Scaffolding base jacks are the small but critical components that sit at the base of nearly every scaffold. They connect the load-bearing standard to the ground, level the structure on uneven sites, and prevent the whole tower from sinking into soft ground. Choose the wrong size, and you risk instability, overload, or a scaffold that simply cannot be erected to the right working height. This guide explains the sizes that matter and how to match a scaffolding base jack size to your project’s real needs.
What defines a base jack’s size?
A base jack is essentially a screw rod fitted into a coupling nut, welded to a flat steel plate at the bottom. Its “size” is not a single number but a combination of three key dimensions you need to check before ordering:
- Screw (rod) diameter — commonly 30 mm, 32 mm, 34 mm, or 38 mm for standard systems. This must match the inside diameter of the standard or ledger tube you are supporting.
- Base plate size — usually 120 mm × 120 mm or 150 mm × 150 mm, with a plate thickness of 5–8 mm. The plate spreads the load onto the ground.
- Total length — typical options range from 450 mm up to 600 mm, 750 mm, or more. This controls how much vertical adjustment your scaffold can make.
Match the screw diameter to your tube size
Before anything else, confirm the internal diameter of the tubes your system uses. A ringlock standard, a frame upright, or a tube-and-clamp pipe each expect a specific rod diameter to sit tightly inside. Using a rod that is too thin leaves slack that weakens the connection; a rod that is too thick will not fit at all. Most modular systems work with 32 mm or 34 mm rods, while heavier-duty applications call for 38 mm. When in doubt, send your existing tube samples or drawings to your supplier so the rod can be matched precisely.
Solid or hollow core carrying the load
Base jacks come in two core forms, and the choice depends on how much weight the jack must carry. A solid base jack uses a solid round steel rod, which delivers the highest load capacity and is the safe default for heavy shoring, slab formwork, and high-load ringlock or frame systems. A hollow base jack instead uses a thick-wall steel pipe, which is noticeably lighter and easier to handle on large jobs with hundreds of jacks, while still offering reliable support for standard access scaffolds. For light maintenance and facade work, hollow jacks cut handling fatigue; for heavy-duty shoring, solid jacks give you extra peace of mind. Both are manufactured from Q235 or Q355 steel in quality production facilities, so the deciding factor is your working load and how much lifting you want your crew to do.
Base plate size depends on your ground
A wider base plate spreads the vertical load over a larger area, which matters most on soft, sandy, or loose soil. On firm concrete or compacted ground, a 120 mm plate is usually enough. On soft ground, step up to a 150 mm plate — or even combine the jack with timber sole boards — to stop the scaffold from sinking and to keep the load distributed evenly. If your project site has poor bearing capacity, always discuss the plate option with your supplier rather than assuming one plate fits every condition.
Total length and the two-thirds rule
Total length sets how much the screw can be raised or lowered. A longer jack gives more adjustment range, which helps on sloping or stepped ground. However, experience and standard practice recommend that the threaded portion exposed in use should not exceed roughly two-thirds of the jack’s total length. Extending too far leaves too little thread engaged in the nut, which can cause the screw to buckle under load. So when you need a wide range of heights, it is safer to choose a longer overall jack than to over-extend a short one.
Check compatibility with your scaffold system
Base jacks sit directly under the standards of nearly every system — ringlock, cuplock, frame, kwikstage, and tube-and-clamp. The rod diameter and the coupling fitting at the top have to line up with the base connection of your chosen system. Ringlock towers, for example, are typically capped with a rosette connection, so the jack must present a compatible spigot or collar. Tell your supplier which system you run and the exact tube sizes you use, so the jack you buy connects cleanly with no field modification.
Surface treatment for your environment
Corrosion resistance is a real part of choosing the right jack. For most civil and building work, hot-dip galvanized coating gives the longest service life and is the common choice for jacks left out on site. Electro-galvanized or painted finishes are lighter and more economical for short-term rentals or indoor use. If your project is coastal, humid, or involves long storage periods, invest in a more corrosion-resistant finish so your jacks can be reused safely across many cycles.
Quick quality checks before you order
- Welds: the weld between base plate and screw must be fully penetrated, with no pores or cracks.
- Thread: the nut should turn and tighten smoothly along the full thread without jamming.
- Documents: request a mill certificate and a load test report so the claimed capacity is backed up by real testing.
Wrapping up: your size checklist
To pick the right base jack size in minutes, work through this checklist: confirm your tube’s internal diameter and pick the matching screw rod; decide solid or hollow based on your working load; choose a base plate that suits your ground conditions; select a total length that gives the adjustment you need while respecting the two-thirds rule; match the fitting to your scaffold system; and choose a galvanizing finish that suits your site environment. Clear every one of these points before ordering and you will avoid re-orders, delays, and on-site safety issues.
Getting the sizing right is the fastest way to keep crews safe and schedules on track. Bythai Scaffolding supplies a complete range of adjustable base jacks in both solid and hollow forms, manufactured to EN and BS standards with ISO 9001 quality control and flexible MOQ. If you can send your tube sizes and site conditions, the Bythai team will help you match the correct base jack specifications and quote a fair price — just reach out through the contact page.