Base jack (adjustable base)
A threaded, height-adjustable foot at the bottom of each scaffold leg that levels the first lift and carries the leg load into the ground.
Also called: adjustable base, screw jack, base screw jack, levelling jack, jack base
No site is truly level. A pavement falls towards the road, a yard has sunken paving, a terrace has a slope. The base jack absorbs these differences. It sits in the bottom of a frame leg or in the base collar of a modular scaffold, and a nut lets you adjust its height steplessly. The first lift ends up level and everything above stands plumb.
The jack is also the point where the whole load of one leg enters the ground: decks, materials, people and the self-weight of twenty metres of scaffold all meet here. A badly set jack is therefore not a cosmetic issue but a stability risk.
Parts and variants
A base jack has three parts: a square steel base plate, often about 0.15 × 0.15 m, the threaded stem and the nut, frequently winged so it can be turned without tools. Many nuts have a lug or recess that centres the leg. Everything is normally hot-dip galvanised.
- Lengths: 0.40 m, 0.60 m and 0.80 m overall are widespread.
- Hollow or solid stems: hollow jacks are lighter, solid or reinforced jacks take higher loads.
- Swivel jacks with a hinged plate for sloping ground, see base plate, fixed and swivel.
The figure that matters most is the permitted extension, measured from the underside of the base plate to the underside of the frame. Depending on system and configuration it often lies somewhere between a little over 0.20 m and a little over 0.40 m. The exact value is in the manufacturer's instructions and applies only to the jacks described there.
Setting out
Check the ground and, where needed, lay a sole board. Start at the highest point of the area and wind the jack out only a little there, so that you keep reserve at the lowest point. Fit the first frames, connect them, then level with a spirit level: rails or ledgers horizontal, frames vertical. If the jack runs out of travel, a compensation frame takes over. With large extensions some instructions also require bracing close to the jacks.
Buying used and common faults
- Run the nut by hand along the full thread; it should turn freely without tight spots.
- Roll the jack on a flat floor: even a slight bow is a reason to reject it.
- Look for bent plates and cracked welds between plate and stem.
- Light surface rust is less critical than a bent stem.
On site, the classic errors are a plate bearing on one edge only, which bends the stem, a jack wound out beyond its limit, and threads clogged with mortar, concrete or sand. Brush the thread clean before storage. Used jacks of the common systems are listed under base jacks.