1.Purpose and scope
This note explains how wind loading should be considered in the design and site control of common temporary works: hoardings and fences, scaffolds (sheeted and unsheeted), falsework and propping, site accommodation, and temporary structures. It is written for temporary works coordinators (TWCs), site engineers and designers working under a BS 5975:2024 temporary works procedure in the UK.
It does not replace a wind assessment to BS EN 1991-1-4 and its UK National Annex, and it does not cover permanent-works wind design, marquees to BS EN 13782, or crane selection (which remains the appointed person's responsibility under BS 7121).
2.Why wind matters for temporary works
Wind is the most common environmental action that fails temporary works. Hoardings blow over onto footpaths, sheeted scaffolds pull their ties, unrestrained falsework racks over, and stacked materials become missiles. Three features make temporary works especially wind-sensitive:
- Light weight, large sail area. Many temporary structures rely on self-weight or ballast for stability, and adding sheeting, netting, banners or signage can multiply the wind load on a structure that was never designed for it.
- Incomplete states. A part-erected structure often has less strength and stability than the completed scheme — the critical wind case is frequently during erection, alteration or dismantle.
- Changing site conditions. Local shielding and funnelling change as the permanent works rise; a scaffold that was sheltered in month one can be exposed in month six.
Common failure trigger
Adding sheeting, monarflex, debris netting or advertising banners to an existing scaffold or fence without a design check. This changes the structure's wind loading fundamentally and must be treated as a design change under the temporary works procedure — not a site-level decision.
3.Key standards and references
| Document | Relevance |
|---|---|
| BS EN 1991-1-4 + UK National Annex | The core method for deriving site wind speeds and peak velocity pressure, and pressure/force coefficients for structures. |
| BS 5975:2024 | Procedural control of temporary works; stability requirements including minimum notional horizontal loads; guidance on wind during working conditions. |
| TG20:21 (NASC) | Wind loading built into compliance sheets for tube-and-fitting scaffolds, including sheeting and netting factors and town/country terrain. |
| PD 6688-1-4 | UK background and supplementary guidance to BS EN 1991-1-4. |
| BS EN 12811-1 / BS EN 12812 | Performance requirements for scaffolds and falsework, including in-service wind assumptions. |
| BS EN 13374 | Temporary edge protection systems. |
4.Deriving the design wind action
The BS EN 1991-1-4 route, in outline:
- Fundamental basic wind velocity — read the mapped value for the site from the UK National Annex wind map, then correct for altitude.
- Directional and seasonal factors — for temporary works with a defined, limited duration, the National Annex permits seasonal and probability factors that can legitimately reduce the design wind speed for short exposure periods. Use them only with a defined erection window and a plan for what happens if the programme slips past it.
- Terrain and orography — classify the site exposure (sea, country, town), account for distance inside town terrain, and check for orographic speed-up on hills, cliffs and escarpments.
- Peak velocity pressure qp(z) — computed at the reference height of the structure; this is the pressure the coefficients are applied to.
- Force/pressure coefficients — select for the structure form: free-standing walls and hoardings, signboards, lattice structures and scaffolds, or clad structures. Net coefficients for free-standing hoardings vary strongly with solidity, length and the presence of return corners — end zones see markedly higher suction/pressure than mid-length zones.
Working wind vs. maximum wind
BS 5975 distinguishes the working state (operations continue up to a stated working wind pressure, with a defined site response above it) from the maximum (out-of-service) state the structure must survive with work stopped. Both cases must be designed for, and the working wind limit must be stated on the design output so site can enforce it — typically via anemometer readings and forecast triggers.
In addition to wind, BS 5975 requires falsework and similar temporary structures to resist a minimum notional horizontal disturbing force — conventionally taken as not less than 2.5% of the applied vertical loads, applied where the loads act — to cover erection tolerances, eccentricities and incidental impacts. Wind is checked in combination; the governing case is whichever is worse.
5.Application to common temporary works
5.1 Hoardings and fencing
- Treat as free-standing walls; net pressure coefficients depend on solidity and end effects. A hoarding return or free end attracts significantly higher load than the run.
- Stability is usually the governing check: overturning and sliding of posts, ballast blocks or ground anchorage. Kentledge on smooth ground can slide before it overturns — check both.
- Ground fixity (post embedment, concrete foundations) must reflect real ground, not assumed ground.
5.2 Scaffolds
- TG20:21 compliance sheets embed wind assessment for standard configurations — but only within the stated limits (height, cladding, town/country terrain, tie pattern). Outside those limits a bespoke design is required.
- Sheeting and debris netting factors are step changes: netting can roughly double, and impermeable sheeting can several-fold increase, the wind load compared with an unclad scaffold.
- Ties resist wind both in pressure and suction — tie capacity, spacing and testing are wind-critical items. Missing or removed ties are a leading cause of scaffold collapse in storms.
5.3 Falsework and propping
- Check the erection sequence: bare towers before beams/soffit are landed, and the just-struck condition, can be the critical wind cases.
- Wind on the permanent works being supported (e.g. a wall or beam on props) loads the temporary works — include the sail area of the supported element, its formwork and any weather protection.
5.4 Site accommodation, materials and small structures
- Stacked cabins, container offices and welfare units need anchorage or ballast checks, particularly when raised on jack-legs or stacked two-high.
- Loose materials, boards and sheet piles on scaffolds or roof edges must be restrained or removed ahead of forecast winds — make this an explicit item in the storm-readiness plan.
6.Illustrative example — hoarding stability (concept)
A 2.4 m timber hoarding in town terrain is assessed for a peak velocity pressure of, say, qp = 0.60 kN/m² with a net pressure coefficient of 1.2 in the run of the wall:
The restoring moment from posts, foundations or kentledge must exceed this with an adequate margin, and end zones must be checked for their higher coefficients. The numbers above are illustrative only — site-specific wind derivation is always required.
7.Site control checklist
- Design output states the design wind speed/pressure, the working wind limit, and any duration or seasonal limits on validity.
- Any change to cladding, sheeting, netting, signage or banners is referred back to the designer before it happens.
- Ties, guys, ballast and anchorages are installed as designed and included in scaffold/TW inspections.
- A named person monitors weather forecasts; trigger levels and pre-storm actions (stop work, clear loose materials, close sheeting gates) are written down.
- Anemometer available where operations have wind limits (lifting, sheeting works, tower cranes).
- Post-storm inspection is carried out and recorded before work restarts on or near the structure.
- Programme slippage beyond the assumed exposure window is flagged to the TWC and designer.
8.References
- BS EN 1991-1-4:2005+A1:2010 and UK National Annex — Wind actions.
- PD 6688-1-4 — Background information to the UK National Annex.
- BS 5975:2024 — Code of practice for temporary works procedures and the permissible stress design of falsework.
- NASC TG20:21 — Guide to good practice for tube and fitting scaffolding.
- BS EN 12811-1, BS EN 12812, BS EN 13374.