Wind and storms: wind load, nets, sheeting and making the scaffold safe
Vertical loads on a scaffold are easy to calculate: people, material and the scaffold's own weight. The real challenge comes from the side. Wind pushes on one face and sucks on the other, shakes nets and sheeting and finds the weakest point, usually a tie or a loose object on a deck. Knowing how wind load builds up lets you prepare a scaffold for a storm and decide in good time when work up top has to stop.
How wind acts on a scaffold
An unclad scaffold offers the wind only tubes, frames and decks; most of the area is open. Even so, the forces are considerable and must be carried into the building through the ties. A facade scaffold is tall and narrow: without ties it would overturn in moderate wind.
Wind acts in two directions. Blowing towards the facade, it presses the scaffold against the building. Blowing from the building side or round a corner, it creates suction that pulls the scaffold away from the wall. Suction is strongest at corners and along the roof edge, which is why manufacturers' instructions often call for additional ties in edge zones. Forces parallel to the facade are taken by V-ties; tie types and patterns are explained in tying scaffolding to the building.
Speed is what matters. The pressure wind exerts on a surface rises with the square of its speed. As a rough guide, 50 km/h (31 mph) gives about 0.12 kN/m², 100 km/h (62 mph) about 0.48 kN/m². Twice the wind speed therefore means roughly four times the force. A scaffold that stands quietly on a breezy day can reach its limits in a single strong gust.
Reading the forecast
Weather services give wind in km/h, mph, m/s or on the Beaufort scale. On site the gust is what counts, not the mean wind. A forecast of “mean 40 km/h, gusts to 85 km/h” is a storm warning, however calm the morning looks.
| Beaufort | Description | km/h (mph) | Typical effect on site |
|---|---|---|---|
| 6 | Strong breeze | 39–49 (25–31) | Sheeting flaps, insulation boards and panels hard to handle |
| 7 | Near gale | 50–61 (32–38) | Walking on exposed lifts uncomfortable, large components hard to control |
| 8 | Gale | 62–74 (39–46) | Loose objects fly; work on the scaffold normally stops |
| 9 | Severe gale | 75–88 (47–54) | Roof tiles can lift, high forces on clad scaffolds |
| 10–12 | Storm to hurricane | 89 and above (55+) | Damage to buildings and trees; scaffold must be secured beforehand, keep away |
The right-hand column is site experience, not a regulation. The wind speed at which erection or work must stop is set by the contractor's risk assessment, and many manufacturers' instructions give their own limits, for example for fitting sheeting or using mobile towers outdoors. Those limits take precedence.
Nets and sheeting: more protection, much more load
Debris netting holds back dust, paint mist and small parts; sheeting also keeps out rain. Both turn an open frame into a surface the wind can push against. A net still lets some air through, sheeting almost none, and the load on the ties rises many times over.
Worked example
An elevation 15 m long with a 10 m high scaffold is fully sheeted: 150 m². In gusts of 90 km/h the wind pressure is roughly 0.39 kN/m², which on the whole area gives about 59 kN, comparable to the weight of six tonnes. All of that has to go through the ties into the wall. This rough calculation ignores shape factors and height effects and is no substitute for a design check, but it shows why a sheeted scaffold is tied differently.
The consequences:
- Manufacturers' instructions give separate tie patterns for unclad, netted and sheeted scaffolds. The sheeted pattern is the densest.
- Adding nets or sheeting later means adjusting the ties first. That is an alteration and is followed by a new inspection.
- Large advertising banners or cladding beyond the standard configurations usually need an individual design.
- Fix nets and sheeting at every intended eyelet. Tying only every second one lets the sheet tear and turn into a sail.
Before a forecast storm
Storm warnings normally come one or two days ahead. Use the last calm working day, not the moment the wind picks up:
- Clear the decks: insulation boards, render bags, buckets, film, battens, tools. Anything the wind can lift goes down or inside.
- Remove waste: rubble sacks and packaging are future projectiles.
- Check the decks: all in place and secured against uplift with deck locks; remove unsecured loose boards.
- Close hatches and secure ladders.
- Check the ties: are all ties required by the pattern present? Any removed for plastering or insulation go back in now.
- Check cladding: nets and sheeting taut, complete, no tears. Whether sheeting is opened or taken down before extreme wind is the erector's decision based on the instructions; improvising in rising wind is dangerous.
- Secure temporary roofs as the manufacturer specifies; they have their own wind limits.
- Secure the surroundings: weigh down and connect fence panels, close containers, check barriers.
- Close the access: sign at the entrance, bottom ladder removed.
Walk every lift once. A single batten blown off the fifth bay can damage a car or injure someone.
Weak points that wind exposes
- Ends and corners: strongest suction, and the places where ties are most often missing.
- Sections above the eaves: a roof edge protection screen above the gutter has no wall behind it.
- Free-standing sections: scaffolds in front of gables, gaps between buildings, stair towers beside the main scaffold.
- Poor anchorage: ties in old brickwork, in insulation or in joints. A storm shows whether the fixings really hold.
- Soft ground: heavy rain before or after a storm softens the ground; a settling base jack changes the load distribution of the whole scaffold.
Thunderstorms and mobile towers
A steel or aluminium scaffold is a large metal structure, often the highest point around. The rule is simple: as soon as you hear thunder, stop work and get off the scaffold. Thunderstorms move fast and bring downbursts that reach storm force within seconds. Shelter in the building or a closed metal-bodied vehicle, not under the scaffold or a fan. Resume only when the storm has clearly passed; many firms set a waiting time after the last thunder in their risk assessment. Whether a scaffold should be earthed is a design question for a qualified electrician; a home-made cable and earth spike is no solution.
A mobile tower is not tied to the building. Its stability comes from its base, outriggers and any ballast, so wind affects it even more. The instructions for EN 1004 towers usually allow lower heights outdoors than indoors and set a wind limit for use. Do not move a tower in rising wind, keep castors braked and outriggers fitted, never hang sheeting on it unless the instructions allow it, and before a storm move it to shelter, tie it to a solid structure or dismantle it. See facade scaffold or mobile tower.
After the storm: inspect before anyone climbs
Storm damage is a classic trigger for a fresh inspection. In Britain, the Work at Height Regulations 2005 name events likely to affect stability as a reason; on US sites a competent person must look at the scaffold before every shift anyway (OSHA, 29 CFR 1926 Subpart L). In other countries the national rules decide. Look especially at:
- ties: tight, not bent, fixings not pulled, no cracks in the wall;
- cladding: tears, loose ties, slipped sheets;
- decks: shifted or lifted, toe boards missing;
- plumb and line: has any section moved out of vertical?
- base points: jacks settled, sole boards undermined;
- debris: branches, tiles, scraps of sheeting on the lifts.
Record the result in the inspection report; see scaffolding inspection. Damaged parts are replaced, never bent straight. Replacement wall ties, decks and guard rails are available among our components.
Agree at the start who does what: the client or site manager watches warnings and tells all trades, the erector handles ties, cladding and alterations and inspects afterwards, the trades clear their own material and report damage. On a private job with an owner-erected scaffold, all of this lies with the owner; if you lack experience, get competent help for the post-storm check.