Laser screed and concrete screeding methods field guide

Screeding, and why it sets the floor before anyone finishes it

Screeding, also called strikeoff, is the step that brings fresh concrete to the right elevation and a flat plane right after it is placed and before any bull float, float, or trowel touches it. It is the first cut at flatness, and it is the one that matters most, because the floor never gets flatter than the screed leaves it. You cannot trowel a wavy screed flat. The high and low spots in the strikeoff are the high and low spots the finished floor keeps.

Two things come out of the screed: grade and flatness. Grade is whether the surface lands at the elevation the drawings call for, so the floor drains where it should and the slab is the thickness it was designed to be. Flatness is whether the surface is true over short and long distances, which is what a forklift feels and what a flooring covering needs. The screed method you pick drives both, and it drives the production rate, which is why the choice of screed is one of the first decisions on a floor pour, not an afterthought.

The finishing that follows the screed, the bull float and the wait for bleed water and the troweling, is its own discipline, covered in the flatwork finishing guide. The slab thickness, the subgrade, and the loads the floor carries are covered in the slab on grade guide. This guide is about the step in between: getting the concrete struck off flat and on grade so the finishers and the floor have something true to work from.

What does screeding do?

Screeding does three things at once: it strikes the surface off to grade, it brings the slab to a flat plane, and on the methods that vibrate, it consolidates the top of the concrete as it goes. Strike off means cutting the excess concrete down and filling the lows so the surface sits at the design elevation across the whole pour. The plane is the flatness, the trueness of that surface over distance. Consolidation is working the entrapped air out of the top and settling the paste and aggregate together so the surface is dense and workable.

The screed sets up everything after it. A surface struck off flat and consolidated gives the bull float a true plane to knock down and the finishers a sound surface to work. A surface left wavy, or left full of voids because nothing vibrated it, fights the crew the rest of the day and shows up as low spots, soft patches, and a floor that fails its flatness check no matter how hard anyone trowels.

What screeding does not do is finish the surface. The strikeoff is rough by design, a flat plane with the aggregate embedded and the cream not yet brought up. The bull float comes right behind it to flatten the ridges and set the surface for the wait. Screed, then bull float, then leave it alone until the bleed water goes. Run the screed step right and the finishing has a chance. Run it wrong and no amount of finishing recovers the floor.

The screeding methods and how they differ

Screeding runs across a spectrum, from a hand-pulled board to a self-propelled machine guided by a laser. They all do the same job, strike off to grade and flatness, but they trade cost, speed, and how flat a floor they leave, and they sort roughly by how much the result depends on the operator's hand and eye versus the machine.

At the low end is the wet screed and the hand straightedge, cheap, simple, and as flat as the operator is good. In the middle are the vibratory truss screed and the roller tube screed, which run form to form and add consolidation and consistency. At the top is the laser screed, a machine that holds grade off a laser plane with almost no dependence on the operator's eye, and places large areas fast and very flat. The table is the shape of the spectrum. The sections after it work down each method.

The pick is driven by the floor. A small pad or a footing strikes off by hand. A driveway or a modest slab runs a roller or a vibratory screed off the forms. A warehouse floor, a data center slab, or any floor with a real flatness spec goes to the laser screed, because that is the method that holds the number over a large area. Match the screed to the flatness the floor has to hit and the area you have to cover in a day.

Wet screeding off the pour

A wet screed is the simplest method: the crew sets wet pads of concrete or screed rails to the right grade across the pour, then pulls a straightedge or a board across them by hand to strike the surface off level with the pads. There are no fixed forms across the field of the slab, just the reference points the crew sets and reads by eye. It is the cheapest way to screed and the one that needs the least equipment, which is why it carries small jobs, pads, equipment bases, and hand pours where a machine cannot get in.

The catch is accuracy. The flatness of a wet screed is only as good as the pads the crew set and the eye that pulled the board, and the pads can settle into the fresh concrete while the crew works the rest of the slab. A wet-screeded floor commonly lands in the range of FF 25, FL 20, give or take, which is fine for a slab that gets carpet or a slab in a back room and nowhere near a floor with a real flatness spec.

Wet screeding has its place and it is not going away. On a small pour, or a complex shape, or a spot a machine cannot reach, it is the right call and a skilled crew gets a respectable floor out of it. The mistake is reaching for it on a floor that has a flatness number to hit, because the method cannot hold a tight number over area. Match the method to the spec.

The hand straightedge screed

The hand straightedge, or hand screed, is a stiff board, often magnesium, pulled across the tops of the forms or the screed rails to strike the surface off level with them. Two people work it on a small slab, riding the board on the forms and dragging it back and forth with a sawing motion as they pull, cutting the highs and filling the lows. It is the method most people picture when they hear screeding, and on formed flatwork, walks, and small slabs it is still the standard.

Magnesium is the common choice for the board because it slides better than wood, does not drag the surface, and stays straight. The skill is in the sawing-and-pulling motion and in keeping the board full of concrete ahead of it so it has something to cut and fill with, rather than dragging across a starved surface that leaves tears and lows.

The hand straightedge depends on the forms being set true, because the screed only ever gets as flat as the rails it rides. Set the forms wavy or let them deflect under the concrete and the screed copies the wave straight into the floor. It also depends on the operator, the same as the wet screed, so it covers the same ground: small, formed, and where the flatness spec is modest.

Vibratory truss and roller tube screeds

The middle of the spectrum is the powered screed that runs form to form and adds something the hand methods do not: a machine that strikes off and consolidates in one pass, with less reliance on the operator. Two types cover most of this work, the vibratory truss screed and the roller tube screed, and they move concrete in different ways.

A vibratory truss screed is a vibrating blade or a trussed beam, driven by a motor, that the crew rides across the forms. The vibration strikes the surface off and consolidates the concrete at the same time, settling the aggregate and bringing the cream up as it passes. The truss is sectional and extends to long widths, well past 40 ft on big paving work, while staying stiff across the span. The vibration is its strength on flat work and its limit on slope: on a steep grade the vibration tends to make the concrete run downhill, so truss screeds are kept to flatter pours.

A roller tube screed is a spinning steel tube, driven by a small motor, that the crew pulls across the surface. The rotation rolls the concrete forward and strikes it off, and because it works by rolling rather than vibrating, it handles slopes that a vibratory screed cannot, and it folds the aggregate in rather than driving it down. It is quick to set up, easy to run, and leaves a consistent result without a highly skilled operator, which makes it a common pick for driveways, ramps, and mid-size slabs. Both types beat the hand screed on consolidation and consistency. Neither holds the flatness of a laser screed over a large floor.

What is a laser screed?

A laser screed is a self-propelled machine that strikes concrete off to a laser-set elevation automatically, holding grade and flatness off a rotating laser plane instead of off forms or an operator's eye. The machine carries a screed head out over the fresh concrete on a telescoping boom or drives across the pour, and the head reads the laser, finds its own height, and cuts the surface to the plane as it pulls back. It is the method that places large, very flat floors fast, and it is the modern workhorse on warehouse, industrial, and data center slabs.

The reason it took over the flat-floor market is that it removes the operator's hand and eye from the flatness. A hand screed is as flat as the person pulling it. A laser screed is as flat as the laser and the calibration, which is far tighter and far more repeatable across a big pour. It also works without screed rails across the field of the slab, so a crew can strike off in the middle of a large floor with nothing to ride on, which is what lets it cover ground a formed method cannot.

The major builders are Somero, which introduced the laser screed, along with Ligchine and Allen Engineering. Models range from large ride-on machines with long booms for big interior floors down to compact walk-behind and boom units for tighter work. The machine is expensive enough that most crews rent it for the pour rather than own it, and the rate it places concrete is what pays for it.

How does a laser screed work?

A laser screed works off three parts: a rotating laser transmitter set up beside the pour, a receiver and grade-control system on the screed head, and the head itself, which carries an auger, a vibrator, and a screed plate. The transmitter spins a level laser plane across the whole work area. The receiver on the head reads where it sits relative to that plane, the control system compares it to the target elevation, and hydraulics raise or lower the head to hold it on the plane as the machine pulls it back through the concrete. The control loop checks and corrects elevation many times a second, so the head tracks the plane continuously rather than in steps.

The head does the physical work in one pass. The auger out front cuts the surface down to grade and pushes the excess concrete to the side, so the head is always working into a consistent amount of material. The vibrator behind it consolidates the concrete, settling the top and working out the entrapped air. The screed plate trails last and strikes the surface off flat to the laser plane. Plow, cut, vibrate, strike: one pass leaves a consolidated surface true to the plane, ready for the bull float.

Because the head finds its own height off the laser, it does not need form rails to ride on. The crew strikes off in the open field of the slab, the machine repositions, and it strikes the next strip, indexing across the floor. That is the mechanism behind both the speed and the form-less placement. The plane in the air, not a form on the ground, is the reference the floor is built to.

Setting up and calibrating the laser

The whole floor is built to the laser plane, so the setup of that plane is the part you do not rush. The transmitter is set on a stable tripod or stand off the pour where it cannot get bumped, leveled, and tied to the project benchmark so the plane sits at the right elevation, not just a flat plane at the wrong height. A flat floor at the wrong grade is still a wrong floor. Get the benchmark and the target elevation right before the first truck.

Calibrate and check the system before the pour and watch it during. The receiver and grade control get checked against a known elevation so the head strikes off to the height you think it does, and a smart crew shoots a check point partway through the pour to confirm nothing has drifted. The classic failure is a transmitter that got bumped, knocked out of level, or set up over a disturbed benchmark, so the machine faithfully builds a floor to the wrong plane all day and nobody catches it until the floor is hard.

Keep the laser where vibration and traffic cannot reach it. A transmitter on a stand near the pour, on ground a loaded truck drives over, will walk out of level from the shaking alone. Set it off to the side on solid ground, and if the floor is large enough that one setup cannot see the whole area, plan the second setup and tie the two together so the planes match at the seam.

How flat can a laser screed floor be?

A laser screed routinely produces a much flatter floor than a hand screed, commonly doubling the flatness numbers a wet screed gets and landing floor levelness in the mid-30s and up, with floor flatness higher still when the crew restraightens behind the screed. Flatness and levelness are measured as FF and FL numbers under ASTM E1155: FF, the flatness number, controls the short bumps you feel every couple of feet, and FL, the levelness number, controls the overall slope across the floor. Higher numbers are flatter and more level.

The tolerances a floor has to meet come from ACI 117, the specification for tolerances in concrete construction, and from the project specification, which can be stricter. A wet-screeded floor sits around FF 25, FL 20. An ordinary laser-screeded warehouse floor runs well above that. The very flat floors, the superflat specs written for defined-traffic very narrow aisle warehouses where a turret truck reaches high up a rack, call for flatness in a different class and are measured in the wheel paths specifically. Confirm the FF and FL targets and how they are measured against ACI 117 and the spec, because the number and the measurement method both control whether the floor passes.

The laser screed gets you into the flat range, but it does not hit a superflat number by itself. The crew restraightens behind the screed, running a highway straightedge to cut the highs and fill the lows while the concrete still moves, and that pass is what separates a flat floor from a very flat one. The flatness is built at the screed and protected through the finishing, covered in the flatwork finishing guide. Chasing it at the trowel is too late.

Productivity: area placed per day

The laser screed earns its rental on production. It strikes off and consolidates a wide strip in one pass with a small crew, where a hand-screed crew would crawl across the same floor pulling boards and chasing grade with far more people. A laser screed head commonly runs 12 to 20 ft wide and can strike off a couple hundred square feet in a minute when the concrete is coming, so a crew places several times the area in a day that a hand operation would, with fewer hands on the screed.

The actual figure varies with the pour, the mix delivery, and the crew, so treat any square-feet-per-day number as a planning estimate, not a promise. The real limit on a good laser screed crew is usually not the machine. It is how fast the concrete shows up. A pour that outruns the trucks stops the machine, and the savings come from keeping the screed fed and moving, which means staging the trucks and the placing to match the rate the machine can take.

Productivity is also where the floor and the schedule meet. Placing a large floor in fewer, larger pours with a laser screed cuts the number of construction joints and the number of days, which is part of why big interior floors are placed this way. The faster placement only pays off if the finishing crew can keep up behind it, because the screed can place concrete faster than a short-handed finish crew can work it before it sets.

Formed, form-less, and what the laser screed changes

Most screeding methods need a reference to strike off to, the forms or the screed rails the board or the truss rides on. The laser screed does not, because it strikes off to the laser plane in the air, so it places concrete form-less in the open field of the slab. That is the change that matters. The crew can pour a large area and strike it off in strips without setting and pulling rails across the whole floor, which is faster and removes the rail lines as a place for the floor to go wavy.

Form-to-form methods still have their place. On a slab bounded by forms or against existing concrete, the vibratory truss or roller screed riding the forms is quick and simple, and on a narrow or oddly shaped pour the forms are already there. The laser screed shines on the large open floor where setting rails everywhere would be slow and where the field of the slab, not the edge, is most of the work.

The edges are where the two approaches meet. A laser screed places the field of the floor fast and form-less, but the perimeter against the forms, the columns, and the walls still gets struck off and worked by hand, because the machine cannot get the head tight into every edge. Plan the hand work at the edges as part of the laser screed pour, not as a surprise at the end.

Where wet screeding still fits, and where it does not

Wet screeding is the least accurate method because it depends entirely on the pads and the eye, and the pads settle. The crew sets reference pads of concrete to grade, then strikes off to them, but those pads sit in fresh concrete and can sink while the crew works the rest of the slab, so the reference itself moves under the screed. Add the operator's eye reading the strikeoff and the result drifts in a way a laser plane never does.

That does not make wet screeding wrong. It makes it a method for the floors where flatness is not on the line: a slab that gets carpet or a thick covering, a back-of-house pad, a footing, a small or complex shape a machine cannot reach. On those, a skilled crew wet screeds a perfectly serviceable floor and the cost of a machine would be wasted.

Where it does not fit is any floor with a flatness spec to hit. The moment the drawings carry an FF and FL number, the method has to be one that can hold that number over area, which the wet screed cannot. The expensive mistake is wet screeding a floor that later gets measured against a flatness spec it was never going to make, and then arguing about it after the concrete is hard. Read the spec before you pick the screed.

Consolidation at the screed and the thick edge

Consolidation is working the entrapped air out of the concrete and settling the solids together, and the vibrating screeds do part of it as they strike off. The vibrator on a laser screed head, and the vibration of a truss screed, consolidate the top of the slab in the same pass that strikes it off, which is part of why the vibrating methods leave a sounder surface than a board dragged across by hand. The hand screed strikes off but adds no vibration, so a wet- or hand-screeded slab leans on the bull float and the finishing to settle the surface.

The screed only consolidates the top, though. On a thick slab, a deep edge, or a section with heavy reinforcement, the depth of the slab needs an internal vibrator worked through it, because the screed vibration does not reach to the bottom. Skip the internal vibration on the thick sections and you get honeycombing and voids down in the slab that the flat screed surface hides completely until something cores it or it cracks.

Over-vibration is its own problem. Too much vibration drives the heavy aggregate down and floats the paste and water up, segregating the mix and leaving a weak, paste-rich top that dusts and cracks. The screed should consolidate enough to close the surface and remove the air, not work the mix until it separates. The placement and consolidation of the body of the slab is its own topic, covered with the slab work; the point here is that the screed handles the top, and the thick sections still need a vibrator down in them.

What slump does a laser screed need?

A laser screed runs best on a consistent, fairly low-slump mix, often in the range of 3 to 4 in, because the head and the powerful hydraulics can hold grade in a stiffer concrete and a low-slump mix shrinks and curls less than a wet one. But the number matters less than the consistency. The single thing that wrecks a laser screed floor from the mix side is slump that wanders load to load, because the machine is set to cut a consistent amount of material and a mix that arrives wet on one truck and stiff on the next changes how the head strikes off and leaves a surface that varies with the delivery.

Too wet is the more common trouble. A high-slump mix bleeds more, segregates more, and runs out from under the screed plate, and the extra water raises shrinkage and curling that the laser flatness then has to fight later. Too stiff and the machine works harder and the surface can tear, but a strong laser screed handles a low-slump mix that a hand crew would struggle to pull. Adding water at the truck to make the concrete easier to place is the wrong fix, because it drops the strength and feeds the shrinkage, the same trap covered in the slab on grade and mix work.

So the order from the mix side is consistency first, then a slump on the low side. Hold the slump tight across the trucks, check it at the point of placement rather than trusting the ticket, and a laser screed will hold grade on it all day. Let the slump bounce and the flattest machine on the market still builds a floor that varies truck to truck.

The base, the grade, and yield

A laser screed strikes off to a flat plane, which means it will faithfully build a flat top on a base that is not flat, and that costs you concrete. The slab thickness is the gap between the flat top the screed cuts and the base underneath. If the base pillows up high in one place and dips low in another, the slab comes out thin where the base is high and thick where it is low, and the extra concrete in the lows is yield you pay for and never planned. A base that runs high can also leave the slab thinner than the design where the load needs it most.

So the base has to be brought in flat and at grade before the laser screed shows up, the same prepared, compacted, consistent subbase the slab on grade guide covers. The flatness of the floor starts with the laser, but the thickness and the yield start with the base. A fine-graded, trimmed base lets the slab come out at the design thickness across the floor with the concrete you ordered. A rough base eats concrete and varies the thickness under a top that looks perfect.

Check the base grade against the same benchmark the laser is set to, so the plane and the base are measured to the same reference and the slab thickness comes out where the design put it. Yield overruns on a big floor trace back to a base that came in high or wavy more often than to anything else, and they are invisible once the flat slab covers them.

After the screed: bull float, finish, and joints

The screed leaves a flat, consolidated, rough surface, and the finishing picks up from there. The bull float comes right behind the screed, before any bleed water shows, to knock down the ridges and embed the aggregate and set the surface for the wait. Then nothing touches the surface until the bleed water leaves, and the floating and troweling run on the timing the bleed water sets. That whole sequence, the bleed-water wait, the float, the trowel passes, and the cure, is covered in the flatwork finishing guide and is where a floor is finished or ruined.

Jointing comes after the finish. The shrinkage cracks get steered into sawcut or tooled contraction joints at the design spacing and depth, inside the window before the slab cracks on its own. On a large laser-screeded floor placed form-less, the joint layout is its own plan, decided before the pour, and it is covered with the slab and joint work.

The point here is only the handoff. The screed sets the flatness and the grade; the finishing sets the surface; the joints control the cracking. They run in order, and a flat screed buys nothing if the finishing crew works the surface over bleed water or the saw crew misses the joint window. Plan the finish and the joints around the laser screed pour, because the machine places concrete faster than a short crew can finish and cut it.

Equipment: ride-on, walk-behind, boom, and rental

Laser screeds come in a few configurations, and they sort by how the head gets out over the concrete and how big a floor they suit. The large ride-on boom machine carries the head on a telescoping boom that reaches roughly 20 ft or more across the slab, so the machine sits on the base or the prepared subgrade and swings the head out over the fresh concrete without driving on it. That is the production machine for big interior floors, and the long boom and machine rotation let it cover a wide area from one position.

Smaller and tighter work runs a walk-behind or a compact boom unit, lighter machines that get into spaces and edges the big ride-on cannot, at a lower placement rate. Some setups drive across the pour rather than reaching over it with a boom, which works on a base that carries the machine. The pick is the floor: a big open warehouse slab wants the long-boom ride-on, a tight or cut-up floor wants the compact unit.

Most crews rent the machine for the pour. A laser screed is a serious capital item, and a contractor who places flat floors now and then comes out ahead renting it, the operator, or both, for the days they need it. When you rent, confirm the head width suits the pour, the boom reaches the area, and the laser and grade control are calibrated and checked before the first truck. The rate the machine places concrete is what justifies it, so a machine that sits waiting on trucks or fights a bad base is not earning the rental.

Safety around the screed and wet concrete

A laser screed is a heavy, self-propelled machine with a swinging boom and a vibrating head working in a crew of people in wet concrete, so the hazards are the machine and the concrete together. The boom swings a wide arc and the head has moving augers and a vibrating plate, so the crew stays clear of the swing and the pinch points, and the operator and the ground crew work to clear signals so nobody is under the boom or in the path when it moves. People walking backward in wet concrete ahead of the machine is how someone gets caught.

Wet concrete burns. Fresh concrete is caustic, and skin in contact with it long enough, kneeling in it, taking it inside a boot, getting it inside a glove, gets a chemical burn that can be serious before it even hurts. The crew wears the gear that keeps concrete off skin, washes it off fast when it gets through, and does not ride out a boot full of concrete to finish the strip.

The rest is the ordinary heavy-equipment discipline. Watch the ground the machine sits on so it does not tip or sink, keep the laser and the people out of each other's way, and treat the machine as the heavy thing it is in a crowded, slick, fast-moving pour. The production rate is part of the hazard, because the floor moves fast and the crew is busy, which is exactly when a clear plan for who stands where keeps people out from under the boom.

QC: checking grade and flatness

The quality check on a screeded floor has two parts, grade and flatness, and they get checked at different times. Grade gets checked during the pour: shoot the surface against the benchmark, confirm the slab is landing at the design elevation and thickness, and catch a base or a laser problem while the concrete is still fresh enough to fix. A check point partway through the pour is how a crew catches a laser that drifted before it builds the whole floor to the wrong plane.

Flatness gets measured after, as FF and FL numbers under ASTM E1155, and the timing matters: measure within the window the standard allows, the first day or so, because the slab curls and the numbers drift after that. A floor that read flat green can read worse once it curls at the joints. The FF and FL results get compared to the tolerances in ACI 117 and the project specification, and on a defined-traffic floor the measurement runs in the wheel paths the way the spec calls out.

Tie the QC back to the record. The benchmark the laser was set to, the grade checks during the pour, the slab thickness confirmed at the screed, and the FF and FL after, all belong in the file, because a floor that fails a flatness check later is a dispute that the record either settles or loses. Confirm the depth and the grade at the screed, not after the floor is hard and the only tool left is a core.

Why did the laser screed floor come out wrong?

When a laser-screeded floor comes out off-grade, wavy, thin, or short on flatness, the cause is almost always one of a short list, and they rank by how often they bite. The most common is the laser: a transmitter not leveled right, set over a disturbed benchmark, or bumped mid-pour, so the machine builds a flat floor to the wrong plane all day. That is the failure the check point during the pour exists to catch.

The mix is next. Slump that wanders load to load changes how the head strikes off and leaves a surface that varies truck to truck, and a too-wet mix bleeds, segregates, and curls. After that is the base: a base that came in high or wavy leaves the slab thin in the highs, thick in the lows, and over yield, with a flat top hiding all of it. Then consolidation: no internal vibration in the thick sections leaves honeycombing and voids down in the slab. And the floor can simply be the wrong method for the spec, wet screeded where a flatness number had to be hit.

The pattern is that the failures are upstream of the machine. The laser screed itself is reliable and flat. What it does is faithfully build whatever it is told to, to a plane it is set to, from concrete it is fed, on a base it is run over. Get the laser, the slump, and the base right and the machine delivers. Get any of those wrong and the flattest machine on the market builds the error into a hard floor.

Warehouse and data center superflat floors

The floors that drove the laser screed into the market are the big, flat ones: distribution warehouses, manufacturing plants, and data centers, where a large area has to come out flat enough for the equipment that runs on it. A random-traffic warehouse floor needs the flatness ACI 117 and the spec call for across the floor. A defined-traffic very narrow aisle floor, where a turret truck reaches high up a rack in a fixed aisle, needs a superflat floor measured in the wheel paths, because a small tilt at the wheels becomes a large sway at the top of the mast.

The laser screed is how those floors get placed flat and fast, but the very flat numbers take more than the machine. The crew restraightens behind the screed with a highway straightedge, controls the mix for low shrinkage and curling, places in a layout that fits the flatness requirement, and on the highest specs may grind the floor after to bring the flatness up the last step. The screed gets the floor into the flat range; the technique and the finishing carry it to superflat.

Data center floors add the load case on top of the flatness. The slab carries heavy equipment racks on small footprints and takes the rolling load of moving a loaded rack across the floor during a build, which the slab on grade guide covers, and it often has to be both flat and strong. The laser screed handles the placement and the flatness; the thickness, the subgrade, and the load design are decided before the pour. A flat floor that is too thin for the rack load is still a failed floor.

What to document

A screeded floor is covered by finish and then by racks and traffic, and most of what set its flatness and grade is out of sight or in the past by the time anyone questions it. The record is what answers the question months out when a floor fails a flatness check or a corner sits low, and it shows the laser was set right, the base was at grade, the mix held, and the floor was measured.

Capture the pour and area, the screed method, the benchmark and the laser setup and calibration check, the base grade confirmation, the mix and the slump checks across the trucks, the slab thickness confirmed at the screed, whether the thick sections got internal vibration, the restraightening if the floor was held to a flatness spec, and the FF and FL results against the tolerance. If anything was wet screeded or done by hand at the edges, note it. The next person reading it, the inspector or the owner, wants to see the flatness was built and checked, not hoped for.

Common mistakes

• Leaving the laser transmitter where traffic or vibration bumps it, or setting it over a disturbed benchmark, so the floor is flat at the wrong plane.
• Skipping the calibration check, and the mid-pour check point, so a drifted laser builds the error into the whole floor.
• Running a mix whose slump wanders load to load, which changes the strikeoff truck to truck.
• Striking off over a base that came in high or wavy, so the slab is thin in spots and over yield in others.
• Relying on the screed vibration alone on thick sections, leaving honeycombing and voids down in the slab.
• Wet screeding a floor that carries a real FF and FL spec it was never going to make.
• Adding water at the truck to place easier, dropping strength and feeding shrinkage and curling.
• Placing faster than the finish crew can work the surface, so the concrete sets before it is finished.
• Measuring flatness late, after the slab has curled, instead of inside the ASTM E1155 window.

Field checklist

Standards and references

The project specification governs the floor, full stop, and everything below is the framework it sits on. Where the contract is stricter or more specific, it wins. ACI 302, the guide to concrete floor and slab construction, covers floor construction including screeding and strikeoff, the finishing that follows, and the placing practices behind a sound floor. ACI 117, the specification for tolerances for concrete construction, carries the tolerances, including the framework behind the floor flatness and levelness limits a floor is held to.

On the test-method side, ASTM E1155 is the test for FF floor flatness and FL floor levelness numbers, the way a specified flat floor is measured and accepted. ACI 117 sets the tolerances; ASTM E1155 measures whether the floor meets them, and the two are usually referenced together. The exact document numbers and contents shift between editions, so confirm the editions the project and the jurisdiction have adopted before citing a provision, and let the engineer of record and the contract documents control.

For the equipment, the laser screed manufacturers, Somero, which introduced the machine, along with Ligchine and Allen Engineering, publish the operating and calibration procedures for their machines, and the calibration and setup follow the manufacturer's instructions for the specific unit. Cite the standard that controls the point, hedge the FF and FL targets and tolerances to ACI 117 and the spec, and remember that the floor is won or lost upstream, at the laser calibration, the slump, and the base and grade, not at the machine.

Units, terms, and conversions

Screeding carries its own vocabulary, and the same step reads differently across a spec, a manufacturer sheet, and the crew talking on the slab. Strikeoff and screeding are the same step. Flatness rides on FF and FL numbers, unitless and higher-is-flatter, measured under ASTM E1155 to tolerances in ACI 117. Slump is in inches in the US and millimeters in metric, where a 4 in slump is about 100 mm. Slab thickness and head width run in inches and feet here, millimeters and meters in metric drawings.

The terms below are the ones that get mixed up, mostly the screed-versus-finish distinction and the parts of the laser screed head. Keep them straight, because the screed is the strikeoff, not the finish, and the floor's flatness is decided in the strikeoff well before the trowel.

Screed / strikeoff

Bringing fresh concrete to grade and a flat plane right after placement, before finishing

Wet screed

Striking off to wet reference pads or form edges by eye; cheap and least accurate

Vibratory truss screed

A vibrating blade or truss run form to form that strikes off and consolidates

Roller tube screed

A spinning tube pulled across the surface; handles slopes, less operator-dependent

Laser screed

Self-propelled machine that strikes off to a rotating laser plane with auger, vibrator, and screed plate

FF / FL

Floor flatness and levelness F-numbers, measured under ASTM E1155 to ACI 117 tolerances

Superflat

Very flat floor for defined-traffic narrow-aisle use, measured in the wheel paths

Yield

Concrete volume actually used; a high or wavy base raises it by thickening the lows

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