Towers
Mobile tower height-to-base ratio check (legacy rule of thumb)
- where
- height to base ratio
- platform height (m)
- least base dimension (m)
- maximum platform height at this base (m)
- height to base limit
- minimum base for this height (m)
- ratio check (1 or less passes)
Explain this equation
Checks a mobile tower against the old height-to-least-base rule of thumb. Use it as a sanity check only.
The platform height divided by the narrowest base dimension shows how tall and slender the tower is. Multiplying the limit by the base gives the highest platform allowed, and dividing the height by the limit gives the base needed. The ratio of actual to limit shows how close you are.
A tipping check: a taller tower on a narrow base topples more easily.
Sense check: a 4 m platform on a 1.45 m base is 2.76 to 1, inside a 3 to 1 limit; a 1.45 m base allows up to about 4.35 m.
Watch out: modern towers to BS EN 1004 are built to the manufacturer's instructions, which set heights, stabilisers and ballast. Those override this rule.
A quick check of a mobile tower’s platform height against its narrowest base using the traditional ratio limit.
Enter your values to see the result.
Good to know
Older guidance limited mobile towers to a height of 3.5 times the least base dimension indoors and 3 times outdoors (static towers slightly more). PASMA now says that rule no longer works on its own: tower stability under BS EN 1004-1:2020 comes from a calculation by the manufacturer, which sets the stabilisers or outriggers and ballast for each height. BS EN 1004-1:2020 towers go up to 8 m outdoors and 12 m indoors. Use this check only as a sense check before reading the instructions. Towers must be built by people trained by PASMA or the manufacturer.
Common mistake
Building to the ratio and leaving off the stabilisers the manufacturer’s instructions call for at that height.
Limits
Rule of thumb only; it ignores wind, side loads, ballast and ground slope. Never use it in place of the manufacturer’s instruction manual and BS EN 1004. For planning and estimating only. This does not replace TG20:21 compliance (a TG20 compliance sheet or eGuide output) or a scaffold design by a competent person.
Sources
- PASMA, Safety FAQs (the old 3:1 rule no longer determines tower stability; follow BS EN 1004 and the manufacturer’s instructions)
- PASMA, Design standards for towers and podiums (BS EN 1004-1:2020: up to 8 m outdoors, 12 m indoors)
- Scaffmag, Scaffold towers (height to base: mobile internal 3.5:1, mobile external 3:1)
- Health and Safety Authority (Ireland), Code of Practice for Access and Working Scaffolds (sole boards ≥ 1000 cm², ties ≥ 6.1 kN, façade bracing at least every six bays, tower height to least base ≤ 3, toe-boards ≥ 150 mm)
Results are calculations from the published methods and figures shown, and depend on the data you enter. For planning and checking only. Scaffolds must be erected to NASC TG20 or a bespoke design by competent persons, and lifting operations planned under LOLER by a competent person. Never rely on these results instead of a design, a lift plan or manufacturer ratings.