How to Support an RSJ During Installation – Acrow Props, Needles, and Temporary Works

Temporary support during RSJ installation is the most critical safety aspect of any load-bearing wall alteration. Inadequate temporary works can lead to catastrophic structural collapse, injury, or death. This comprehensive guide explains proper temporary support systems, equipment requirements, installation procedures, and safety protocols for 2026 UK construction practice.

Why Temporary Support is Critical

When installing an RSJ beam in a load-bearing wall, you’re removing structural elements that currently support significant loads. Before the new steel beam can take over this role, you must temporarily transfer these loads to a temporary support system.

Loads to be supported:

• Weight of structure above opening (walls, floors, roofs)
• Live loads (occupants, furniture, snow)
• Dynamic loads (movement, vibration)

Consequences of inadequate support:

• Progressive collapse of upper floors
• Injury or death to workers/occupants
• Catastrophic property damage
• Legal prosecution for negligence

UK Health & Safety Executive (HSE): Temporary works failures are a leading cause of construction deaths. Proper design and installation of temporary support is NOT optional.

Types of Temporary Support Systems

1. Acrow Props (Adjustable Steel Props)

Description:

• Telescopic steel tubes with screw adjustment
• Load capacity: 10-50 kN typical (check specific prop rating)
• Heights: 1.0m - 5.0m adjustable range
• Most common temporary support for residential alterations

Advantages:

• Quick to install
• Adjustable for different heights
• Reusable
• Readily available for hire/purchase

Limitations:

• Must be vertical or near-vertical for full capacity
• Require solid support top and bottom
• Can slip if not properly secured

Costs (2026 UK):

• Hire: £12-20 per prop per week
• Purchase: £40-80 per prop

2. Needle Beams

Description:

• Horizontal steel or timber beams passed through wall
• Support structure above opening from outside wall line
• Enable removal of wall section while maintaining support

Typical configuration:

• 150 × 100mm timber or small RSJ (152×127 or similar)
• Pass through wall at 900-1200mm above floor
• Project 900mm+ each side of wall
• Supported on acrow props each end

Advantages:

• Support load from outside opening perimeter
• Allow complete removal of wall section
• Standard method for large openings

Limitations:

• Require holes through wall (must be made good after)
• Need access both sides of wall
• More complex to install than simple propping

3. Strongbacks (Load Distribution Beams)

Description:

• Horizontal timber beams (typically 100 ×50mm) placed perpendicular to supported elements
• Distribute point loads from props across multiple joists/rafters
• Prevent local crushing or failure

Applications:

• Under floor joists when propping from below
• Over ceiling joists when propping above
• Spreading load from needle beams

Critical importance:

• Props without strongbacks concentrate load at single point
• Can punch through floors or crush single joists
• ALWAYS use strongbacks for residential propping

4. Scaffolding Towers

Description:

• Free-standing scaffold structures
• Adjustable height platforms
• More stable than individual props for tall structures

Applications:

• Multi-story buildings
• When working height >3m
• External support for needle beams
• Where access from below restricted

Costs: £150-300/week hire for basic tower

Designing Temporary Support

Load Calculation

Step 1: Identify loads toSupport

Masonry above opening:

• Assume 45° load dispersion triangle
• OR full height if opening very wide relative to height above
• Brick/block density: 2.3 kN/m³ typical

Floor loads:

• Full dead load (permanent construction)
• Reduced live load (25-50% of design live load typical)
• People will vacate during structural work

Roof loads:

• Dead load only usually (unlikely to snow during work)
• Wind loads negligible for temporary works timescale

Step 2: Calculate Total Load

Example: 4m wide opening, bedroom above

• Masonry (assume 2m high triangle): 4m × 1m × 2.3 kN/m³ = 9.2 kN
• Floor (joists spanning 2.5m onto beam): 2.5 kN/m² × 4m × 2.5m = 25 kN
• Total load: ~34 kN

Step 3: Determine Number of Props

Standard acrow prop capacity: 20 kN (check specific prop!)

Number required: 34 kN / 20 kN = 1.7, round up to 2 props minimum

Add safety margin: Use 3-4 props (50-100% extra capacity)

Recommended: 3-4 props for 4m opening

Prop Spacing

General rules:

• Space props evenly across span
• Maximum spacing: 1.5-2.0m
• Minimum spacing: 600mm (avoid crowding)
• Always use odd numbers for symmetry (3, 5, 7 etc.)

Example prop layout for 4m opening:

Wall ----|-1.0m-|----|-1.0m-|----|-1.0m-|----|-1.0m-|---- Wall
        Prop 1    Prop 2    Prop 3    Prop 4

Installation Procedure

Phase 1: Preparation (BEFORE any demolition)

1. Clear area above and below opening:

• Remove furniture, occupants
• Protect finishes with dustsheets
• Establish exclusion zone (no entry during structural work)

2. Inspect existing structure:

• Check for hidden defects, rot, previous alterations
• Verify load path understood
• Identify services (pipes, cables) - isolate if necessary

3. Assemble equipment:

• Correct number of props
• Strongbacks sufficient length
• Scaffold boards for spreading loads
• Spirit levels, measuring tape
• Timber wedges/packings

4. Building Control notification:

• Notify 24-48 hours before starting work
• Inspection of temporary works may be required

Phase 2: Install needles (if required)

For large openings or complex support:

1. Mark needle positions:

• Usually 2-3 needles for 4m opening
• First needle 600-900mm from edge of proposed opening
• Additional needles at ~1.5m spacing

2. Cut needle holes:

• 150 × 150mm through wall
• 900-1200mm above floor level typically (structural engineer specifies)
• Work from both sides to prevent breakout

3. Insert needle beams:

• Push beam through hole
• Equal projection each side (900mm minimum)
• Ensure level

4. Support needle ends:

• Place scaffold boards on floor each side
• Position acrow props under needle ends
• Adjust to just touch (no load yet)

5. Load needles progressively:

• Tighten each prop quarter-turn
• Work systematically (left side, right side, repeat)
• Continue until needles fully loaded
• Wall above should be supported (test by attempting to remove a brick - should be loose)

Phase 3: Install Floor Props (if propping from below)

1. Position strongbacks:

• 100 × 50mm timber planks perpendicular to floor joists
• If floor joists run parallel to wall: 2-3 strongbacks spanning from wall to wall
• If joists perpendicular: strongbacks along line of props

2. Place scaffold boards:

• On floor below, spread load
• Boards perpendicular to strongbacks
• Wider spread = better load distribution

3. Position props:

• Under strongback-scaffold board intersection
• Vertical orientation (critical for capacity)
• Baseplate square to floor

4. Extend props:

• Adjust to height just touching strongback
• Lock adjustment collar
• Verify all props equally loaded (hand-tight, not force)

5. Load props progressively:

• Start from one end
• Quarter-turn each prop
• Move along line
• Repeat multiple times until fully loaded
• Check with spirit level - ceiling should rise slightly (1-2mm) when props fully loaded

Safety check: Once fully loaded, floor above should show no deflection when walked on above props. If movement detected, add more props!

Phase 4: Monitor During Demolition

Critical: Temporary support is a dynamic system

Continuously monitor:

• Check props remain vertical
• No slipping at base or top
• No extension beyond safe limits (check prop safety labels)
• Strongbacks not bowing
• No cracking or distress in structure

If any concerns: STOP work immediately, add props, consult engineer

Phase 5: Install RSJ Beam

With temporary support in place:

• Remove masonry to create opening
• Install padstones
• Lift RSJ into position
• Verify level and bearing
• Make permanent connections

Don’t rush: RSJ installation is critical phase

Phase 6: Progressive Removal of Temporary Works

Critical: NEVER remove all temporary support at once!

Procedure:

1. Verify RSJ properly installed:

• Minimum 48 hours after beam seated (allow masonry above to cure)
• All connections complete
• Building Control inspected

2. Release props gradually:

• Start with props furthest from beam
• One quarter-turn all props
• Wait 10-15 minutes
• Observe for movement, cracking, sounds
• Repeat

3. Remove needles:

• Only after props fully released
• Remove needles from walls
• Make good holes

4. Final inspection:

• Check beam has not deflected excessively
• No cracks in walls or ceiling
• Floor solid when walked on

Safety margin: Many contractors leave one or two props in place for extra 24-48 hours for extra safety

Equipment Checklist

For typical 4m opening residential project:

Equipment	Quantity	Hire/Buy	Cost
Acrow props	4-6	Hire 2 weeks	£140-200
Strongbacks (100×50 timber)	2-3 @ 4-5m	Buy	£40-70
Scaffold boards	4-6	Hire	£40-80
Needles (if required)	2-3 @ 2.5-3m	Buy (timber) or hire (steel)	£60-180
Spirit level (1200mm+)	1	Buy	£25-50
Timber packings/wedges	Various	Buy	£15-30
Total			£320-610

Reusable for future projects: Props, boards worth buying if doing multiple projects

Safety Protocols

Pre-Work Safety Checks

• Structural engineer involved in design
• Temporary works design approved
• All equipment inspected and rated
• Exclusion zone established
• Emergency procedures known
• Building Control notified

During Work Safety

• Hard hats worn at all times
• No occupants in building during demolition
• Props checked hourly minimum
• Any concerns → immediate stop work
• Weather monitored (wind, rain can affect stability)
• [ ] Daily photographic record of temporary works

Common Mistakes to Avoid

1. Insufficient props: Trying to save hire cost by using minimum number Result: Overloaded props, potential collapse Solution: Always use extra props (50-100% over calculated minimum)

2. No strongbacks: Props placed directly under ceiling/floor Result: Punt through plasterboard, crush single joist Solution: ALWAYS use strongbacks to distribute load

3. Uneven loading: Some props tight, others loose Result: Eccentric loading, potential twist/movement Solution: Load all props progressively and evenly

4. Props not vertical: Angled props used due to obstructions Result: Reduced capacity, potential buckling Solution: Remove obstructions or use needles to support from clear area

5. Removing support too soon: Impatient to complete project Result: Settlement, cracking, potential collapse Solution: Follow removal procedure, allow adequate curing time

Building Control Requirements

UK Building Control will typically inspect:

Before work starts:

• Temporary support design adequate
• Correct equipment in place
• Proper installation method planned

During work:

• May visit to verify temporary works in place
• Observe demolition procedures

After beam installation:

• Verify RSJ correctly installed before approving removal of temporary works

Never proceed without Building Control approval at each stage!

Professional vs. DIY

When to hire professionals:

• Opening >4m span
• Multi-story building
• Complex load paths
• Any uncertainty or lack of confidence
• Limited construction experience

Professional structural scaffold/falsework company:

• Design temporary works
• Install and certify
• Supervise demolition
• Progressive removal
• Cost: £800-2,000 for typical domestic opening
• Value: Peace of mind, safety assurance, insurance coverage

When DIY might be acceptable (with engineer oversight):

• Small opening (<3m)
• Single story structure
• Simple load path well understood
• Competent person supervises
• Full engineer involvement

Never DIY temporary works without:

• Structural engineer approval of method
• Building Control notification and inspection
• Proper equipment (never improvise with Inappropriate materials)
• Understanding of procedures

Emergency Procedures

If any of these occur, STOP work immediately:

• Cracking in walls or ceiling
• Props leaning or slipping
• Unusual sounds (creaking, groaning)
• Visible deflection or movement
• Prop over-extended (check safety labels)

Emergency action:

1. Evacuate building immediately
2. Establish exclusion zone (no entry)
3. Contact structural engineer urgently
4. Do NOT remove any props
5. Add additional props if safe to do so briefly
6. Notify Building Control
7. Do not resume until engineer approves

Conclusion

Temporary support during RSJ installation is safety-critical work that must be taken seriously. Proper design, installation, and monitoring of acrow props, needles, and strongbacks prevents structural collapse and ensures worker and occupant safety.

Key principles:

1. Design temporary works properly - calculate loads, specify equipment
2. Use adequate capacity - over-specify by 50-100% for safety margin
3. Install systematically - follow proper procedures, no shortcuts
4. Monitor continuously - check props hourly minimum during work
5. Remove progressively - never rush this final critical step
6. Engage professionals when uncertain - cost is trivial compared to collapse consequences

For complex or large projects, hiring professional falsework/scaffold companies provides designed, certified, and insured temporary support - often worth the cost for peace of mind alone.

Disclaimer: This guide provides general information only. Temporary works must be designed by a competent person (structural engineer or experienced contractor) for each specific project. Never undertake load-bearing structural alterations without professional involvement. Inadequate temporary support can result in structural collapse, injury, or death.