The base jack is the first piece of scaffolding to touch the ground, and its size decides more than people expect. The spindle diameter has to match the standard it sits inside, the base plate has to spread the load across the ground, and the thread and height range determine how much leveling you can actually do on site. Get the size right and the whole scaffold builds square and stable; get it wrong and you are fighting compatibility problems from the first lift. This guide explains the standard scaffolding base jack size for the common systems, so you can match the right jack to the right scaffold.
What Is a Scaffolding Base Jack?
A base jack is a screw jack with a flat base plate at the bottom. It sits under the bottom standard of a scaffold and does three jobs at once: it levels the scaffold on uneven ground, it spreads the weight of the structure across the ground so the legs do not sink, and it transfers the load from the standard into the foundation. Every scaffold, whether it is ringlock, frame, or tube and clamp, starts on a set of base jacks.
A base jack is made of three parts. The threaded spindle, or screw rod, is the long threaded stem that slides inside the hollow bottom of a standard. The base plate is the flat square plate welded to the bottom of the spindle that stands on the ground or on a sole board. The jack nut, sometimes called the handle, is the wing nut threaded onto the spindle; turning it raises or lowers the standard above it. Turn the nut clockwise to raise the scaffold leg and counter-clockwise to lower it, and once the scaffold is level, the nut locks the adjustment in place.
Why the Size of a Base Jack Matters
The size of a base jack is not a style choice; it is a compatibility and safety decision. The spindle outside diameter must fit the inside of the standard it goes into. A 38 mm spindle is the common size for most system scaffolding because the standard tubes are made to accept it, while frame scaffolding uses smaller solid spindles in the 26 to 34 mm range. If the spindle is too small for the standard, the leg rocks inside the tube; if it is too large, it will not fit at all.
Size also controls load capacity and adjustment range. A thicker spindle and a thicker base plate carry more load, which is why heavy-duty shoring uses 48 mm spindles with thicker plates. The length of the spindle sets how much height the jack can adjust, and the safe extension limit matters just as much as the overall length, because winding a spindle out to its last thread turns the jack into a lever arm and is unsafe. Finally, the base plate area decides how the load is spread on the ground, so a bigger plate is needed on soft ground to stop the leg sinking.
Standard Base Jack Sizes by Scaffolding System
There is no single universal base jack size, but each scaffolding system has a well-established standard. The table below summarizes the common sizes for the main systems.
| Scaffolding System | Spindle Diameter | Type | Typical Height | Common Base Plate |
|---|---|---|---|---|
| Ringlock system | 38 mm | Hollow | 600–800 mm | 150 × 150 mm |
| Frame scaffolding | 26–34 mm | Solid | 600 mm | 120 × 120 to 150 × 150 mm |
| Tube & clamps | 38 mm | Hollow | 600 mm | 150 × 120 mm |
| Cuplock system | 38 mm | Hollow | 600–760 mm | 150 × 150 mm |
| Shoring / props | 48 mm | Hollow | 600–1200 mm | 150 × 150 mm or larger |
The 38 mm hollow base jack is the workhorse of the industry because it fits the 48.3 mm standard tubes used across ringlock, cuplock, and most modular systems. If you are buying for a single system, confirm the spindle diameter against your standard’s internal bore before ordering, because a few millimetres make the difference between a solid seat and a wobbly leg.
Solid vs. Hollow Base Jacks
Base jacks come in two spindle types, and the choice is driven by the scaffold system and the load. A solid base jack uses a solid threaded bar for the spindle. It is the standard choice for frame scaffolding, where the smaller spindles in the 26 to 34 mm range carry the load directly, and it is generally the stronger option for heavy, concentrated loads on firm ground. A hollow base jack uses a threaded tube instead of a solid bar. It is lighter for the same diameter, which makes it easier to handle and cheaper to ship, and it is the standard choice for ringlock, cuplock, and other system scaffolding where the 38 mm hollow spindle is the norm.
As a rule of thumb, match the jack type to the system: solid spindles for frame scaffolding, hollow spindles for modular system scaffolding. If you are not sure which one your scaffold needs, check the bottom standard, because the internal bore of the standard is designed for one or the other. Bythai supplies both an adjustable solid base jack in 30, 32, and 34 mm spindles and an adjustable hollow base jack in 38 mm, so the right size is available whichever system you run.
Base Plate Size and Thickness
The base plate is what actually stands on the ground, so its size and thickness are part of the standard size too. Common plate sizes run from 120 × 120 mm up to 180 × 180 mm, with thicknesses from 4 mm to 8 mm. A 120 × 120 mm plate with a 4 mm thickness is typical for lighter frame scaffolding, while a 150 × 150 mm plate with a 5 mm or 6 mm thickness is the common choice for system scaffolding, and heavy-duty shoring uses the largest plates with the thickest steel.
The plate area matters most on soft or uneven ground, because a larger plate spreads the point load of the leg over more ground and stops it sinking. On tested concrete or asphalt a smaller plate is usually fine; on soil, fill, or anything that might soften after rain, use the largest plate the system allows and put a sole board under it. Bythai’s solid base jacks come with 120 × 120 × 4 mm and 150 × 150 × 5 mm plates, and the hollow base jack with a 115 × 110 × 5 mm plate, all hot-dip or electro-galvanized for corrosion resistance.
Thread Types: 4 TPI and Coarse Thread
The thread on the spindle is another part of the size specification, because it controls how fast the jack adjusts and how well it holds under load. The two common options are 4 TPI and coarse thread. TPI stands for threads per inch, and a 4 TPI thread has four thread peaks in every inch of spindle, which works out to a pitch of about 6.35 mm. It is the traditional standard for base jacks and gives fine, precise adjustment.
A coarse thread has a larger pitch, typically around 8 mm, so the nut travels further with each turn. That makes it faster to wind a scaffold up to height, which is why coarse-thread jacks are often called quick-thread jacks, and the deeper thread also tends to hold better under heavy load and resist stripping. If you are leveling a large scaffold where speed matters, coarse thread is worth asking for; if you want fine control on a smaller job, 4 TPI is the dependable default.
How to Choose the Right Base Jack Size
- Match the spindle to the standard. Confirm the spindle outside diameter fits the internal bore of your bottom standard. 38 mm for most system scaffolding, 26 to 34 mm solid for frame scaffolding, 48 mm for heavy-duty shoring.
- Choose the right type. Hollow spindles for ringlock, cuplock, and modular systems; solid spindles for frame scaffolding and concentrated heavy loads.
- Size the base plate for the ground. Use a larger plate on soft or uneven ground to spread the load, and always pair the plate with a sole board unless you are on tested concrete or asphalt.
- Check the height and extension range. Pick a spindle length that covers the height difference across your footprint, and never wind a jack out to its last thread to chase a level.
- Match the thread to the job. Coarse thread for fast leveling on large scaffolds, 4 TPI for fine adjustment on smaller work.
- Confirm the finish. Hot-dip galvanized jacks last far longer on damp or coastal sites than painted ones, so the surface treatment is part of the specification too.
If you are buying for a mixed fleet, it is worth standardizing on the 38 mm hollow base jack, because it covers ringlock, cuplock, and tube and clamp systems, and keeping one common size simplifies spares and reduces the risk of grabbing the wrong jack on site. For frame scaffolding, keep the matching solid jacks separate and clearly marked.
Frequently Asked Questions
What is the most common scaffolding base jack size? The 38 mm hollow base jack is the most common size in the industry. It fits the 48.3 mm standard tubes used across ringlock, cuplock, and most modular systems, with a typical height of 600 to 800 mm and a 150 × 150 mm base plate.
What is the difference between a solid and a hollow base jack? A solid base jack uses a solid threaded bar and is the standard for frame scaffolding, with spindles in the 26 to 34 mm range. A hollow base jack uses a threaded tube, is lighter for the same diameter, and is the standard for ringlock, cuplock, and other system scaffolding in 38 mm.
What base plate size do I need? It depends on the system and the ground. Common sizes run from 120 × 120 mm up to 180 × 180 mm, with thicknesses from 4 to 8 mm. Use a larger plate on soft ground to spread the load and prevent the leg sinking, and put a sole board underneath unless you are on tested concrete or asphalt.
Can I use the same base jack for different scaffolding systems? Often yes, if the spindle diameter matches. The 38 mm hollow base jack fits ringlock, cuplock, and tube and clamp systems, which is why it is the standard to standardize on. Frame scaffolding uses smaller solid spindles, so those jacks are not interchangeable with system jacks.
What is a u head jack? A u head jack is the top-end version of the same screw jack, with a U-shaped head instead of a flat base plate. It is used under beams, formwork, and ledgers to support them from below, and it is common in tube and clamp and shoring work. Bythai supplies an adjustable u head jack with a 38 × 4 × 600 mm spindle for tube and clamp systems.
Conclusion
The standard scaffolding base jack size is not one number but a set of matching specifications: spindle diameter, type, base plate, thread, and height. The 38 mm hollow base jack covers most system scaffolding, solid 26 to 34 mm jacks cover frame scaffolding, and 48 mm jacks handle heavy-duty shoring. Match the jack to the standard, size the plate for the ground, and never over-extend the spindle, and the base of your scaffold will be as solid as the rest of it. If you are planning an order and want to confirm the right sizes for your systems, Bythai Scaffolding supplies adjustable solid and hollow base jacks, u head jacks, and the full range of base jack and u head jack accessories, manufactured to EN and BS standards with galvanized finishes. Contact Bythai for a quote and let them help you pick the right base for the job.