Updated 2 Nov 2026
Under the Saudi Aramco Construction Safety Manual figures in the AL-YASSIN question bank, every scaffold post needs a base plate of at least 150 by 150 mm and 6 mm thick, on a timber sill at least 225 mm wide and 38 mm thick under at least two posts, on sound, compacted, level ground.
The figures
| Item | Value | Source |
|---|---|---|
| Base plate under every post | at least 150 × 150 mm, 6 mm thick | Saudi Aramco CSM II-2 §2.5.2, as cited in the AL-YASSIN question bank |
| Timber sill | at least 225 mm wide, 38 mm thick, under at least 2 posts | same |
| Ground | sound, compacted, level | same |
| Screwjack extension | maximum two thirds of the threaded length; level with the jack, never bricks or packing | same |
Verify every value against the latest edition of the Construction Safety Manual.
The OSHA principle
OSHA's letter of 1 August 2000 explains that an adequate foundation keeps a scaffold from settling, that mud sills spread the load over a larger area, and that concrete is the only material it treats as firm without mud sills. The letter gives no sizes, so use the owner's or designer's values for soft ground.
Soft sand and made ground in the Gulf
No source read for this page gives a bearing capacity or a sole board size for sand. For sand, fill or trench backfill, ask the designer for the ground bearing assumption and the board size: confirm with your client or the site HSE team.
Checks before the first lift
- Walk the footprint: is the ground compacted, level and drained?
- Check each post sits on a base plate, on a sill that spans at least two posts.
- Check sill dimensions with a tape, not by eye.
- Check jacks are not extended past two thirds of the thread.
- Re-check after rain, washouts or nearby excavation: OSHJ-CoP-23 lists excavation near the base as an inspection trigger.
Common mistakes on site
- Bricks or loose packing under a base plate to level it.
- A sill under only one post, so the board tilts.
- Plates placed on fresh backfill or a pipe trench.
- Jacks wound to the end of the thread to avoid digging out.
Questions
What size base plate does Saudi Aramco require?
At least 150 × 150 mm and 6 mm thick, per the CSM figures cited in the AL-YASSIN question bank. Verify against the latest edition.
How wide must a timber sole board be?
At least 225 mm wide and 38 mm thick, under at least two posts, in the same source.
Is concrete needed under a scaffold?
OSHA treats concrete as the only material that is firm without mud sills; other ground needs sills or a design.
Can I level with bricks?
No. Level with the jack, never with bricks or packing.
- AL-YASSIN page /interview-questions/scaffold-supervisor/ Q1, Q3, Q4 (own data, read 2026-11-02)
- OSHA standard interpretations (read 2026-11-02)
- Sharjah Prevention and Safety Authority, OSHJ-CoP-23 Scaffolding Code of Practice, v2 rev 0, June/July 2024 (read 2026-11-02)
General guidance for learning, not legal or engineering advice. Verify every value against the latest edition and your client's own rules. AL-YASSIN is independent and is not endorsed or approved by Saudi Aramco, OSHA, BSI, NASC, CISRS, CSCS or any other body.