How to Level a Concrete Slab with a Vibrating Screed (Step by Step)

What a vibrating screed does

Once concrete has been placed and compacted in depth with a poker vibrator, the top of the slab still needs to be compacted and levelled. That is the job of a vibrating screed. A vibrating steel or aluminium beam is drawn across the surface while it vibrates; the vibration brings the cement paste up to the top, closes the surface, and works the slab to a flat, even plane in a single pass. In other words, a poker consolidates the depth of the pour and the screed finishes the surface — the two tools work together on a floor. At Lievers we've designed and built concrete compaction and finishing machinery in Mijdrecht, the Netherlands, since 1954, and the method below is how we recommend levelling a slab.

Setting up and setting the level

Before you start, decide what defines your finished level. On a form-to-form pour the side forms or screed rails are set to the finished height and the beam simply rides on top of them. On an open floor without rails, the finished level is usually set with a laser and checked as you work. Fit a beam width that suits the pour and the formwork, spray the beam with a form-release agent so paste doesn't stick, and place the machine on the fresh concrete close to where you'll begin. On a battery finishing screed such as the Lievers K150Li — a rail-free 1.5 m beam that a single operator can carry and run cordless — you start the vibration at its slowest setting and bring it up gradually until the blade begins to glide on its own.

The technique, step by step

1. Rest the beam on the wet concrete. Set the screed down flat on the freshly placed, poker-compacted concrete at the edge of the pour. Don't let it sink — get it moving as soon as the vibration starts.
2. Start the vibration. Bring the vibration up until the beam settles and starts to float on the surface rather than digging in.
3. Draw it steadily across. Pull or walk the screed backwards across the slab at an even, unhurried pace. Move slower for stiff, low-slump mixes and a little faster for wetter mixes.
4. Keep a roll of concrete ahead of the blade. Maintain a small, even roll of concrete — roughly 20–30 mm — rolling along the leading edge. That roll fills every low spot as the beam passes; too little leaves hollows, too much smothers the vibration and hurts the finish.
5. Overlap each pass. Lap adjacent passes by about 15 cm so no untouched strip is left between them and the surface stays flat across the joins.
6. Check and repeat if needed. If a strip isn't flat and closed enough after one pass, run the beam back over the same area. A correctly set screed leaves a level, tight, slightly sheened surface ready for floating or trowelling.

Rail screeds vs rail-free and handheld screeds

There are two broad ways to guide the beam. A rail (form-to-form) screed spans between fixed forms or screed rails and rides on them, which gives a very repeatable level and suits large, open slabs — at the cost of setting and later pulling those rails. A rail-free or handheld screed rests directly on the concrete and is guided by the operator, so there are no rails to set, strike or work around. Lightweight cordless finishing screeds like the Lievers HAL200Li and K150Li are rail-free by design: the 1.5 m beam floats on the surface, its rounded, upturned edges avoid ridges and start/stop marks, and one person can finish an open pour without any guide rails at all. Rail-free screeds give flexibility and speed on floors; rail screeds give the tightest tolerance on big, form-defined pours.

Levelling wide pours

When a slab is wider than a single handheld beam, split the floor into screeding lanes and finish them one after another, overlapping each lane by roughly 15 cm to keep the surface continuous. For genuinely wide bays, a longer vibrating beam or a truss screed spans the full width in one pass; these ride on rails or edge forms and are pulled across the whole lane at once, which keeps large floors flat with far fewer joins. On very wide beams the vibration has to be matched to the extra length so the whole blade works evenly. Whichever you use, an extra worker with a shovel to feed the roll of concrete and clear excess behind the beam keeps a wide pour moving.

When to screed

Timing makes or breaks the finish. Screed right after the concrete has been placed and internally compacted with a poker, while the mix is still fresh and plastic and the vibration can still draw cement paste to the surface. Crucially, screed before bleed water rises: working the surface once water has bled to the top drags that weak, watery layer into the finish and leaves a soft, dusty slab. As a rule of thumb, for slabs thicker than about 10 cm compact with a poker first, then screed straight away — place, poker, screed, in a steady rhythm across the pour.