How to Screed Concrete Accurately to Grade

Screeding—also called strikeoff—removes excess fresh concrete and establishes the slab’s initial elevation and shape. The correct method is to set reliable grade references, choose a screed that suits the span and placement rate, maintain a manageable roll of concrete ahead of it, and stop before vibration or repeated passes bring excessive mortar to the surface.

Choose the placement type and slump condition to identify the appropriate screed and its main limitation.

Strikeoff is not the final finish. The American Cement Association distinguishes the operations: strikeoff brings the surface to grade, while bull floating immediately afterward removes smaller highs and lows and embeds large aggregate near the surface (American Cement Association). Depending on the slab specification, later work may include edging, jointing, floating, troweling or brooming, followed by curing.

Grade References Control the Result

A screed can reproduce only the elevation and shape of the supports or guidance system it follows. Before concrete arrives, verify:

• form, rail or wet-screed elevations against the project benchmark;
• intended drainage slope and direction;
• form stiffness and stake spacing;
• slab thickness at transitions, drains and embedded items;
• screed span, bearing length and operator access;
• reinforcement and embedments, including whether workers or equipment could displace them; and
• a placement route that keeps delivery ahead of strikeoff without burying the screed in an oversized roll.

Level is not always the target. Exterior paving and washdown areas may require planned drainage, while an interior floor may have separate elevation and flatness requirements. Follow the drawings and specifications rather than assuming that a visibly smooth surface complies.

For a specified floor, establish the acceptance method before placement. A local straightedge check, an elevation survey and a formal floor-profile test answer different questions. A field check can guide the placing crew, but it does not replace the test required by the specification.

If the floor will receive a coating, the completed slab will also need the substrate checks appropriate to the epoxy flooring installation.

Match the Screed to the Placement

Screed	Typical Fit	Main Constraint
Hand straightedge	Steps, pads, narrow walks, small placements	Labor; long members can flex or twist
Lightweight vibratory screed	Sidewalks, driveways, moderate slab strips	Mixture compatibility and travel rate
Vibratory truss screed	Wide strips and larger paving placements	Setup, handling and substantial supports
Laser-guided or ride-on screed	Large floors with production and elevation demands	Access, mobilization, reach and logistics

Select the screed by clear span, slab geometry, required surface tolerance, concrete consistency, delivery rate, access and crew capacity. Slab area alone is not enough. A powered machine cannot correct weak forms, misplaced rails or a supply rate that does not match the placing crew.

Vibration can perform strikeoff and some consolidation, but it must suit the mixture and slab. NRMCA’s flatwork guidance says not to use a jitterbug or vibrating screed with concrete above a 3-inch (75 mm) slump and warns that vibration can work excessive mortar to the surface (NRMCA CIP 14). That is guidance from this document, not permission to alter an approved mixture or disregard the project specification and screed manufacturer’s instructions.

Use a Controlled Screeding Sequence

1. Make the Placement Screed-Ready

Complete subgrade preparation, formwork, reinforcement and embedments before delivery. Check a hand screed for straightness and confirm that it will maintain adequate bearing on both supports throughout the pass.

For a sectional truss screed, alignment is part of assembly—not a correction to attempt over fresh concrete. Multiquip’s current WSHE/A manual directs the crew to stretch a string line along the assembled screed, compare blade height at each splice and adjust the turnbuckles until adjoining blades are equal (Multiquip WSHE/A manual). Follow the assembly and alignment procedure for the actual model on site.

2. Deposit Concrete Near Its Final Position

Place concrete only as fast as the crew can handle it and as close as practical to its final location. The American Cement Association recommends placing each batch against previously placed concrete rather than dumping isolated piles and moving them horizontally.

Bring the concrete slightly high in front of the screed. Assign workers to feed visible low spots and remove excessive buildup. Do not add water at the surface or casually change the delivered mixture to compensate for an undermanned placement.

The delivery rate, spreading crew and screed must operate as one system. If concrete accumulates faster than the screed can advance, the roll becomes difficult to control. If delivery falls behind, the screed can starve and leave low areas.

3. Strike Off Continuously

With a hand screed, rest the straightedge on both guides, tilt it slightly forward and pull it with a short sawing motion. Maintain a small roll of concrete ahead of the board so material remains available to fill low areas. If a void opens behind it, add concrete and re-strike the area while it remains workable. This method follows NRMCA’s flatwork guidance.

With a powered unit, keep both ends advancing evenly and match travel speed to the mixture and concrete supply. For its WSHE/A truss screed, Multiquip instructs operators to turn both winches simultaneously, slow the machine if supply falls behind and prevent concrete from rising above the blade-attachment bolts. The manual also warns against over-vibration. These controls and limits are specific to that model.

Do not repeatedly churn an area because it looks wet. NRMCA identifies vibratory screeds, jitterbugs and excessive or improper floating as possible causes of a paste-rich surface and crazing when they depress coarse aggregate or overwork the concrete (NRMCA CIP 3).

4. Bull Float Promptly, Then Wait

Bull float or darby as soon as strikeoff is complete and before bleed water appears. Keep the tool relatively flat so it corrects minor irregularities without prematurely sealing the surface. Bull floating cannot repair an incorrect rail elevation or a major hollow left during strikeoff.

After this initial leveling pass, wait until bleeding has stopped and the water sheen has disappeared before later finishing. Finishing while concrete is still bleeding can contribute to dusting, scaling, crazing, delamination and blisters. NRMCA also says never to sprinkle water or dry cement on concrete during finishing.

5. Check Before Correction Becomes Impractical

Inspect directly behind the screed for:

• gaps beneath the specified straightedge;
• abrupt elevation changes at forms and construction joints;
• low areas around drains;
• displaced edge forms or rails;
• ridges between adjoining passes; and
• exposed aggregate, torn areas or excessive surface paste.

Use the test instrument, length, locations and timing required by the project. Appearance alone does not establish compliance, and a local straightedge check does not substitute for a specified survey or floor-profile test.

Surface Problems Point Back to the Process

A Ridge Follows the Pass

The screed may be flexing, one end may be leading, or adjoining runs may not overlap properly. Check the screed member and the operator’s advance before working the ridge repeatedly.

The Slab Is Uniformly Off Elevation

Recheck the forms, rails and benchmark. A straight screed following the wrong reference still produces the wrong grade.

Low Pockets Trail the Screed

The concrete roll may be too small, delivery may be inconsistent, or travel may be too fast. Feed the low area and restore a controlled roll rather than forcing the screed onward.

Excess Mortar Rises to the Surface

Check for excessive vibration, a mixture unsuitable for the method or repeated reworking. More vibration is not the correction.

The Surface Tears

Concrete may be setting ahead of the crew, the method may not suit the mixture, or material may not be feeding beneath the blade. Correct the supply or operating issue instead of making repeated dry passes.

Placement Outruns Finishing

Reduce the placement width or delivery rate, add capable labor, or use equipment that matches the placement plan. Continuing at the same rate transfers the delay to floating and finishing, when correction is more difficult.

Protect Workers From Concrete and Equipment Hazards

Fresh concrete is a chemical-burn hazard, not merely wet mud. OSHA says wet portland cement is caustic, abrasive and moisture-absorbing. A cement burn may not hurt immediately and can continue worsening after contact ends. OSHA requires appropriate PPE and accessible washing facilities for exposed workers (OSHA).

Use suitable alkali-resistant gloves, waterproof boots, long sleeves, long pants and eye protection. Promptly remove contaminated clothing and wash exposed skin with clean water and appropriate soap.

For a powered screed, address moving parts, noise, fuel, exhaust, cables and manual handling under the machine manual and site plan. Do not run a gasoline engine in an enclosed or poorly ventilated area. Inspect guards, blades, fasteners and cables before use, and keep workers clear of moving parts.

Before maintenance or adjustment, shut the machine down, allow it to cool and follow the manufacturer’s isolation procedure. The cited Multiquip WSHE/A manual specifically requires disconnecting and securing the spark-plug wire away from the plug.

Effective screeding depends on four controls: the correct elevation reference, a screed suited to the span and mixture, a manageable concrete roll, and an advance rate the whole crew can support. Once strikeoff and initial leveling are complete, unnecessary manipulation puts the surface at risk rather than improving it.