Mobile towers look simple. Aluminium frames on castors. But there’s more to their design than meets the eye. BS EN 1004‑1:2020 is Part 1 of the EN 1004 series. It sets the materials, dimensions, design loads and structural performance requirements for mobile access and working towers made of prefabricated elements. It supersedes the older EN 1004:2004 and is written mainly for designers and manufacturers. But the numbers in it (platform sizes, wind limits, overturning safety factors) are exactly what determine whether a tower on site is safe to work on. It’s used alongside EN 1004‑2, which sets out the requirements for the tower instruction manual.
A mobile tower is only as safe as the ground it stands on, the wind its exposed to, and whether it was actually built the way the manufacturer designed it. EN 1004‑1 fixes the engineering side of that equation. The load it’s designed to carry, how far apart platforms can be, how much side protection is required, and, critically, the wind speed above which a free‑standing tower is no longer considered stable.
The standard applies to mobile access and working towers made of prefabricated elements, with dimensions fixed by the design, up to 12 m indoors and 8 m outdoors. It covers the choice of main dimensions and stabilising methods, safety and performance requirements, and information on complete towers. It does not apply to façade or working scaffolds covered by EN 12810‑1 and EN 12811‑1. Mobile towers are a different animal, built as a single‑bay, free‑standing structure on castors rather than tied into a building.
| Term | Simplified definition |
|---|---|
| Mobile access and working tower | A temporary, free‑standing structure of prefabricated elements, fixed dimensions, four legs with castors, providing one or more platforms. |
| Height (h) | The distance from the ground to the upper surface of the uppermost platform. |
| Adjustable leg | A component at the base used to level the tower on uneven or sloping ground. |
| Outrigger / stabiliser | A component that increases the tower's effective base footprint – an outrigger has a castor fitted, a stabiliser doesn't. |
| Ballast | Weights placed at the base of the tower to increase resistance to overturning. |
| Wall strut | A horizontal member connecting the tower to a wall or other structure to resist overturning. |
| Working platform / intermediate platform | The single platform work is actually carried out from, versus any platform below it that isn't currently in use. |
| Positive locking device | A mechanical device – a pin, bolt or protrusion – that prevents unintentional disconnection without relying on friction. |
Every tower built to the standard is classified by 3 categories:
| Classification | What is means |
|---|---|
| Load class | Uniformly distributed load of 1.50 kN/m² (Class 2) or 2.00 kN/m² (Class 3) |
| Access class (A–D) | A = stairway, B = stairladder, C = inclined ladder (60°–75°), D = vertical ladder (90°). A tower's designation can combine types, e.g. “AXCX” means stairways and inclined ladders only. |
| Height class H1 / H2 | Minimum clear height between platforms: 1.85 m (H1) or 1.90 m (H2). |
The part of the standard that most directly affects site safety is the overturning check. Two load cases are run, both directions (parallel and perpendicular to the tower):
| Load case | Safety factor | What's applied |
|---|---|---|
| 1 – in use | S ≥ 1.5 | Self‑weight, a vertical service load near the platform edge (0.75 kN, doubled over 4 m platforms), plus the worse of a 0.3 kN horizontal service load or the 0.1 kN/m² wind load with people standing on the platform, plus a 1% out‑of‑level allowance. |
| 2 – unattended | S ≥ 1.3 | Self‑weight plus the 0.1 kN/m² wind load alone, plus the 1% out‑of‑level allowance. |
Overturning resistance can come from self‑weight, ballast, stabilisers, outriggers, or a combination. Against a wall, wall struts can be used instead.
Each component must carry a symbol or letters identifying the manufacturer and the year of manufacture, legible for the component’s life. The assembled tower needs a manufacturer’s label visible from ground level showing the manufacturer’s name, the tower’s designation (load/access/height class), and an instruction to read the manual. The standard also requires an independent assessment (carried out by someone other than the designer) confirming compliance with the standard.
EN 1004‑1 is the engineering backbone behind every mobile tower in Europe: the load it’s rated for, how the platforms and side protection are arranged, and above all, the wind speed at which it stops being safe to leave standing. It’s a manufacturer‑facing standard, but the numbers in it are exactly what a competent person is checking when they inspect a tower before use. Paired with EN 1004‑2, which governs the instruction manual that comes with the tower, it covers both how the thing is built and how the people assembling and using it are told to do so safely.
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