Buttress and outrigger

Inclined props from scaffold to ground that take over the job of ties where the wall may not be drilled, or steady the scaffold during erection.

Also called: raker, raking brace, push-pull prop, stability brace, outrigger

Normally a facade scaffold is anchored to the wall with ties. Sometimes that is impossible: a listed stone facade nobody may drill, a glass front, a gable exposed after the neighbouring building was demolished, or a scaffold standing with no structure at all. Then a buttress takes over from the anchor. Inclined props carry the horizontal forces from wind and use from the scaffold down into the ground.

A more everyday task is stability during erection. Until the first lift is tied, the scaffold can topple, for instance where material is passed up; a temporary raker at first-deck level prevents that.

Variants

• Raker of scaffold tube: the simplest solution – a tube fixed at a slope to the upright with swivel couplers, with a base plate or another tube at the foot.
• Telescopic buttresses: two sliding tubes with pin or screw adjustment and fixed couplers. The permitted extension is limited and marked on the part.
• Push-pull props: screw props known from formwork, fixed to the upright with a suitable connector.
• Foot points: base plates fixed with ground pins, dowels or anchor bolts, or ballast.
• Outriggers on mobile towers: the stabilisers specified by the tower manufacturer under EN 1004 do a similar job for freestanding towers.

The word "post" appears elsewhere too – guard rail posts or fan posts are plug-in uprights, not structural props.

Propping a scaffold

1. Get the design done: a buttress replacing ties needs a calculation of prop forces, angles and ground.
2. Check the bearing capacity of the ground and prepare the foot, on soft ground with a load-spreading sole board.
3. Connect to the scaffold as close as possible to a node or ledger, so the upright is not loaded in bending.
4. Fix the prop and set the required angle – props that are too steep achieve little.
5. Secure the foot and tighten the couplers to the specified torque, commonly 50 Nm.
6. Brace the props to each other horizontally and vertically where needed.
7. Mark the props as an obstacle.

Buttresses grow with the scaffold and only come out when no lift above depends on them.

What to watch

• Ground: the best connection is worthless if the foot slides in softened soil.
• Extension: never pull telescopic props past the marked limit.
• Impact: props reach into traffic areas – fence them off or mark them clearly.
• No guesswork: a buttress "instead of ties" without calculation is one of the riskiest improvisations on a scaffold.
• Used: tubes straight, without dents or rust; couplers with free-running bolts; screw props with clean threads; telescopic pins and holes without play.

Where possible, ties are simpler – they need no space in front of the scaffold and no firm ground. Buttresses come into their own where the facade is off limits or missing, but on tall scaffolds they reach several metres outwards and belong in the site plan from the start. Listed facades are discussed in the guide on period and listed buildings.