Screeding is the process of dragging a straight board across freshly poured concrete to strike it off level and remove excess material. Done correctly, it gives you a flat, even surface that's ready for finishing — whether that's a broom texture, a power-float polish, or tiles. With the right preparation and technique, a competent DIYer can screed a small domestic slab successfully.
What Is a Screed Board?
A screed board (also called a strike-off board or straight edge) is simply a straight, rigid board — typically a length of 90 × 45 mm or 90 × 35 mm timber, or an aluminium straight edge — that spans across your formwork or screed rails. It's dragged along the top of the forms in a sawing motion to level the concrete to the correct height. For slabs wider than about 2.5 m, a longer aluminium screed bar or a vibrating screed (motorised) is recommended for accuracy.
Tools and Materials You'll Need
- Screed board (timber or aluminium straight edge, at least 300 mm longer than the slab width)
- Timber or steel formwork set to the correct finished height
- Screed rails or wet screed guides (for wider slabs)
- Concrete rake or come-along hoe
- Bull float or darby float
- Rubber boots and gloves (concrete is caustic — protect your skin)
- Spirit level or laser level for setting forms
Setting Your Screed Rails or Formwork
Before you pour a single barrow of concrete, your formwork must be set accurately. This is the most critical step — the screed board can only be as accurate as the guides it rides on.
- Set the height of your formwork or screed rails to the desired finished slab level. For a typical domestic floor slab, this is usually 100 mm thick (check your engineer's specification).
- Check for level along the full length using a spirit level or, better still, a laser level. For slabs requiring drainage fall (e.g., a garage or outdoor pad), set a deliberate gradient — typically a minimum of 1:100 fall away from the house, as guided by MBIE Building Performance requirements under the NZ Building Code.
- For wider slabs, set intermediate screed rails (lengths of steel pipe or timber) across the slab at intervals your screed board can span. Bed them into a small ridge of concrete and check they're level before the main pour.
Pouring and Raking the Concrete
- Pour concrete slightly above the finished level — you need material to screed off.
- Use a concrete rake or come-along hoe to spread the concrete roughly level, working it into corners and around any reinforcing mesh or rebar.
- Avoid over-raking, which can cause segregation (aggregate sinking, water rising).
How to Screed: Step-by-Step Technique
- Position the screed board across the formwork or screed rails at the near end of the slab.
- Apply downward pressure on both ends of the board so it sits firmly on the guides.
- Using a side-to-side sawing motion, draw the board slowly towards you. The sawing action helps the board cut through high spots and fill low spots.
- As you move forward, keep a small roll of concrete building up in front of the board — this fills any voids as you go. If the roll disappears, add more concrete ahead of the board.
- Make a second pass in the opposite direction over any areas that look uneven.
- Fill any low spots (bird baths) with a shovel of concrete, then re-screed that section.
Tips for a Better Result
- Work quickly — concrete begins to stiffen within 30–60 minutes depending on temperature and mix. Hot, windy NZ summer days accelerate this significantly.
- Have a helper rake and shovel concrete ahead of you while you screed.
- Don't add water to concrete to make it easier to work — this weakens the finished slab.
- Check your finished surface with a long straight edge and fill any dips before the concrete sets.
Finishing After Screeding
Once screeded, the surface still needs finishing:
- Bull float or darby: Immediately after screeding, push a bull float across the surface to embed aggregate and close the surface. This is essential before any trowelling.
- Broom finish: For outdoor slabs (driveways, paths), drag a stiff broom across the surface once the bleed water has evaporated for a slip-resistant texture.
- Steel trowel or power float: For smooth interior floors, wait until the concrete can support your weight without sinking more than a few millimetres, then trowel or power-float.
- Curing: Cover with plastic sheeting or apply a curing compound to retain moisture for at least 3–7 days. Proper curing is critical to slab strength — refer to BRANZ guidance for best practice.
Building Consent Considerations
Many domestic concrete slabs (such as garden paths or small shed pads) may be exempt from building consent under Schedule 1 of the Building Act — but always confirm with your local council before starting. Structural slabs, house foundations, and retaining walls almost always require consent and must comply with NZS 3604 or a specific engineering design.
When to Call a Professional
Screeding a large slab, a structural floor, or a slab with underfloor heating or post-tension cables is not a DIY job. Hire a qualified concreter if:
- The slab is larger than about 20–25 m²
- The pour requires a concrete pump or ready-mix truck
- The slab is structural (house foundation, garage floor with heavy loads)
- You need a power-float finish to a high tolerance
- The job involves reinforcing design or engineer's specifications
A skilled concreter will also have access to a vibrating screed, which consolidates the concrete as it levels — far superior to a hand screed board on larger pours. You can find qualified local concreters on tradedirectory.co.nz.
Safety Note
Fresh concrete is highly alkaline and can cause serious chemical burns. Always wear waterproof gloves, rubber boots, and eye protection. If concrete contacts skin, wash immediately with clean water. Refer to WorkSafe NZ for safe handling guidelines.
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