Independent Tied Scaffold: Tie Patterns, Bay Widths, Lift Heights and TG20 Design Limits

Summary

The independent tied scaffold is the most common scaffold configuration in UK construction. Unlike a putlog scaffold, it has two rows of vertical standards (inner and outer) and is structurally independent of the building it serves — the ties to the building prevent overturning and resist wind loads, but the scaffold's weight and working loads are borne by the scaffold itself.

"Tied" refers to the horizontal restraints between the scaffold and the building face. These ties are critical components — removing or omitting ties is one of the most common causes of scaffold collapse. The number and pattern of ties required depends on the height of the scaffold, the bay width, the applied loads, and the wind exposure.

TG20:21 pre-calculates tie patterns for all standard configurations within the system's scope, meaning contractors can generate a Compliance Sheet confirming tie requirements without site-specific engineering. For scaffolds outside TG20 scope (above 50m, bay widths over 2.4m, or other non-standard parameters), a structural engineer's design is required.

Key Facts

• Independent tied scaffold — two rows of standards; structurally independent of building; tied for lateral restraint
• Standard TG20 bay width — maximum 2.4m (centre to centre of transoms/ledgers in the direction along the facade)
• Standard TG20 lift height — maximum 2.0m (bottom of boards to bottom of boards of next lift)
• Standard TG20 facade height — up to 50m; beyond this, engineer's design required
• Tie types — reveal tie (tube wedged across window reveal), box tie (tube through window or opening), lip tie (tube hooked over window sill)
• Minimum tie frequency — TG20 eSP calculates exactly; typically alternating bays, alternating lifts as minimum
• Outer standard spacing — typically 2.4m bay width (ledger spacing) and 2.7m standard spacing longitudinally; exact to TG20
• Inner standard — set back from facade, typically 100–300mm clearance from wall face
• First lift height from ground — first board level (working platform) typically 2.0m above ground; adjust for site conditions
• Sole plates — 225mm × 50mm minimum timber sole plates under base plates on soft or distributed ground
• Base plates — 150mm × 150mm × 5mm minimum per BS EN 12811; must be centred under the standard
• Kentledge / ballast — only used as temporary measure during erection before ties are installed; must not be used as permanent substitute for ties

Quick Reference Table: TG20 Standard Independent Scaffold Parameters

Detailed Guidance

Scaffold Geometry: Standards, Ledgers and Transoms

Standards are the vertical tubes. In an independent tied scaffold:

• Outer standards — the row closest to the building at working height; typically 200–300mm from the wall face
• Inner standards — the row further from the wall, forming the outer edge of the working platform

Bay width is the distance between adjacent standards along the facade, typically 2.4m for general purpose work. Board spans determine transom spacing; 38mm scaffold boards (to BS 2482) can span up to 1.5m but 1.2m is standard for comfort.

Ledgers are horizontal tubes parallel to the facade, running along the length of the scaffold and connecting standards at each lift level.

Transoms run perpendicular to the facade, connecting inner and outer ledgers. They support the boards of the working platform. A main transom sits at each standard position; intermediate transoms sit between standards.

Tie Types and Pull-Out Requirements

TG20 distinguishes three tie types based on how they are anchored to the building:

Reveal ties are common on new-build sites where the scaffold is erected before windows are installed. They rely on friction against the reveal and must be periodically checked and adjusted. TG20 provides pull-out calculation for reveal ties; the resulting required tie density is higher than for through-bolted anchors.

TG20 Tie Pattern Examples

The eSP tool will output the exact pattern. Typical patterns for a standard duty class 3 independent scaffold at medium height in Wind Zone 1:

• Every alternate bay, every other lift — e.g., ties in bays 1, 3, 5... on lift 1, bays 2, 4, 6... on lift 2 (staggered)
• Every bay, every third lift — for heavier duty or higher structures

Closer tie spacing is required for:

• Taller scaffolds (wind moment increases with height)
• Higher wind zones (Zone 2–3 vs Zone 1)
• Heavier duty classes (more load = more overturning force)
• Exposed conditions vs sheltered

Never omit ties to simplify erection. Missing a single tie in a critical position can reduce the overall tie capacity significantly.

The Importance of the First Lift

The first lift — from ground level to the first working platform — sets the tone for the entire scaffold. Key considerations:

• Ground bearing — standards must bear on sole plates if ground is soft, granular, or compressible. Never rest base plates directly on loose backfill or tarmac over services
• Sole plate sizing — TG20 specifies 225mm × 38mm minimum timber; wider on poor ground; concrete pads or adjustable base jack extensions where needed
• Base lift height — typically 2.0m above existing ground to allow passage; confirm with client. Consideration for loading bays
• Reveal tie installation — first ties should be installed as erection progresses, not all at the end

Scaffold Access: Ladders and Stair Towers

Access to working platforms must comply with WAH Regulations and TG20:

• Internal access ladder — standard for most scaffolds; ladder inside scaffold frame at one or both ends; secured at top; pitched at 75° (1:4 ratio)
• External stair tower — preferred for scaffolds in use for extended periods or accessed by multiple trades; reduced risk vs ladder
• Ladder bay — dedicated bay with an access through-board hatch at each working level
• Access must be on the scaffold, not via adjacent roofline or building features

Protecting the Ground Floor

Independent scaffolds on public footways or near traffic require:

• Fansifolds/fans — projecting fans or debris netting at first lift to catch falling materials
• Brick guards — mesh or solid protection fitted to guardrails to contain small items (screws, offcuts) on working platforms adjacent to pedestrian areas
• Lighting — red lights on projecting tubes at foot level after dark; consult highway authority guidance
• Pedestrian protection boards — overhead protection canopy at ground level over footway

Frequently Asked Questions

What is the difference between an independent scaffold and a putlog scaffold?

In a putlog scaffold, the inner row of standards is replaced by putlog tubes (single horizontal tubes) that rest in the perpendicular joints of brick courses, using the building wall as partial structural support. This means putlog scaffolds rely on the building being structurally sound and in a suitable construction state. Independent scaffolds have two full rows of standards and do not rely on the building for vertical load support — only for lateral tie restraint.

Can I increase the bay width to 3.0m to give bricklayers more working room?

Not without an engineer's design. TG20:21 only covers bay widths up to 2.4m (centre to centre of standards). A 3.0m bay width takes the scaffold outside TG20 scope and requires a structural engineer's design certificate. The design must confirm standards, couplings, ties, and boards are adequate for the wider configuration.

How many standards per bay does a typical independent scaffold have?

Typically two standards per bay end (one inner, one outer), giving 4 standards at each bay boundary (two bays share standards). For a 40m long scaffold at 2.4m bay widths, there will be approximately 17 standard positions × 2 rows = approximately 34 standards per lift. Exact numbers depend on bay configuration, corners, and loading bays.

Does a working platform need to be boarded to the edge of the scaffold on all sides?

Yes. The working platform should be fully boarded with no gaps exceeding 25mm between boards or between boards and standards. At the outer edge, a toeboard (minimum 150mm) and guardrail system must be fitted. At the inner edge (adjacent to the building), a toeboard is also required unless the gap is small enough to be non-hazardous (typically less than 100mm gap from the wall face at board level).

Regulations & Standards

• Work at Height Regulations 2005 (SI 2005/735) — Schedule 2: working platform requirements; guardrail specifications

• BS EN 12811-1:2003 — Performance requirements and general design for scaffolding

• BS EN 39:2001 — Steel tube specification for scaffolding (48.3mm OD, Grade S235/S275)

• BS EN 74-1:2005 — Couplings: right-angle, swivel, and sleeve couplers

• BS 2482:2009 — Specification for timber scaffold boards

• NASC TG20:21 — Comprehensive Guide to Good Practice for Tube and Fitting Scaffolding

• NASC SG4:22 — Preventing Falls in Scaffolding; advance guardrail methods during erection

• NASC SG6:17 — Loading of Scaffolds; duty class loading

• NASC TG20:21 and eSP Tool — comprehensive guide and free eSP calculator

• HSE Scaffolding Safety — guidance and statistics

• NASC SG4:22 — preventing falls during erection

• tg20 compliance guide — generating TG20 compliance sheets for tube and fitting scaffolds

• scaffold loading limits — duty class loading and board types

• scaffold edge protection — guardrail and toeboard specifications

• scaffold inspection records — inspection and handover documentation