Temporary Works in London Construction: Propping, Shoring and Structural Support During an Extension

**What are temporary works?**

Temporary works are structures, plant, and equipment installed to support, protect, or provide access during construction — and removed after the permanent works are complete. In the context of London residential extensions and refurbishments, 'temporary works' primarily means structural temporary works: systems that temporarily carry loads that would otherwise be carried by the permanent structure being modified.

The key characteristic of temporary works is that they are load-carrying — the forces they resist are real structural forces, not just the weight of scaffolding boards or sheeting. A scaffold (which primarily supports operatives and materials, not the structural loads of the building) is not typically classified as temporary works in the structural sense. Temporary propping systems, shoring systems, and excavation support systems are.

For London residential projects, temporary works most commonly arise in the following situations:

• *1. Structural openings in load-bearing walls*:
• When a structural opening is formed in a load-bearing wall — for a wide door opening, an opening into an extension, or a beam pocket installation — the wall must be temporarily propped to carry the loads above while the opening is formed. Temporary propping systems for domestic structural openings typically use:
• •Acrow props (adjustable steel screw-jack props) positioned on each side of the wall
• •Timber needles threaded through temporary openings in the wall to carry the floor joists above
• •Steel frame temporary support (for heavier-duty applications or wider openings)

The propping must be in place and load-tested before any masonry is removed. The propping scheme is designed by the structural engineer or a temporary works designer.

*2. Chimney breast removal*: Removing a chimney breast (particularly on an intermediate floor, leaving the stack supported above) requires temporary support for the chimney stack and the loads it carries while the breast is removed. Depending on the height and weight of the remaining stack, the temporary support system may use Acrow props and timber spreader boards, or a bespoke steel frame if the loads are high.

*3. Basement and underpinning projects*: Basement conversions and underpinning schemes require the most extensive temporary works in London residential construction. When excavation proceeds below the level of the existing foundations (to create a new basement or to lower the existing floor level), the existing walls lose their foundation support — and must be temporarily supported by 'dead shoring' (vertical shores carrying the wall loads while the excavation proceeds), 'raking shores' (diagonal shores transmitting loads to a temporary foundation at lower level), or 'flying shores' (horizontal shores transmitting loads across a gap to the adjacent structure).

For a London basement conversion beneath an existing Victorian terrace, the temporary shoring system is often the most complex and costly temporary works item — and must be designed by a competent temporary works engineer.

*4. Adjacent structure support during deep excavation (Section 6 party wall works)*: Where an extension foundation is to be excavated below the depth of the adjoining owner's foundation (Section 6 of the Party Wall Act), the excavation must be designed so that it does not undermine the adjoining foundation. This may require underpinning of the adjoining owner's foundation before excavation proceeds, or a retaining wall system designed to prevent movement of the ground beneath the adjoining foundation.

*5. Demolition and strip-out support*: Where part of the existing structure is to be demolished (e.g., an existing outbuilding whose roof is to be removed before a new extension roof is constructed over the same footprint), the remaining walls may require temporary support until the new permanent roof structure is in place.

**Who designs temporary works for London residential projects?**

The design of temporary works is governed by BS 5975:2019 (Code of Practice for Temporary Works Procedures and the Permissible Stress Design of Falsework) and the CDM Regulations 2015.

For temporary works on domestic London residential projects:

• •*Simple temporary propping for domestic structural openings*: Can typically be designed by the structural engineer as part of their structural drawings and specification — the SE will include a temporary propping layout drawing showing prop positions, needle positions, and loading. This is standard practice for most domestic structural opening works.

• •*Complex shoring and excavation support (basement conversions, deep foundations near neighbours)*: Must be designed by a competent temporary works engineer or a structural engineer with demonstrable temporary works experience. The design is typically a separate commission from the permanent structural works design.

• •*Any temporary works where prop or shore failure could cause injury or structural damage*: Must be formally designed, checked by a third-party independent checker (as required by BS 5975), and approved by the principal contractor's temporary works supervisor before installation.

The question of 'who designs the temporary works' matters for liability — if temporary works fail and cause injury or structural damage, the party responsible for the design and supervision of the temporary works bears significant liability. RCB ensures that all temporary works involving structural loads are formally designed before installation.

**Acrow props — the most common temporary support system in London residential construction**:

Acrow props (also known as screw props or adjustable steel props) are adjustable steel columns used to carry point loads during construction. The standard Acrow prop consists of an outer tube (typically 48mm or 60mm diameter) and an inner tube that slides inside it, adjusted by a central nut on a screwed section — allowing the prop height to be extended and tightened to apply a pre-load against the structure above.

For a domestic London extension structural opening, Acrow props are typically used in the following configuration:

*Standard needle-and-prop arrangement for a ground-floor structural opening*:

1. A timber needle (typically 150×75mm or 150×100mm C24 softwood, or steel RSJ needle for heavier loads) is threaded through a temporary 'needle hole' cut in the wall at first-floor joist level — the needle is positioned directly under or adjacent to the first-floor joists that bear on the wall being modified 2. Acrow props are positioned on each side of the wall (ground floor and first floor), with timber spreader plates (typically 50mm plywood or scaffold boards, min 450×450mm area) under the prop feet to distribute the prop load across the floor 3. The props are tightened to apply a pre-load against the needle and the structure above 4. The needle-and-prop system transfers the joist loads from the wall into the temporary props — releasing the wall section below from structural duty 5. The wall opening can now be formed (masonry removed) and the permanent steel beam installed 6. The permanent beam is installed on its padstones, confirmed in position and bearing correctly, and then the temporary props are released and removed

*Prop loading capacities*:

The loading capacity of Acrow props varies with the extended height (taller props have lower buckling resistance and therefore lower capacity). Typical capacities:

| Prop type | Max load at recommended extension | |---|---| | Standard No. 3 prop (extended to 2.6m) | ~18 kN (1.8 tonnes) | | Standard No. 4 prop (extended to 3.9m) | ~12 kN (1.2 tonnes) | | Heavy duty prop (large diameter) | ~35–50 kN (3.5–5 tonnes) |

For typical London Victorian terrace floor loadings, standard No. 3 or No. 4 props used in pairs (one on each side of the wall) provide adequate capacity for most single-storey and two-storey situations.

**Timber needle design considerations**:

The timber needle must be sized to span across the wall from one side to the other while carrying the joist loads distributed along its length. For a standard 225mm (9-inch) brick wall, the needle spans approximately 400–600mm (the wall width plus the bearing lengths on each side). For a typical domestic floor loading, 150×75mm C24 timber needles at 600–800mm centres (matching the joist spacing above) are typically adequate. The structural engineer will confirm needle sizes and spacing.

Needle holes are made in the wall by drilling a series of holes in an L-pattern and using a cold chisel to break out the remaining masonry. The holes should be just large enough for the needle timber to thread through — excess clearance reduces the effectiveness of the needle in bearing on the wall courses above and below.

**Dead shoring — for basement and underpinning works**:

'Dead shoring' (vertical temporary shoring) is used in London residential projects primarily for basement conversions and underpinning schemes where the existing wall's foundation support is temporarily lost during excavation. Dead shores:

• •Vertical timber or steel posts positioned against the wall face (or through the wall where through-bolted)
• •Carrying the wall loads from the structure above down to temporary foundations at a level below the excavation (or to a solid floor base)
• •Combined with a horizontal needle or spreader at the top (bearing against the wall or threading through the wall) and a spreader plate at the base

Dead shoring for a basement project in a London Victorian terrace is significantly more complex and expensive than Acrow prop arrangements for a ground-floor structural opening — the loads are larger (the full height of the building above), the excavation depth is greater, and the risk of failure is higher. A complete dead shoring scheme for a basement conversion typically costs £5,000–£20,000+ for the design, materials, and installation.

**CDM 2015 and temporary works**:

The Construction (Design and Management) Regulations 2015 (CDM 2015) impose duties on the principal contractor in relation to the management of temporary works:

• •The principal contractor must ensure that all temporary works are designed by a competent person
• •The principal contractor must appoint a Temporary Works Supervisor (TWS) — a competent person responsible for overseeing the installation, use, and dismantling of temporary works
• •The principal contractor must maintain a register of temporary works on the project
• •Temporary works must be checked and approved (often with an independent check for complex systems) before loading is applied

For a domestic London extension, the Temporary Works Supervisor is typically the principal contractor's site manager — who must be competent to oversee the installation and operation of the specific temporary works on the project. For simple Acrow prop arrangements on domestic structural openings, the site manager's competence is typically sufficient. For basement shoring or complex multi-level temporary works, a dedicated temporary works engineer may be engaged to act as TWS.

**Building Control and temporary works**:

Building Control does not typically inspect or approve temporary works directly — temporary works are not part of the permanent building works that Building Control regulates. However:

• •Building Control will expect to see the structural engineer's temporary propping drawings as part of the structural information package (to confirm that the temporary support is adequate for the specific loading condition)
• •If a temporary works failure occurs and causes structural damage, Building Control can require investigation and remediation of any permanent structural elements affected
• •Building Control inspectors will often observe the temporary works arrangement during a site inspection visit and flag concerns if the propping appears inadequate — though this is an advisory observation rather than a formal approval

**Temporary works and the party wall surveyor**:

Where temporary works involve any activity that could affect the adjoining owner's property (e.g., needling through the party wall; installing temporary shores against the party wall face; excavating below the adjoining foundation level), the party wall surveyor should be informed and the temporary works method should be approved as part of the party wall award conditions. This protects both the building owner and the adjoining owner in the event of any temporary works incident.

**What happens when temporary works fail**:

Temporary works failures in London residential construction — while relatively rare when properly designed and supervised — have severe consequences:

• •At best: partial collapse of the structure above the opening, requiring emergency structural works and significant programme delay (typically 2–6 weeks)
• •At worst: injury to operatives or building occupants; structural damage to the adjoining property; legal and insurance consequences for the contractor and the principal

• The most common causes of temporary works failures in London residential projects are:
• •Insufficient prop capacity for the actual load
• •Inadequate base for props (props standing on soft ground or on floorboards that are not checked for bearing capacity)
• •Props removed before the permanent structure is capable of carrying the loads
• •Inadequate number of props or needles (one needle where two are needed)
• •Unauthorised modification of the propping arrangement by a subcontractor

RCB's approach: all temporary propping arrangements for structural opening works are formally designed by the structural engineer and approved by the site manager before installation. No wall section is removed without confirmed propping in place and checked.