Compliance · 10 min read

TG20:21 tie patterns explained: types, spacing and testing

Ties are the difference between a scaffold that stands and one that peels off the building in a gale. TG20:21 sets out how many ties you need, where they go, and how they're checked, but the theory behind tie types, spacing and testing rarely gets explained in plain English. Here's the version you can actually use on site.

For the wider load-class and design-envelope picture, see our TG20:21 compliance guide. This article stays focused on ties: types, spacing logic, the effect of sheeting and netting, testing frequency, and the failures that actually happen.

Why ties exist

An independent tied scaffold is inherently unstable on its own, it's a tall, narrow structure standing on relatively small base plates. Wind load on the façade (and much more if it's sheeted or netted) tries to push it away from or into the building, and overturning moments build up with height. Ties transfer that load into the building structure so the standards, ledgers and bracing don't have to resist it alone. Remove or weaken enough ties and the scaffold can rack, lean, or in the worst cases come away from the wall entirely.

The five tie types

Every tie, whatever the type, is there to resist a design tie force, commonly taken as around 6.1 kN in tension for a standard compliant scaffold, higher for sheeted or heavily loaded configurations. The tie type doesn't change the force it needs to resist; it changes how that force gets into the building.

Spacing logic: why ties go where they go

Tie spacing isn't arbitrary, it follows from the scaffold's height, load class, and exposure. TG20:21's eGuide works this out for you for standard configurations, but the underlying logic is:

• Taller scaffolds and higher wind exposure need more ties per unit area, the overturning moment and total wind load both increase with height.
• Sheeted or netted scaffolds present far more surface area to the wind, so tie density has to go up sharply, a pattern that was compliant unsheeted can be dangerously under-tied once sheeting goes on.
• Heavier load classes (Class 4 and above) increase the loads the tie network has to help stabilise, particularly at returns and corners.
• Corners, returns and free-standing ends always need closer tie spacing than a long straight run, these are the points most likely to be under-tied on site.

In practice this shows up as three named tie patterns: a light pattern (roughly one tie every other lift, alternate bays) for low-rise, low-exposure, light-duty work; a standard pattern (one tie every lift, alternate bays) for most general-access scaffolds; and a close pattern (one tie every lift, every bay) for sheeted, netted or heavily loaded scaffolds. Which one applies to your job is stated on the compliance sheet, see our guide to the TG20:21 compliance sheet for how that document records the pattern you're built to.

Worked example: sheeting changes the answer

Take a two-lift independent scaffold, 24 m run, Class 2 loading, on a moderately exposed site. Unsheeted, that might be compliant on a standard tie pattern, say 1 tie per 2 lifts per bay, roughly 6–8 ties along the run. The contractor is then asked to sheet the whole elevation for weather protection.

Sheeting can increase the effective wind load on that elevation by a large multiple compared with the open tube-and-board scaffold. The same 24 m run now needs a close tie pattern, potentially one tie every lift, every bay, pushing the tie count from around 7 up to 24 or more. That's not a paperwork adjustment; it's a different structure, and if the eGuide's sheeted envelope doesn't cover the height and exposure, it tips into bespoke design territory (see when a scaffold needs bespoke design). Sheeting "for the weekend" without re-checking ties is one of the most common causes of scaffold failures in high wind.

Tie testing frequency and pull-out loads

Not all ties are tested the same way, and this is where site practice varies a lot:

• Drilled mechanical anchors should be proof-tested before being relied on, typically pulled to a set proportion above the design tie force with a calibrated tension gauge, and the result logged against the tie's location.
• Through, box, lip and reveal ties aren't load-tested with a gauge in the same way, they rely on correct installation (proper coupling, full engagement, no crushed reveals) checked visually at every statutory inspection.
• Every tie, of every type, gets checked at the 7-day statutory inspection and after any event that could have disturbed it. High winds, a vehicle strike, or any alteration nearby.

Typical anchor pull-out test loads used on UK sites sit somewhere around 1.5–2× the design tie force, giving a safety margin above the 6.1 kN (or higher, sheeted) figure the tie is meant to resist in service, always confirm the exact test load with your scaffold designer or the anchor manufacturer's data, since it varies by substrate and anchor type.

Common tie failures on site

• Ties removed for access and not reinstated, a window tie taken out so a delivery could pass through, never put back before the next inspection.
• Reveal ties in soft or damaged masonry, the reveal itself crumbles or the wedge slips, giving no real resistance despite looking installed.
• Anchors drilled into unsuitable substrate, render, thin brick slips, or cavity walls without checking what's actually behind the face.
• Sheeting added without re-checking the tie pattern . Covered above, and still one of the most frequent causes of scaffold movement in wind.
• Corner and return bays under-tied, general pattern applied uniformly along a run without extra ties at ends and corners where load concentrates.
• No test record for anchor ties, anchors installed but never pull-tested, or tested without the result logged, leaving nothing to show an inspector.

Common questions

Can I mix tie types on the same scaffold?

Yes, and it's normal, through ties where openings exist, box or anchor ties on solid runs between them. What matters is that every tie, whatever type, is at the spacing and frequency the compliance sheet or design calls for.

Who decides which tie pattern applies?

For a standard configuration, the TG20:21 eGuide determines the pattern from the scaffold's height, load class, exposure and whether it's sheeted. Outside that envelope, a scaffold designer sets the pattern as part of a bespoke design.

Does box tying avoid the need to test anything?

It avoids drilling and pull-out testing, but it doesn't avoid inspection. A box tie still has to be checked at every statutory inspection to confirm it's tight, correctly coupled and hasn't slipped on the pier it's wrapped around.

What happens if a client won't allow any drilled ties?

Box ties and through/reveal ties at openings are the usual fallback. If the elevation can't provide enough non-drilled tie points for the required pattern, you may need rakers, a freestanding design, or a bespoke solution from a designer, flag this at quoting stage, not after the scaffold is up.

TG20:21 is a trademark of the National Access & Scaffolding Confederation (NASC). Figures given are typical/indicative, always confirm exact tie forces and test loads against the current TG20:21 eGuide output or your scaffold designer's specification.