Scaffolding Foundations

1.Purpose and scope

This note covers the foundations of access and support scaffolds: estimating leg loads, sizing soleboards and spreaders, assessing the ground or structure the scaffold stands on, and controlling settlement. It is aimed at TWCs, site engineers, scaffold designers and supervisors working to TG20:21, BS EN 12811-1 and BS 5975:2024.

Tying, bracing and the scaffold structure itself are outside this note except where they interact with foundations (they do — a settling scaffold redistributes load into ties and adjacent standards).

2.Why scaffold foundations matter

• Scaffold leg loads are routinely underestimated: a standard in a loaded independent scaffold can easily carry 10–40 kN or more, and much higher in support scaffolds, loading bays and beneath cantilevers or beamwork.
• A baseplate alone bears on roughly 0.02–0.03 m². 25 kN through a bare 150 mm baseplate is around 1,100 kN/m² — more than most surfaces can take. The soleboard, not the baseplate, is the foundation.
• Settlement of one leg does not simply lower the scaffold: it throws load onto neighbouring standards and ties, cracks boards and can initiate progressive failure — especially in sheeted scaffolds in wind.

3.Key standards and references

4.Estimating leg loads

• For TG20:21 compliant scaffolds, take leg loads from the compliance sheet for the configuration (height, bays, load class, cladding).
• For designed scaffolds, leg loads come from the design output — and should be stated on it so foundations can be checked.
• Watch the concentrators: loading bays, beamed openings, cantilever and truss-out scaffolds, buttress bays, rubbish chutes, hoist towers and sheeted elevations all push leg loads well above the typical run of standards.
• Include what the scaffold will actually carry: stacked boards and materials at working lifts, mast/hoist reactions, weather protection roofs.

5.Baseplates, soleboards and spreaders

• Every standard gets a baseplate. On anything other than sound, designed hardstanding, standards also get a soleboard sized to spread the leg load to a pressure the ground can take.
• Soleboards should be sound timber (or proprietary pads) laid flat on prepared ground — not on debris, frozen crust or bridging across ruts. A common minimum practice is at least two standards per soleboard where practicable, improving robustness against local soft spots.
• Size by calculation when loads or ground demand it: required soleboard bearing area = leg load ÷ allowable ground pressure; then check the board itself in bending/bearing (a long thin board does not usefully spread load far beyond the baseplate unless it is stiff enough).
• On slopes, cut level steps — never wedge baseplates on packs on a slope.

6.Assessing what the scaffold stands on

6.1 Ground

• Use the same discipline as any foundation: what is the material, is it compacted, what happens when it gets wet, what is buried beneath it?
• Be suspicious of: made ground and recent backfill, service trenches along the scaffold line (they usually are along the building line), landscaping topsoil, and winter softening.
• Protect foundations in use: drainage away from soleboards, no excavation beside standards without TWC approval, vehicle protection where plant passes close.

6.2 Paved and hard surfaces

• Footways and block paving often overlie poorly compacted trench backfill and shallow services — the paving hides the problem; it does not solve it.
• Check for cellars, vaults and basements under footpaths in urban streets, and for manhole covers, gully pots and duct runs directly beneath standards.

6.3 Scaffolds on structures

• Standing scaffolds on suspended slabs, podium decks, bridges or roofs is a designed condition: the structure's capacity at the actual leg positions must be confirmed by a competent engineer, and back-propping provided where required (and itself designed).
• Spreader beams/grillages may be needed to move leg loads to beams or walls rather than mid-slab.

7.Settlement: prevention and response

• Prevent: prepared, drained bearing; sized soleboards; base lift braced and level; adjustable bases within their allowed extension.
• Monitor: include foundations explicitly in the statutory scaffold inspections (before first use, then at intervals not exceeding 7 days, and after weather or events likely to affect stability).
• Respond: any settled, undermined or washed-out base is a stop-and-refer item — relieving and repacking a loaded standard is itself a temporary works operation that needs a method from a competent person, not a hammer and an extra wedge.

8.Illustrative example — soleboard sizing (concept)

Design leg load 25 kN per standard; ground assessed as firm clay, allowable pressure taken as 100 kN/m² for the temporary case:

So a 2.4 m × 225 mm board under two standards is reasonable if the board is thick enough to distribute the load over that length and the ground is as assumed — on softer ground (say 50 kN/m²) the required area doubles and a wider/doubled board or improved bearing is needed. Illustrative only; use design leg loads and verified ground values.

9.Site control checklist

• Leg loads known — from the TG20:21 compliance sheet or the scaffold design — including loading bays and other concentrations.
• Bearing surface identified and assessed: ground type, backfill/services below, cellars/voids, drainage.
• Baseplates on every standard; soleboards sized and bedded on prepared ground (no debris, no bridging, level steps on slopes).
• Scaffolds on structures: capacity confirmed by an engineer; back-propping designed and installed before loading.
• Adjustable legs within permitted extension; base lift complete and braced before erecting above.
• Foundations included in 7-day and post-weather inspections; settlement is a stop-and-refer trigger.
• No excavation, dewatering or heavy plant next to standards without TWC review.
• Any change of use (extra lifts, sheeting, loading bays, hoists) triggers a foundation re-check.

10.References

• NASC TG20:21 — Guide to good practice for tube and fitting scaffolding.
• BS EN 12811-1 — Temporary works equipment: scaffolds.
• BS 5975:2024 — Temporary works procedures.
• Work at Height Regulations 2005 — scaffold inspection requirements.
• NASC SG/TG guidance series.