, specifies beams
- Submit plans and calculations to Building Control
- Pay fee (£300-500 typically)
- Await approval (2-4 weeks)
- Notify Building Control before starting work
- Arrange inspections:
- Foundations
- DPC level
- Structural frame
- Completion
DO NOT skip Building Control - implications:
- Illegal construction
- Insurance invalid
- Cannot sell property
- Enforcement action and potential demolition
- Fines up to £5,000
Foundation Requirements
Steel beams impose concentrated loads requiring adequate foundations:
Typical requirements:
- Strip foundations under walls: 600mm wide × 900mm deep minimum
- Pad foundations under columns: 1200 × 1200 × 600mm typical
- Concrete: C20/25 grade minimum
- Reinforcement if specified by engineer
Ground conditions matter:
- Clay soils: Deeper foundations (heave risk)
- Sandy/gravelly soils: Standard depths usually OK
- Sloping sites: Stepped foundations
- Poor ground: May need engineered solutions (piles, ground improvement)
Foundation cost: £2,000-4,500 for typical single/double garage
Complete Cost Breakdown (2026 UK)
Single Garage (3m door, no room above)
| Item | Cost |
|---|---|
| Professional design & approval | |
| Structural engineer | £400-600 |
| Building plans/drawings | £200-400 |
| Building Control fee | £300-450 |
| Foundations | |
| Excavation | £400-650 |
| Concrete (ready-mix) | £400-600 |
| Reinforcement | £100-200 |
| Structure | |
| RSJ header beam (203×133×30) | £270-350 |
| Blockwork walls | £1,800-2,800 |
| Roof structure (timber/tiles) | £1,500-2,500 |
| Garage door | £400-1,200 |
| Finishing | |
| Floor slab | £600-900 |
| Electrics | £400-700 |
| Gutters/drainage | £250-400 |
| Decoration/finishing | £300-600 |
| Total | £7,320-11,350 |
Double Garage (5m door, storage above)
| Item | Cost |
|---|---|
| Professional design & approval | |
| Structural engineer | £600-900 |
| Architectural plans | £300-550 |
| Building Control fee | £350-500 |
| Foundations | |
| Excavation | £650-1,000 |
| Concrete | £650-950 |
| Reinforcement | £150-300 |
| Structure | |
| RSJ header beam (305×165×40) | £460-600 |
| Mezzanine beams (203×133×25 ×3) | £240-350 |
| Blockwork/brickwork | £3,200-4,800 |
| Roof structure | £2,200-3,500 |
| Mezzanine floor | £1,200-2,000 |
| Garage door (5m sectional) | £1,200-2,500 |
| Stairs to mezzanine | £400-800 |
| Finishing | |
| Floor slab | £900-1,400 |
| Electrics (inc. mezzanine) | £700-1,200 |
| Insulation | £400-700 |
| Gutters/drainage | £350-550 |
| Decoration | £500-900 |
| Total | £14,250-22,500 |
Garage with Room Above
£18,000-35,000 depending on specification
- Significantly more complex
- Larger beams required
- Full first-floor construction
- Stairs, heating, plumbing
- Finishing to habitable standard
DIY vs. Professional Build
Professional Build
Advantages:
- Full guarantee/warranty
- Building Control liaison handled
- Professional insurance
- Faster completion (4-8 weeks typical)
- Better finish quality
Disadvantages:
- Higher cost (£8,000-25,000+)
- Less control over specification
- Finding reputable builder takes time
DIY Build
What you can DIY:
- Some groundworks (under supervision)
- Laying blocks (if skilled)
- Roof covering
- Interior finishing
- Electrics (if qualified)
What you should NOT DIY:
- Structural engineering design
- Foundation design
- Critical structural connections
- Gas work (if applicable)
- Building Control inspections (can’t bypass these)
Realistic DIY approach:
- Hire structural engineer for design
- Hire professional for foundations and structural frame (£4,000-8,000)
- DIY finishing works (£1,000-2,000 savings)
- Net project cost: £6,000-16,000 depending on spec
Installation Tips
Beam Installation Best Practices
1. Adequate bearing:
- Minimum 100mm, prefer 150mm each end
- On properly sized padstones
- Level and true
2. DPC protection:
- Place damp-proof membrane between concrete and steel
- Prevents moisture transfer andcorrosion
3. Positive fixing:
- Bolt wall plates to beam top flange
- Use joist hangers for floor joists
- Don’t rely on gravity alone
4. Fire protection:
- Non-habitable garage may not require protection
- Room above garage: Protect steel to achieve rating
- Intumescent paint or plasterboard encasement
5. Corrosion protection:
- Prime and paint steel
- Particularly important in coastal areas
- Galvanized beams for exposed situations
Common Mistakes to Avoid
1. Undersizing header beam:
- Trying to save money on beam size
- Forgetting future loads (room conversion)
- Result: Structural inadequacy, expensive remediation
2. Inadequate foundations:
- Designing for current loads only
- Ignoring ground conditions
- Result: Settlement, cracking
3. Skipping Building Control:
- “No one will know” attitude
- Result: Illegal structure, can’t sell property, enforcement
4. Poor drainage:
- No falls on floor slab
- Inadequate guttering
- Result: Flooding, damp issues
5. Access oversight:
- Not planning beam delivery/installation
- Result: Crane costs, delays
Conclusion
Building a garage with RSJ steel beams provides a strong, durable structure with design flexibility. Careful planning, proper beam sizing, and Building Regulations compliance are essential for success.
Keys to successful garage project:
- Engage structural engineer early - designs beams, checks loading
- Obtain all necessary approvals - planning (if needed) and Building Control (always)
- Select appropriate beam sizes - don’t undersize to save money
- Use quality foundations - structure is only as good as its base
- Arrange inspections - Building Control sign-off essential
- Consider future uses - beam spec for potential future loads
Budget realistically:
- Single garage: £7,000-11,000
- Double garage: £14,000-22,000
- Garage with room above: £18,000-35,000
The investment adds significant property value and functionality, making it worthwhile when done properly.
Disclaimer: This guide provides general information only. All garage construction must comply with local Planning and Building Regulations. Structural design must be performed by a chartered structural engineer. Costs are indicative 2026 UK estimates and vary by location and specification.
