Introduction
This procedure is limited to a manufacturer-approved bonded, hand-placed cementitious screed or topping over an existing, sound concrete substrate, with temporary screed rails. A screed rail is a straight, rigid guide whose top establishes the finished plane for the straightedge.
Do not use this as a design for a structural slab, slab-on-ground, unbonded screed, floating screed over insulation, or a layer over a slip membrane or vapor-control layer. Those assemblies require separate design decisions on thickness, joints, reinforcement, moisture control, and placement. Before starting, select the exact product and confirm that it permits bonded installation, the required thickness, temporary rail supports and removal, the specified finish, curing method, and the intended floor covering.
Manufacturer guidance distinguishes bonded, unbonded, and floating screed construction because their requirements are not interchangeable. See manufacturer screed guidance.
Key takeaways
- Set rail tops at the required finished plane or designed fall; do not set them above finish height.
- Use temporary rails only when the selected product permits their support and removal method.
- Use the specified screed or topping exactly as directed; do not substitute generic concrete or alter water and aggregate proportions.
- Check flatness separately from elevation or slope. A floor can be flat while intentionally sloped to a drain.
- Remove rails only at the stage stated in the product instructions or written supplier guidance; fill, bond, compact where required, and re-strike every channel.
- Cure, moisture-test, and install the floor covering only as required by the selected material and flooring manufacturer.
Project overview and goals
The goal is a bonded, nonstructural topping or screed at the required thickness and finished elevation, ready for its specified floor covering. Do not proceed until the product technical data sheet (TDS), substrate-preparation instructions, joint and perimeter-isolation details, rail-support and rail-removal instructions, curing instructions, and covering/adhesive requirements are available. If any of these documents do not define the work, stop and obtain written manufacturer or supplier guidance.
| Confirm before ordering material | Why it matters |
|---|---|
| Exact bonded product and TDS | It controls permitted thickness, substrate preparation, mixing water, working time, placement, consolidation, finish, rail treatment, and curing. |
| Finished elevation and any fall | Rail tops follow the required plane. Thresholds, door clearances, drains, and penetrations can control it. |
| Floor covering and adhesive | The flooring system supplies the required flatness method, moisture-test method, and acceptance limit. |
| Joints and perimeter isolation | The product documents, approved drawing, existing substrate-joint layout, and flooring plan must define treatment at walls, columns, pipes, thresholds, drains, and substrate joints. |
| Quantity and working time | Wet volume = room length × room width × average installed thickness. Use the product’s published yield, then allow for measured grade variation and project-specific waste. |
When to use screed rails vs. other leveling methods
Use rails when the selected hand-placed bonded product must be brought to a controlled plane, the substrate variation requires fixed guides, and the manufacturer permits temporary rails. Use the product only within its published thickness range. Do not assume a thin smoothing compound can replace a deeper screed, or that bagged concrete is appropriate for a bonded topping.
Stop for structural cracking, settlement, active moisture, widespread debonding, unknown embedded services, unknown vapor-control conditions, or any required structural thickness. These conditions require professional or manufacturer review before work begins.
Acceptable flatness and level tolerances
Flatness is local surface variation under a specified straightedge. Levelness is the overall horizontal plane or specified elevation plane. A floor that falls to a drain is intentionally not level. Obtain the required acceptance method, straightedge length, and limit from the flooring or project specification before layout.
Check local flatness with that specified straightedge; check elevation or fall separately with a laser, level, or datum marks. Do not convert a straightedge result into an FF/FL number. ASTM FF/FL measurements are statistical floor-profile results produced by a specific method, not a universal equivalent of a gap under a straightedge; see ASTM E1155/E1155M. Related guidance: How to level a floor with concrete.
Tools and materials checklist
Essential hand tools and power tools
- Straight screed: a rigid aluminum or magnesium straightedge long enough to span two rails without excessive flex.
- Laser, water level, or reliable level: for transferring a datum and checking rail elevation and fall.
- Tape, pencil, stringline, and specified straightedge: for layout and acceptance checks.
- Mixing equipment: a mixer or drill and paddle sized for the product, batch size, and stated mixing procedure.
- Shovels, gauge rake, buckets, margin trowel, float, and finishing trowel: for placing, filling, and making the permitted finish.
- Temporary rail hardware: only the supports or fasteners expressly permitted by the product system; do not improvise dabs, shims, or anchors.
- Wet saw, grinder, or scarifier only where required: use the required silica controls.
Materials and hardware
- Specified bonded screed or topping: order using its published coverage rate, not a generic cement:sand ratio.
- Specified primer, bond coat, or bonding system: use only if the product requires it for the prepared concrete.
- Perimeter isolation and joint materials: exactly as required by the product documents, approved drawing, substrate-joint layout, and flooring plan.
- Straight, rigid temporary rails: use only rails approved or accepted by the selected system.
- Compatible rail-channel fill material: the same material or a manufacturer-approved material.
- Specified curing and protection materials: use only measures permitted by the product and the planned floor covering.
Plan labor and batch size so mixing, moving, striking off, rail removal, channel filling, and finishing all occur within the stated working time. Divide a large area only at planned construction joints; do not create an unintended cold joint because mixing cannot keep up.
Safety and site preparation
Wear eye protection, impervious gloves, long sleeves, and waterproof boots when handling wet cementitious material. Wet concrete and mortar can irritate skin and cause chemical burns. For cutting, grinding, drilling, or removing hardened material, use wet methods or local extraction. Use HEPA vacuuming or wet cleanup rather than routine dry sweeping. OSHA’s construction silica guidance addresses exposure controls and dust-generating cleanup.
Keep cementitious wash water out of drains, soil, and waterways unless local rules permit disposal. Identify embedded services before drilling, protect nearby finishes, provide ventilation, and keep clear walk paths. Use respiratory protection only within an appropriate respiratory-protection program where required.
Subfloor inspection and repair
Inspect the concrete substrate before layout. It must be sound, clean, dry or conditioned as the selected bonded system requires, and compatible with that system. Remove weak material, dust, oil, paint, adhesive residue, and loose patches by the specified preparation method. Do not cover hollow or delaminated concrete with a thin layer.
Repair holes and broken edges with a compatible repair material, then allow that repair to cure and be accepted as required by the repair and screed systems before screed placement. For the bonded topping, prepare the concrete profile and bonding method exactly as specified. Do not add a primer or bonding agent by habit.
Layout, isolation, joints, and access planning
Measure the room and locate thresholds, walls, columns, pipes, penetrations, drains, existing substrate joints, movement joints, and the exit path. Confirm the highest substrate point and verify that the selected product remains within its minimum and maximum thickness at every measured location.
Before placing, check the selected product TDS, approved drawing, existing substrate-joint layout, and flooring specification for required perimeter isolation and joint treatment. These documents must define treatment at walls, columns, pipes, thresholds, drains, and existing substrate or movement joints. Stop and obtain direction if the treatment is not defined; do not bury, bridge, or relocate a joint by guesswork. Coordinate screed joints with the flooring layout where required.
Lay out rail lines so the straightedge spans them comfortably and the crew can work backward toward the exit without stepping in fresh material. Rail spacing depends on straightedge stiffness and length; use a spacing that prevents noticeable deflection.
Setting reference elevations and installing screed rails
Transferring elevations and creating a screed plan
- Set a laser, water level, or other reliable datum and transfer reference marks around the room.
- Mark the required finished plane. If drainage is required, mark the designed fall from the drain or other controlling point.
- Measure from the marks to the substrate at representative locations, including high and low points, edges, thresholds, and drains. Confirm the resulting thickness is within the selected product’s limits everywhere.
- Mark rail lines, bay sequence, planned joints, and exit path. Calculate material from the average installed thickness and the manufacturer’s yield; confirm that mixing and placement can finish within working time.
Fastening rails and ensuring rigidity
Use temporary rails only if the selected product’s TDS expressly describes or permits the rail-support and removal method. If it does not, obtain written supplier guidance before placing. Do not assume that compatible-material dabs, shims, anchors, or fasteners are acceptable. Never puncture a membrane, fasten blindly into a slab, damage embedded services, or use frost-line staking for ordinary interior work.
Set the top of every rail at the finished elevation. Check each rail at several points for elevation, straightness, slope, and twist; then push on it firmly. It must not rock, spring, or bend under the straightedge. Recheck adjacent rails with the screed board before mixing. Reset a moving rail; do not compensate for it during finishing. Related guidance: Concrete Screeding on Small Slabs: Straightedge Choices and How to Check Flatness.
Mixing and placing the screed concrete
Best mix designs and additives
Use the complete instructions for the selected product. Do not rely on a generic 1:3 or 1:2.5 cement-to-sand rule, and do not substitute bagged concrete for a screed or topping without approval. Product systems can have substantially different permitted depths, aggregate extensions, water quantities, and opening times; for example, UZIN SC 946 product data applies only to that named flow-screed product.
Measure water consistently, mix for the stated time, and keep batches within the stated working time. Do not add water to revive stiffening material or sprinkle water onto the surface. Use admixtures, fibers, aggregate, or reinforcement only when they are part of the selected system or design.
Placing technique and handling workability
- Apply the specified primer or bond coat immediately before placement only when required, and follow its stated timing window.
- Place material in the first bay, keeping a small head of material ahead of the screed board so the board can fill the plane continuously.
- Spread without dropping loads from height or separating aggregate from paste. If the batch segregates, has free water, or does not behave as the product instructions require, stop and correct the batch rather than placing it.
- Use only the p