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Darby

A long, straight hand float used for initial smoothing of concrete after screeding

A darby is a long, straight hand float (typically 3-5 feet long) used for initial smoothing of concrete immediately after screeding. Made of wood, magnesium, or aluminum, the darby removes screed marks, fills low spots, and begins bringing paste to the surface before bull floating or power troweling.

Why It Matters

The darby bridges the gap between rough screeding and final finishing. After striking off concrete, ridges and valleys remain. The darby smooths these out while concrete is still quite wet, creating a more uniform surface for subsequent finishing operations. For slabs too narrow for a bull float or areas where a bull float won't reach, the darby is the primary leveling tool.

DIYers often skip this step, moving directly from screeding to bull floating. This works for very small pours, but larger slabs benefit from darby work. The few minutes spent darby-ing create a flatter, more uniform surface that requires less trowel work later. It's especially valuable when pouring solo—the darby is lighter and more maneuverable than a bull float.

Technical Details

Darby characteristics:

  • Length: 30-60 inches (shorter for small areas, longer for reach)
  • Width: 3-4 inches typical
  • Material:
    • Wood: Traditional, requires maintenance, can stick to concrete
    • Magnesium: Lightweight, won't stick, preferred for most work
    • Aluminum: Durable, slightly heavier than magnesium
  • Edge: Beveled leading edge prevents digging

Darby technique:

  1. Use immediately after screeding
  2. Hold at slight angle to leading edge
  3. Push forward with sawing motion
  4. Pull back flat to smooth
  5. Overlap strokes by half blade width
  6. Work systematically across entire surface

When to use darby vs. bull float:

  • Darby for: Narrow slabs (under 6 feet wide), areas bull float won't reach, solo work
  • Bull float for: Large open areas, faster coverage, less physical effort

Timing:

  • Use while concrete still quite wet
  • Before bleed water appears
  • If bleed water present, wait for it to evaporate before continuing
  • Typically within 15-30 minutes of screeding

The darby's primary job is removing imperfections before they set up. It doesn't create the final finish—just prepares the surface for bull floating and troweling. Think of it as rough leveling versus the precision work that follows.

For very small pours (under 50 square feet), a darby alone may provide adequate leveling. Medium slabs (50-200 square feet) benefit from darby followed by hand floating or small bull float. Large slabs need darby or bull float plus power equipment for efficient finishing.

  • Float - General term for smoothing tools darby belongs to
  • Screeding - Process done before darby use
  • Bull Float - Larger tool often used after or instead of darby

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