5.1 Scaffolding Types, Components, Foundations, Bracing and Tower Scaffolds

Legal basis after BOWEC

Scaffolds used to be governed by Part X of the BOWEC Regulations 1986 (regulations 72 to 98), which were repealed on 1 June 2024. The current duties are:

• CDM 2024 regulation 21: supports, temporary supports and temporary structures must be designed, installed and maintained to withstand any foreseeable load, and used for the purpose for which they were designed. No structure may be loaded until it becomes unsafe.
• CDM 2024 regulation 19: safe access and egress, and guarded edges and openings.
• OSHA 1994 sections 15 and 18B: safe plant and systems of work, and a risk assessment that is acted on.
• DOSH Guidelines for the Prevention of Falls at Workplaces 2007, section 5: employers shall ensure all scaffolding is suitable and safe for employees to use (5.1). This section also contains detailed rules for standing, suspended, hung and tower scaffolds.

Main scaffold types

DOSH's guideline (section 5.6) calls scaffolds not built to an accepted international standard, because of their construction, design or loads, special scaffolds. Expert advice from a professional engineer should be obtained before they are built and used, and every user must be told the maximum safe live load.

Components and what they do

Guardrail ─┐            ┌── Toe-board
Midrail ───┤  PLATFORM  │
           └── boards on transoms ──┘
Transom (across)   Ledger (along)   Brace (diagonal)   Tie (to building)
Standard (vertical) ─ carries load down
Base plate ─ spreads load from standard
Sole board / sole plate ─ spreads load into ground
Firm, level, drained ground

• Standards are the vertical tubes that carry the load to the ground. They must be plumb and properly jointed; joints are staggered so they do not all fall in the same lift.
• Ledgers are horizontal tubes running along the scaffold. They tie standards together and carry transoms.
• Transoms are horizontal tubes across the scaffold that support the boards. Their spacing depends on board thickness and duty.
• Base plates are steel plates that spread each standard's load. Use adjustable base jacks within the manufacturer's maximum extension.
• Sole boards (sole plates) are timber or steel spread pieces under base plates on soil, fill or asphalt. DOSH's tower guidance requires metal base plates and timber sole plates unless the foundation is concrete or other solid material. Never use loose bricks, hollow blocks or offcuts.
• Couplers come in three load-bearing roles: right-angle couplers (ledgers to standards, transoms to ledgers), swivel couplers (bracing at angles) and sleeve couplers (joining tubes end to end). Putlog couplers only hold putlogs or transoms against movement; never use them where they must carry load.
• Bracing turns rectangles into triangles. Façade (longitudinal) bracing runs diagonally along the face, and ledger (transverse) bracing runs across. Without bracing a scaffold sways and can fold.
• Ties connect the scaffold to the permanent structure to resist wind and outward pull. Sheeting or netting greatly increases wind load, so the design must allow for it.

The standard steel scaffold tube is 48.3 mm outside diameter. Mixing tube types, or steel and aluminium, in one structure is unsafe unless the design permits it.

Working platforms

Platforms are where most scaffold falls start. From DOSH's guideline:

• standing scaffolds must have guardrails and working platforms in place on all areas in use, safe access to every platform, sufficient ties and braces, load-bearing members in place with the correct fittings, and platforms fully decked with planks secured against movement (section 5.2)
• the guardrail on scaffolds must be within 200 mm horizontally of the platform edge (section 4.2)
• if instability or damage is found, the scaffold should not be used until repaired by a certificated scaffolder (section 5.2)

Common good-practice rules, used widely and taken from British and European scaffold codes:

• Each board end rests on at least the specified number of supports and overhangs its last support by at least 50 mm, but not so far that it tips if stepped on. A common limit is four times the board thickness, about 150 mm for a 38 mm board.
• No gaps in the deck large enough to drop tools through, and no trip steps between boards.
• Width suited to the work and the load.

Tower scaffolds: DOSH falls guideline section 5.5

Tower scaffolds cause many falls and topple-overs because they are moved often and used by non-scaffolders.

Worked example: a mobile aluminium tower used outdoors has a 1.45 m x 1.45 m base. The maximum platform height at 3:1 is 3 x 1.45 = 4.35 m, unless the manufacturer's instructions are more restrictive (aluminium towers, being lighter, may have lower permitted ratios) or outriggers increase the effective base as the manufacturer specifies.

Scenario: the façade scaffold on new backfill

An independent tied scaffold has been erected on freshly backfilled ground beside a drain. The SSS finds:

• standards on broken bricks
• swivel couplers used to fix load-bearing transoms
• boards overhanging by about 300 mm
• two ties removed by the window installers

Actions:

1. Withdraw the scaffold from use (red tag at every access point) and keep people off.
2. Have the scaffolding contractor's competent scaffolders prop the affected bays, re-base them on compacted ground with sole boards and base plates, replace the couplers with right-angle couplers, re-lay the boards, and reinstate the ties to the design.
3. Before handing the scaffold back, have a competent person inspect it and record the inspection (Section 5.3).
4. Under the construction phase plan's site rules, only the scaffolding contractor may alter the scaffold. Brief the window installers at a toolbox meeting.