Parts of a Scaffold – Labeled Frame Scaffold Diagram

All 13 parts on the diagram

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On the ground 3

Mud sills (sole boards) A plank on the ground under the legs. It gives the scaffold a firm footing, above all on soft ground. Also called a sole board.
Base plates The flat plate that each leg stands on. It sits on the mud sill.
Screw jacks A threaded leg between the base plate and the frame. It is screwed up or down to level the scaffold.

Frames, deck and access 5

End frames A welded frame of two legs joined by crosspieces. One stands at each end of a bay, and frames stack on top of each other to go higher.
Cross braces Diagonal bars that join one end frame to the next. They hold the legs steady sideways.
Platform (planks) The raised work surface. It is made of wood planks or of factory-made planks and decks laid across the frames.
Access ladder A ladder made to hook onto the scaffold. It is for climbing up to the platform. The cross braces are not for climbing.
Ties They fasten the scaffold to the building. A tall scaffold then cannot tip away from it.

The rolling tower 5

Casters A swivel wheel under each leg of a mobile scaffold. Each has a lock that is set while the scaffold is in use.
Outriggers Members that widen the base of the scaffold, which makes it steadier.
Guardrail (top rail) The top rail of the barrier round the platform's open sides. With the midrail and the posts it makes up the guardrail system.
Midrail The rail about halfway between the top rail and the platform.
Toeboard A low barrier along the edge of the platform. It keeps tools and other small things from being kicked off onto people below.

How this list was compiled: Part names checked on 2026-09-30, a name kept only when two independent sources use it. (1) US Occupational Safety and Health Administration, Safety Standards for Scaffolds Used in the Construction Industry, 29 CFR 1926.450 (definitions), 1926.451 and 1926.452, read at https://www.law.cornell.edu/cfr/text/29/1926.450 , https://www.law.cornell.edu/cfr/text/29/1926.451 and https://www.law.cornell.edu/cfr/text/29/1926.452 , with OSHA's Scaffolding eTool page on frame scaffolds, https://www.osha.gov/etools/scaffolding/supported/frame (base plates and mud sills, screw jacks and adjustable screw legs, fabricated end frames, cross braces, coupling or stacking pins, platform of planks or decks, guardrail system of toprails, midrails and posts, toeboards, hook-on and attachable ladders, guys, ties and braces, casters, outrigger). (2) Wikipedia, Scaffolding (base plate or base jack, sole boards, cross braces, boards of the working platform, guard rails and toe boards, ladders, scaffold ties, outriggers on rolling sections). (3) A maker's manual for a portable rolling scaffold, full text at https://archive.org/details/manualzilla-id-7036626 (end frame, platform, swivel casters, guard rails, mid rails and toe boards, ties). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's system, nothing traced. Licence: own work

How it works

The parts of a scaffold are named here from the ground up, on a frame scaffold standing against a wall and on a rolling tower. The names are the ones the US safety standard uses.

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From the ground up

Each leg stands on a base plate on a mud sill, with a screw jack to level it. The end frames carry the platform, and cross braces join one frame to the next. Ties hold the scaffold to the wall, and a ladder gives the way up. On the rolling tower, find the casters and outriggers, then the guardrail, midrail and toeboard round the platform.

Top rail, midrail and posts together are called the guardrail system.
This page names the parts. Putting up a scaffold is a trained job.

Questions and answers

What are the parts of a scaffold called?

From the ground up: mud sills, base plates and screw jacks; end frames joined by cross braces; a platform of planks; a guardrail, midrail and toeboard round it; ties to the building; and a ladder for access. A rolling scaffold adds casters and outriggers.

What is a mud sill on a scaffold?

A plank laid on the ground under the legs. The base plates stand on it, and it gives the scaffold a firm footing on soft ground. It is also called a sole board.

What is the difference between a guardrail, a midrail and a toeboard?

All three run round the open sides of the platform. The guardrail's top rail is the high bar, the midrail sits about halfway down, and the toeboard is the low board at the platform's edge that keeps small things from falling off.

What do outriggers do on a scaffold?

They make the base wider. An outrigger is a member that reaches out from the scaffold to the ground, which gives the scaffold more support and makes it steadier.