What Screeding and Bull Floating Actually Do
Screeding is the process of striking off freshly placed concrete to the correct elevation using the form edges (or set screed rails) as guides. The goal is a flat plane (or a consistent slope) at the right height, with no humps, birdbaths, or dips.
Bull floating happens immediately after screeding. It removes screed marks and ridges, pushes down coarse aggregate just under the surface, and brings up enough paste to fill small voids and create a uniform surface ready for edging/jointing and later finishing steps.
Think of screeding as getting the concrete to grade, and bull floating as making that graded surface continuous and workable—without adding water or overworking.
Screeding Fundamentals
Use the Form Edges as Your Grade Guides
For small slabs and walkways, the top of the forms is your reference plane. If the forms are set correctly, screeding is simply cutting the concrete down to match them.
- Keep the screed board riding on both form edges whenever possible. If one end drops inside the form, you’ll cut a low spot.
- Don’t “chase” the surface visually alone. Trust the guides (forms/rails) and correct the concrete to match them.
- Maintain slope by staying locked to the guides. If the forms establish a slope (for drainage), the screed board automatically transfers that slope—unless you lift one end or dig in.
Screed Board Selection (What Works and Why)
Your screed needs to be straight, stiff, and long enough to span the guides.
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- Narrow walkways (2–4 ft wide): a straight 2x4 or 2x6 works well. A 2x4 is lighter; a 2x6 is stiffer for longer spans.
- Larger pads: consider a longer, stiffer screed (straight 2x6, laminated wood screed, magnesium straightedge, or aluminum box screed). The longer the span, the more stiffness matters.
- Check straightness: sight down the board or place it on a known flat surface. A crowned or twisted board will create waves.
- Edge condition: a slightly eased edge glides; a damaged edge can gouge and leave deep lines.
Body Position and the Sawing Motion
Screeding is not a single pull. It’s a controlled pull + saw action that cuts high spots and rolls excess concrete forward.
- Set the screed on the forms with both ends supported.
- Pull toward you while moving the board side-to-side 2–6 inches (a gentle sawing motion).
- Keep a slight forward tilt (top edge tipped slightly toward the direction you’re moving). This helps the board cut rather than plow.
- Let excess concrete build a small “roll” in front of the screed. That roll helps fill minor lows as you move.
- Advance in short sections (often 2–4 ft at a time), especially if you’re working alone or the concrete is stiffening.
Key feel: If the screed chatters and leaves deep grooves, the mix may be too stiff or you’re pulling too aggressively. If it feels like it’s hydroplaning and the surface turns soupy, the mix may be too wet or you’re overworking.
Filling Low Spots Without Adding Surface Water
Low spots show up as areas the screed doesn’t touch, leaving a hollow or a “shadow” below the plane. Fix them immediately—before the concrete tightens up.
- Identify the low as soon as the screed passes.
- Add concrete (not water) into the low area using a shovel or come-along.
- Re-screed that section with the board riding the forms again.
- Repeat as needed until the screed consistently contacts the surface across the width.
Do not sprinkle water on top to “help it close.” Surface water raises the local water-cement ratio, which can weaken the top layer and increase dusting, scaling, or flaking later. If you need lubrication for tools, lightly dampen the tool—not the slab surface.
Reading the Surface: Highs, Lows, and Immediate Corrections
How to Spot High Areas
- Screed drags hard or stops: you’re hitting a high spot or a pile of coarse aggregate.
- Ridges parallel to the forms: often from lifting one end of the screed or from uneven pressure.
- Concrete “roll” becomes too large: you’re pushing excess forward instead of cutting it down.
Correction: Increase the sawing motion slightly, keep the board flatter on the forms, and shave the high down. If the high is a localized mound, pull some material away and re-screed.
How to Spot Low Areas
- Hollows behind the screed: the board passes and leaves a depression or exposed aggregate with little paste.
- “Shadows” or dull patches: areas that look lower and less worked compared to surrounding concrete.
- Water/paste pooling later: a sign of a birdbath (low spot) that wasn’t corrected early.
Correction: Add concrete, then re-screed. Don’t try to bull float a true low spot out of existence; bull floating can smooth texture, but it won’t reliably fix elevation errors.
Managing Concrete That Is Stiffening
As concrete begins to set, screeding becomes harder and corrections become less forgiving. Your strategy should change from “perfecting” to “getting it right quickly.”
- Work in smaller bays: place and screed in manageable sections so you’re not trying to correct a large area that has already tightened.
- Reduce reworking: repeated passes can tear the surface and bring up excess paste.
- Prioritize grade and slope first: fix obvious highs/lows immediately; cosmetic smoothing comes later with the bull float (at the right time).
If the concrete is stiffening and you still have lows, it’s better to add concrete and re-screed promptly than to accept a birdbath you’ll fight forever.
Bull Floating: Timing, Purpose, and Technique
When to Start Bull Floating
Bull float right after screeding, while the surface is still plastic enough to respond, but not so wet that the float sinks and churns water to the top.
Practical timing cues:
- Start when screed marks are visible and the surface can be smoothed with light pressure.
- Delay slightly if bleed water is already pooling. Bull floating through standing water can trap water under a paste layer and weaken the surface.
- If the float leaves deep tracks and the surface pumps, it’s too wet or you’re pushing too hard—wait a bit or lighten pressure.
What Bull Floating Accomplishes
- Embeds aggregate slightly so stones aren’t sticking up and tearing later tools.
- Levels ridges and fills shallow voids left by the screed board.
- Brings paste to the surface just enough to create a uniform, workable layer for edging and jointing.
Done correctly, bull floating improves flatness and surface continuity without changing the slab’s intended slope.
How to Bull Float Without Overworking
Overworking is one of the most common finishing mistakes. Too many passes (especially while the surface is wet) can trap water and air, create a weak, paste-rich top layer, and lead to dusting or scaling.
Technique:
- Keep the leading edge slightly up (tilt the handle so the float doesn’t dig in). You want it to glide, not plow.
- Push and pull in long strokes, overlapping each pass by a few inches.
- Use minimal pressure. Let the tool do the work; heavy pressure brings up too much paste.
- Limit passes: typically 1–2 passes in each direction is enough for small work. Add a third pass only where needed to remove ridges.
- Watch the surface response: if paste is building up and looking “creamy” or water is being worked in, stop and let the slab tighten.
Rule of thumb: bull float to remove screed marks and seat aggregate, not to create the final finish.
Techniques by Project Type
Narrow Walkways (2–4 ft Wide)
Walkways are often easiest because the forms are close together and act as reliable guides.
- Screeding: Use a straight 2x4/2x6 spanning both forms. Work in short pulls so the board stays stable on the edges.
- Controlling elevation: Keep both ends on the forms at all times; even a small drop on one side shows up as a dip.
- Bull floating: A standard bull float works, but a magnesium hand float can be useful along edges or tight spots where the bull float head can’t reach cleanly.
- Maintain slope: Don’t “correct” the slope out of the slab by eye. If the forms are sloped, your job is to match them consistently.
Larger Pads (Sheds, Patios, Equipment Pads)
Larger pads demand a plan to keep the surface plane consistent across a wider area.
- Screeding: Use a stiffer, longer screed. Consider screeding in lanes: strike off one strip, then the next, keeping the roll of concrete consistent.
- Check for waves: After the first pass, look across the surface at a low angle. Ridges show up quickly in raking light.
- Bull floating pattern: Bull float perpendicular to your screed direction first to knock down ridges, then make a second set of passes parallel to blend.
- Edge control: Don’t let the bull float dip near edges; keep the head flat and the leading edge up to avoid creating a low “gutter” along the form.
Maintaining Slope While Achieving a Smooth Plane
The goal is not “level” unless the design calls for it. The goal is a consistent plane that follows the intended slope.
- Let the forms define the slope and keep the screed riding them.
- Avoid localized overfilling that forces you to dig down later (digging often creates low spots).
- Correct immediately: a small hump can be shaved during screeding; after the slab tightens, that hump becomes much harder to remove without damaging the surface.
- Use the bull float to blend, not reshape. If you try to “float in” slope changes, you’ll usually just create waves.
Troubleshooting During Screeding and Bull Floating
If the Concrete Is Too Wet
Symptoms: The screed hydroplanes; the surface turns soupy; aggregate sinks; bull float leaves deep swirls; bleed water appears quickly and in large amounts.
What to do:
- Stop adding water (especially on the surface). Don’t sprinkle to “help it finish.”
- Use lighter tool pressure and fewer passes. Overworking wet concrete is especially damaging.
- Wait for it to tighten before additional floating. If water is standing, pause and let it evaporate or be reabsorbed naturally.
- Remove only what you must during screeding—avoid repeatedly churning the top.
If the Concrete Is Too Dry (Too Stiff to Screed Cleanly)
Symptoms: Screed chatters; surface tears; voids open behind the screed; it’s hard to pull a roll of concrete; bull float skips and leaves rough texture.
What to do:
- Work smaller sections so you can strike off and float before it tightens further.
- Add concrete to lows immediately and re-screed—don’t try to “massage” stiff concrete into place with the bull float.
- Use a more aggressive sawing motion with steady pull to cut highs without gouging.
- Lightly dampen tools (not the slab) if sticking is a problem.
If It’s Setting Too Fast (Heat, Wind, Sun, or Rapid Mix Behavior)
Symptoms: Edges stiffen while the middle is still workable; screeding becomes difficult within minutes; bull floating stops improving the surface; the top starts to crust.
What to do during screeding:
- Prioritize grade first: get the slab struck off correctly even if the surface isn’t pretty yet.
- Reduce the size of your working area and keep a steady pace—avoid long delays between placement and screeding.
- Minimize rework: repeated passes waste time and can tear a crusting surface.
What to do during bull floating:
- Float sooner, but gently—just enough to knock down ridges before the surface locks up.
- If the surface is crusting and the float is dragging, stop forcing it; forcing can delaminate the surface layer.
Quick Diagnostic Table
| Problem | What you see/feel | Best immediate move | What to avoid |
|---|---|---|---|
| Low spots | Hollows behind screed, later puddles | Add concrete, re-screed on forms | Trying to float it flat without adding mix |
| High spots/ridges | Screed drags, ridges parallel to forms | Saw and shave during screed; re-screed | Leaving it for later (harder to fix) |
| Too wet | Soupy surface, deep float tracks, fast bleed | Pause/let tighten; fewer, lighter passes | Adding surface water; overworking |
| Too dry | Tearing, chatter, open voids | Smaller sections; add concrete to lows | Sprinkling water to “fix” workability |
| Setting too fast | Crusting, sudden loss of workability | Prioritize grade; gentle early float | Forcing tools; repeated passes |
