Knocking Through a Wall: RSJ Installation Guide for Open-Plan Living (With Photos)

Creating open-plan living space by removing a wall is one of the most transformative home improvements. Installing an RSJ steel beam enables you to safely remove load-bearing walls while maintaining structural integrity. This guide covers design considerations, beam selection, installation procedures, costs, and Building Regulations for creating your ideal open-plan space in 2026.

Planning Your Knockthrough

Determining if Wall is Load-Bearing

Signs wall IS load-bearing:

• Runs perpendicular to floor joists
• Walls directly above/below on other floors
• Central spine wall (typical in terraced houses)
• Thick wall (>150mm typically)
• Original building plans show as structural

Signs wall is NON-load bearing:

• Thin partition (<100mm)
• Parallel to floor joists with no walls above
• Recent addition not on original plans
• Stud wall construction

If uncertain: Hire structural surveyor (£150-300) - never guess!

Design Considerations

Size of opening:

• Full wall removal (typical 3-5m)
• Partial removal with piers/columns at ends
• Multiple openings with intermediate supports

What stays, what goes:

• Floor to ceiling removal (maximum space)
• Retain bottom section for services/storage
• Keep partial walls for structural/aesthetic reasons

Services relocation:

• Electrical (rewire before demolition)
• Plumbing (reroute pipes)
• Heating (radiator relocation)
• Cost: £500-1,500 for services

Beam Selection by Project Type

Small Knockthrough (2.5-3.5m)

Typical scenario: Victorian terrace, combining living/dining rooms

Configuration:

• Span: 3.5m
• Solid brick walls (215mm)
• Floor joists parallel to wall (no load transfer)
• Triangle of brickwork above
• Bedroom above

Loads:

• Masonry: ~4 kN/m
• Minimal floor loads (joists don’t bear on wall)
• Total: ~4-5 kN/m

Beam: 203×133×25 RSJ

• Cost: £255-330 (3.8m length)
• Capacity: ~8-10 kN/m at 3.5m (adequate safety margin)

Medium Knockthrough (3.5-4.5m)

Typical scenario: 1930s semi-detached, living room to dining room

Configuration:

• Span: 4.0m
• Cavity wall construction
• Floor joists perpendicular (span 2.5m onto wall)
• Two bedrooms above

Loads:

• Cavity wall: ~6 kN/m
• Floor dead load: 0.6 kN/m² × 1.25m = 0.75 kN/m
• Floor live load: 1.5 kN/m² × 1.25m = 1.88 kN/m
• Total: {8.6 kN/m

Beam: 203×133×30 RSJ

• Cost: £350-450 (4.3m length)
• Capacity: ~8-9 kN/m at 4m (adequate)
• Or 254×146×31 for better deflection control

Large Knockthrough (4.5-6m)

Typical scenario: New build, kitchen-dining-living open plan

Configuration:

• Span: 5.5m
• Timber frame with brick facing
• Complex roof loads above
• Multiple rooms combined

Loads:

• Walls and roof: ~10-12 kN/m
• Floor loads: 6-8 kN/m
• Total: ~18 kN/m

Beam: 305×165×40 UB

• Cost: £550-720 (5.8m length)
• Heavy beam requires crane/mechanical lifting
• Professional installation essential

Step-by-Step Installation

Phase 1: Design and Approval (Weeks 1-4)

Week 1: Engage structural engineer

• Site visit and assessment
• Discuss design intentions
• Identify services
• Fee: £350-600

Week 2: Engineer produces design

• Calculates loads
• Specifies beam size
• Details padstones and supports
• Provides drawings

Week 3: Building Control application

• Submit engineer’s calculations
• Floor plans showing alteration
• Specification of materials
• Fee: £220-400

Week 4: Await approval

• Review period
• May require clarifications
• Approval certificate issued

Phase 2: Preparation (Week 5)

Order materials:

• RSJ beam (lead time 1-2 weeks)
• Padstones (pre-cast or materials)
• Acrow props and strongbacks (hire or buy)
• DPC membrane
• Fire protection materials

Prepare site:

• Clear rooms both sides
• Protect floors and furniture
• Arrange skip for debris (£150-250)
• Set up dust barriers

Relocate services:

• Electrician rewire circuits
• Plumber reroute pipes
• Confirm all services clear of work area

Phase 3: Install Temporary Support (Week 6, Day 1)

Critical safety step - see comprehensive guide in related article

Below opening:

1. Place strongbacks perpendicular to joists
2. Position acrow props (typically 4-6 props for 4m opening)
3. Progressively load props until fully supporting structure

Above opening (if required):

1. Install needles through wall
2. Support needles on props each side
3. Transfer load from wall to needles

Building Control inspection: Verify temporary works adequate

Phase 4: Demolition (Week 6, Day 2-3)

Mark out opening:

• Use engineer’s dimensions exactly
• Mark plumb lines for verticality
• Identify padstone positions

Remove plaster/finishes:

• Both sides of wall
• Extend 300mm beyond opening each side
• Reveal structure beneath

Cut masonry:

1. Score cutting lines with angle grinder
2. Cut vertically at sides of opening
3. Cut horizontally at top (leave temporary support of brickwork above)
4. Work from top down within supported area

Remove bricks/blocks:

• Carefully remove masonry
• Keep rubble for filling/disposal
• Clean out mortar and debris

Create padstone recesses:

• Each end of opening
• Depth: Padstone thickness + 10mm mortar
• Width/length per engineer’s specification
• Clean and level base

Phase 5: Install Padstones (Week 6, Day 3-4)

Prepare:

• Clean recesses thoroughly
• Dampen (not saturate) concrete surfaces

Mortar bed:

• 10mm thick 1:3 cement:sand mix
• Level carefully with spirit level
• Work quickly before setting

Set padstones:

• Lower into position
• Press into mortar bed
• Level in all directions
• Critical: Both padstones must be exactly same height
• Allow 24-48 hours curing

DPC layer:

• Bitumen felt or plastic membrane
• Prevents moisture transfer to steel

Building Control inspection: Padstones before beam installation

Phase 6: Beam Installation (Week 7, Day 1)

Beam preparation:

• Apply rust-inhibiting primer if not pre-treated
• Mark bearing positions
• Attach lifting straps/equipment

Lifting:

Light beams (<100kg):

• 3-4 strong people can manually lift
• Coordinated lifting on count
• Careful maneuvering through opening

Heavy beams (>100kg):

• Engine crane (hire £60-100/day)
• Block and tackle system
• OR hire professional riggers

Positioning:

1. Lift into opening
2. Rest on padstones
3. Verify 100mm+ bearing each end
4. Check level (critical for floors above)
5. Add steel packing shims if needed (never wood)

Temporary support:

• Add props directly under beam ends
• Keep all existing temporary support in place
• Don’t release until making good complete

Phase 7: Make Good Above Beam (Week 7, Day 2-3)

Install permanent support above:

• May be concrete lintel
• Or rebuild masonry directly on beam
• Follow engineer’s specification

Rebuild masonry:

• Strong mortar (1:3 or 1:4)
• Good bond to existing
• Tooth into existing brickwork
• Leave weep holes if external wall

Curing period:

• Minimum 7 days before removing temporary support
• 14 days preferred
• Check weather (avoid freezing conditions)

Phase 8: Remove Temporary Works (Week 8)

After adequate curing:

Progressive release:

1. Release props quarter-turn each
2. Work systematically across all props
3. Pause 10-15 minutes between rounds
4. Observe for movement/cracking
5. Repeat until all props unloaded

Remove equipment:

• Dismantle props and strongbacks
• Remove needles (if used)
• Fill needle holes

Final check:

• No visible deflection
• No cracks developed
• Beam remaining level

Building Control final inspection

Phase 9: Finishing (Weeks 9-10)

Below beam:

• Box in with plasterboard (fire protection if required)
• OR leave exposed for industrial aesthetic
• Skim plaster to match existing
• Paint/decorate

Floor repair:

• Make good where props stood
• Level with existing floor
• Replace flooring

New features:

• Install spotlights if planned
• Built-in shelving
• Different flooring in zones

Final decoration:

• Paint throughout
• Install skirting
• Ceiling cornice/details

Cost Breakdown Example

Typical 4m knockthrough project (UK 2026):

Item	Cost
Professional fees
Structural engineer	£380-580
Building Control	£250-380
Materials
RSJ beam (203×133×30, 4.3m)	£340-430
Padstones (2)	£75-120
Acrow props hire (4, 3 weeks)	£150-240
Strongbacks, boards, fixings	£80-130
Skip hire	£150-230
Fire protection	£60-120
Plasterboard and materials	£120-200
Labour (if hiring)
Services relocation	£500-900
Beam installation	£800-1,400
Plastering/finishing	£400-750
Decoration	£350-650
Total (professional install)	£3,655-6,130
Total (supervised DIY)	£2,405-3,090

Value added to property: £8,000-15,000 (typical London/SE)

Design Ideas for Knocked-Through Spaces

Floor Level Changes

Create visual zones:

• Raised dining area (1-2 steps up)
• Sunken lounge (1 step down)
• Cost: £800-1,500 additional

Benefits:

• Defines spaces without walls
• Adds architectural interest
• Can conceal services under raised areas

Partial Height Walls

Retain lower section of wall:

• 1.0-1.2m height
• Creates breakfast bar/serving hatch
• Hides services and storage
• Retains some separation

Beam exposed above:

• Industrial aesthetic
• Paint/polish exposed steel
• Feature lighting

Column Features

Instead of full-width beam:

• Steel columns at 1/3 and 2/3 points
• Shorter beam spans between columns
• Creates archway effect
• Columns can be boxed decoratively

Cost: Similar to single beam, butmore architectural impact

Building Regulations Compliance

Structural (Part A)

• Structural engineer calculations (mandatory)
• Adequate beam size and supports
• Proper temporary works during installation
• Building Control inspections

Fire Safety (Part B)

If beam creates new escape route:

• May require fire protection
• 30-minute rating typical
• Intumescent paint or plasterboard boxing

Existing escape routes:

• Usually no additional requirements
• Check with Building Control

Sound Insulation (Part E)

Single dwelling:

• Usually not affected by knockthrough

Flats/conversion:

• May require acoustic treatment
• Particularly if altering party walls

Ventilation (Part F)

Combining rooms may require:

• Additional ventilation
• Larger extractor fans in kitchen
• Check Part F compliance

Energy Efficiency (Part L)

If altering external wall:

• Maintain/improve insulation
• U-value compliance
• Thermal bridging at beam

Common Problems and Solutions

Problem 1: Services in wall

Discovery: Pipes/cables running through wall to be removed

Solution:

• Identified during survey (should be known before starting)
• Reroute before demolition
• Cost: £300-800 depending on complexity

Prevention: Thorough survey before work

Problem 2: Beam won’t fit through door

Cause: Beam length exceeds door/stair width

Solutions:

• Pass through window (remove window temporarily)
• Crane lift to first floor then lower through ceiling opening
• In extreme cases, cut external hole in wall

Prevention: Plan delivery route during design phase

Problem 3: Uneven floor levels

Discovery: Floors either side of wall not level

Causes:

• Settlement over time
• Different construction periods
• Previous alterations

Solutions:

• Level with screed/self-leveling compound
• Build up lower side with additional flooring layers
• Accept slight step (threshold bar)

Cost: £200-800

Problem 4: Cracking after beam installation

Minor hairline cracks:

• Normal settlement
• Fill after 2-3 months when stable
• Not structural concern

Progressive/widening cracks:

• Potential structural issue
• Contact engineer immediately
• May indicate inadequate support or foundation issues

Professional vs. DIY

Realistic DIY scope:

You CAN do:

• Demolition (under supervision, with proper temporary support)
• Finishing plasterwork
• Decoration
• Floor repairs

You should NOT DIY:

• Structural engineering (illegal without qualifications)
• Beam installation without experience (safety risk)
• Building Control application/liaison (technical knowledge required)
• Electrical work (qualified electrician only)
• Gas work (registered engineer only)

Recommended approach:

1. Hire engineer for design (essential, £350-600)
2. Hire experienced builder for structural work (£1,500-2,500)
3. DIY finishing to save money (£800-1,500 savings)

Total cost: £2,900-4,700 (vs. £4,000-6,000 fully professional)

Conclusion

Knocking through a wall to create open-plan living is a transformative project that adds value and improves lifestyle. Proper planning, structural engineering, appropriate beam selection, and Building Regulations compliance ensure a safe, successful outcome.

Success factors:

1. Engage structural engineer - non-negotiable
2. Obtain Building Control approval - before any work
3. Use adequate temporary support - safety critical
4. Select correct beam size - don’t undersize to save money
5. Allow proper curing times - don’t rush
6. Professional installation for structural work - worth the cost

Budget £3,500-6,000 for typical 4m knockthrough with professional structural installation and DIY finishing.

Value added: £8,000-15,000 - excellent ROI making this one of the best home improvements.

Disclaimer: This guide provides general information only. All structural work requires design by a chartered structural engineer and Building Regulations approval. Never undertake load-bearing structural alterations without professional involvement.