The 3 to 1 rule is an old rule of thumb for free-standing scaffold towers: the height should be no more than three times the shortest side of the base. A tower with a 1.2 metre wide base, for example, would be kept under 3.6 metres. It is still widely quoted, but for modern mobile towers it has been replaced by the manufacturer's instructions.
Where the 3 to 1 rule came from
Stability depends on how tall a free-standing structure is compared with its base. The traditional guidance was 3 to 1 for towers used outdoors, where wind is a factor, and around 3.5 to 1 indoors. Industry guidance for free-standing birdcage scaffolds still uses ratios like these.
Why it no longer applies to mobile towers
PASMA, the industry body for mobile access towers, says the old 3:1 rule no longer works for working out whether a mobile tower is stable. Base sizes are now set by a calculation in the EN 1004 standard, so the right answer is whatever the manufacturer's instructions say for that height, often with outriggers or stabilisers.
Does it apply to house scaffolding?
Not in the same way. The scaffolding we put up round houses in Epsom and Surrey is tied into the building, which is what keeps it stable, rather than its own base width. Tube and fitting scaffolds are designed to TG20 guidance from the NASC, which sets out tie patterns, loads and when a bespoke design is needed. Free-standing scaffolds, such as some chimney or birdcage structures, are designed with their height-to-base ratio in mind.
Do not confuse it with the ladder 1 in 4 rule
People often mix the 3 to 1 rule up with the ladder rule: a leaning ladder should be set at about 75 degrees, which works out as 1 unit out at the base for every 4 units up.
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Common questions
What is the 3 to 1 rule in scaffolding?
It is the traditional rule that a free-standing scaffold tower should be no taller than three times the shortest side of its base. For modern mobile towers, PASMA says to follow the manufacturer's instructions instead.
Does the 3 to 1 rule apply to scaffolding tied to a house?
No. Scaffolding tied into a building gets its stability from the ties, and is designed to TG20 guidance rather than a simple height-to-base ratio.