Screeding

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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Screeding sets the level, slope and thickness of the surface right after placement. Everything after it depends on it.

The big idea

Screeding sets the level, slope and thickness of the surface right after placement. Everything after it depends on it, so a bad screed cannot be fixed with a good float.

Part 9 of the National Building Code calls for a slab on ground at least 75 mm (3 in) thick (9.16.4.3). Your drawings set the actual thickness and slope.

Types of screed

• Hand screed board: a straight 2x4 or an aluminum strike-off, riding on the forms.
• Screed rails or pipes: fixed guides set to level, for flatness away from the forms.
• Vibrating screed: a powered screed for large slabs. The NRMCA flatwork guide says not to use one when slump is above 75 mm (3 in), and to move it rapidly so consolidation is even.
• Roller screed: for long, wide placements.

Before you screed

• Forms or rails are level and secure, and checked against the drawings.
• The slope is set for exterior slabs.
• Reinforcement sits on chairs, so it will not be dragged.
• The concrete is placed evenly, with a little extra above the forms.
• Spread with a square-ended shovel or a come-along. The NRMCA guide says never use a garden rake to move concrete horizontally, because it causes segregation.

Hand screeding

1. Rest the board Set the board on edge on the forms or rails.
2. Tilt and pull Tilt it forward and draw it toward you with a slight sawing motion.
3. Keep a little in front Keep a little concrete ahead of the board to fill low spots. Shovel more in where the board skips.
4. Fill and re-pass Fill low spots right away and go over them again. Take down high spots before the concrete stiffens.
5. Keep it clean Scrape the board often. A dirty board leaves ridges.

Powered screeds and rails

• Set the screed on rails or forms and move it at a steady speed.
• Let the vibration level and consolidate the surface, but do not build up a thick layer of mortar on top.
• Check elevation often with a laser or a rod.
• Keep the roller clean on roller screeds.

Checking level and thickness

• Use a laser or a grade rod.
• Check corners, edges and mid-spans.
• Check the slope on exterior slabs, against the drawings.
• Make sure thickened edges are full.

Example. A 4 m by 6 m (13 ft 1 in by 19 ft 8 in) slab is ordered at 100 mm (4 in): 4 × 6 × 0.100 = 2.4 m³ (3.14 yd³). If the surface ends up 10 mm (3/8 in) too thick on average, that is 24 m² × 0.010 = 0.24 m³ (0.31 yd³) of concrete you paid for and buried. The sizes are made up.

Hand-off to floating

The NRMCA guide says to begin leveling with a bull float or darby as soon as the concrete is struck off, and to finish that before bleed water appears. Keep the bull float as flat as you can so the surface is not sealed too early.

Then stop. Wait until the bleed water has disappeared before any other finishing. Floating or troweling while it is still bleeding can cause dusting, scaling, crazing, delamination and blisters. The wait depends on the mix and the weather, so test a small area. The next page covers floating.

Common mistakes

• Pulling too fast, leaving waves
• Not filling low spots at once
• Dragging steel with the screed
• Working the surface too much and bringing up cream
• Wrong slope
• A dirty board leaving ridges