TG20 and SG4: the two UK scaffolding references in plain terms

TG20 and SG4 are the two NASC guidance documents behind most scaffolds in the United Kingdom. TG20 deals with tube and fitting scaffolds: where a scaffold matches one of its standard configurations it needs no individual calculation, and where it does not, a competent person has to design it. SG4 deals with the scaffolders themselves and how falls are prevented while a scaffold is erected, altered or struck; the current edition, SG4:22, also covers system scaffolds and is free to download. This guide explains what both mean for the people who order, buy or depend on scaffolding. It relies on the public summaries from the Health and Safety Executive (HSE), because TG20:21 is a paid document and is not reproduced here.

Guidance, not law: how the pieces fit together

The legal duties sit in the Work at Height Regulations 2005. They apply to work at height of any kind, with no minimum height below which they stop applying. Schedule 2 of the Regulations deals with guard-rails, toe-boards and barriers: the top guard-rail has to be at least 950 mm above the edge from which someone could fall (guard-rails installed to the older 910 mm figure may remain), and no unprotected gap between rails and toe-board may exceed 470 mm.

TG20 and SG4 are industry documents from the National Access and Scaffolding Confederation (NASC). They turn the legal duties into design rules and working methods, and HSE's scaffolding guidance points to both. A scaffolding contractor who cannot show that a scaffold follows a TG20 configuration, a manufacturer's configuration or a specific design will find it hard to explain how the scaffold meets the law.

TG20: what a standard configuration means

Tube and fitting can be assembled in almost endless ways, which is its strength and also its risk. TG20 narrows that freedom down to arrangements whose strength and stability have already been checked. If the scaffold planned for a job lies within one of them, the contractor can rely on that check instead of commissioning calculations. If it lies outside, HSE expects a competent person to design the scaffold before it goes up.

System scaffolds follow the same logic with a different reference: the manufacturer's instructions take the place that TG20 holds for tube and fitting. A system scaffold built within the manufacturer's configurations needs no separate design; one that departs from them does. Our guide to manufacturer's instructions explains what these documents usually contain.

Two working lifts, any number of boarded lifts

One TG20 limit catches out many site teams. According to the HSE summary of TG20:21, a standard scaffold may carry as many boarded lifts as the job needs, but only two of them may serve as working lifts at the same time. A boarded lift simply has decking in place; a working lift is one where people are working and materials are standing. Once three or more lifts are in use simultaneously, the scaffold needs its own design.

Picture a four-storey terrace scaffolded for repointing. The contractor boards every lift so the bricklayers can move up as they progress, and that is fine. Then the programme slips, and the client wants window fitters on the second lift and roofers on the top lift while the bricklayers are still busy on the first. Three working lifts at once take the scaffold outside the standard configuration, even though not a single tube has moved.

Duty: the load a scaffold is built for

HSE describes three common load categories. Unless the scaffolding contractor states something else, a scaffold counts as general purpose.

For a sense of scale: one person is usually reckoned at about 1 kN, roughly 100 kg. On a platform section with 3 m² of decking, general purpose duty allows about 6 kN in total, evenly spread, which is two workers plus roughly 400 kg of material and tools distributed across the boards. A pallet of blocks set down in one spot is another matter, because a concentrated load does not behave like an evenly spread one. Whenever the planned materials do not fit comfortably inside the stated duty, say so before the scaffold is designed. For the European way of classifying platform loads, see scaffold load classes.

When a bespoke design is due

Based on the HSE guidance, the following situations should prompt the question “does this need a design?” before anything is erected:

• More than two working lifts at the same time. The standard configurations stop at two.
• Sheeting or netting. Coverings catch the wind and raise the forces on ties and structure. HSE is explicit that sheets or nets must not be fixed without the scaffold designer knowing.
• Loads beyond the stated duty. Loading bays, stacked materials, hoists or rubbish chutes can add forces that a standard arrangement does not allow for.
• Unusual geometry. Bridging over openings, cantilevered sections, scaffolds standing on roofs or carrying temporary roofs may fall outside the configurations; get the contractor to confirm in writing whether they do.
• System scaffolds outside the manual. Any departure from the manufacturer's permitted arrangements.

A design is more than a drawing for the file. The scaffold has to be built and loaded as designed, and HSE expects a plan for erection, use and dismantling, drawn up by a competent person, to stay available until the scaffold comes down. If a tie must be taken out early, for instance to let a window fitter work, an equivalent replacement goes in first. Our guide to scaffold ties and anchoring covers the practical side.

Who designs?

HSE speaks of a competent person: someone with the knowledge and experience to calculate the scaffold. Larger contractors often employ their own designers; smaller firms tend to use an external scaffold design engineer. As a buyer or principal contractor you do not have to check the calculation yourself. What you should check is that a design exists, that it describes the job as it really is, and that somebody tells the designer when the job changes.

SG4:22: keeping scaffolders safe while they build

A finished scaffold with complete edge protection is a safe place to work. The dangerous phase comes earlier: while lifts are going up, while bays are added or altered and while the scaffold is struck. SG4, titled “Preventing Falls in Scaffolding Operations”, is the industry's answer to that phase. The 2022 edition widened its scope to system scaffolds and, as the trade press reported in December 2022, NASC made it free to download.

Fall prevention in scaffolding, in the UK as elsewhere in Europe, follows a simple order: collective protection before personal protection. An advance guard-rail placed from the protected level below shields everyone who steps onto the next lift; a harness protects only its wearer, and only when it is clipped to a suitable point. SG4 describes how UK scaffolders turn fall prevention into daily practice. Since the document costs nothing, contractors and buyers alike can read the current edition rather than relying on second-hand summaries such as this one.

Questions a buyer or principal contractor can ask

• How will the scaffolders be protected against falls while erecting, altering and dismantling, and does the method follow SG4:22?
• For a system scaffold: does the system provide an advance guard-rail or a comparable collective measure, and is it included in the delivery?
• Who supervises the gang, and does every gang include at least one competent scaffolder with suitable training? HSE refers to CISRS cards as the way to check competence levels, and trainees must work under supervision.
• What happens if someone does fall and is held by a harness: who gets them down, and with what?

What this means when you buy equipment for UK work

For a buyer, TG20 and SG4 are not just the contractor's paperwork. They influence which equipment makes sense in the first place.

• Tube and fitting: standard configurations and designs assume tubes and couplers of known quality. Couplers are covered by EN 74 and loose steel tubes by EN 39, published in the UK as BS EN standards. Bent, cracked or badly corroded items have no place in any configuration.
• System scaffold: without the manufacturer's instructions there is no configuration to work to, so make sure the instructions for the exact system are available when you buy used stock. Keep different systems apart unless a designer has approved a combination.
• Fall prevention components: if the contractor's SG4 method depends on advance guard-rails or similar parts, they belong in the package from the outset rather than being bought after the first job.

Used components sold by CETRAC are examined for cracks, deformation and corrosion before they leave the yard; details are on the page about our quality check.

Common misunderstandings

• “TG20 is the law.” It is industry guidance. The duties come from the Work at Height Regulations 2005, and TG20 is a recognised way of showing how a tube and fitting scaffold meets them.
• “Fully boarded means fully usable.” Boards on every lift do not turn every lift into a working lift. The two-lift limit still applies.
• “The sheeting can go on later.” Fixing sheets or nets without telling the designer changes the wind load on a structure that was never checked for it.
• “SG4 is only for tube and fitting.” Since the 2022 edition it applies to system scaffolds as well.
• “Once handed over, the scaffold is done.” Inspection continues: a first check before anyone uses it, a routine check at least once a week, and an extra one after storms, alterations or damage, each written up with defects and actions.

Sources

• Work at Height Regulations 2005, Schedule 2 (SI 2005/735)
• Health and Safety Executive: Scaffolds, updated 19 May 2026
• NASC: SG4:22 Preventing Falls in Scaffolding Operations
• ScaffMag: SG4:22 now free, 19 December 2022