Applying floor levelling compound (also called self-levelling compound or floor screed) looks straightforward, but ridges and bumps are a common frustration for DIYers. The good news is that with the right preparation, correct mixing, and a confident technique, you can achieve a smooth, flat surface that's ready for tiles, vinyl, or timber flooring.
Why Ridges and Bumps Happen
Most problems with floor levelling compound come down to a few root causes:
- Incorrect water-to-powder ratio — too much water weakens the compound and causes it to shrink unevenly; too little makes it stiff and hard to spread.
- Pouring too slowly or in separate batches — the first pour starts to set before the second arrives, creating a visible ridge at the join.
- Skipping primer — without a proper bonding primer, the compound can pull away from the substrate, creating high and low spots.
- Working on a dirty or dusty floor — contamination stops the compound bonding correctly.
- Using the wrong product for the depth — some compounds are only suitable for thin applications (2–5 mm), while others can fill deeper voids.
Step 1 — Choose the Right Product
Not all levelling compounds are the same. Check the manufacturer's data sheet for the minimum and maximum pour depth before you buy. Products from Ardex and Mapei are widely used by NZ flooring professionals and are available at trade and hardware stores. For deep fills (over 10 mm), you may need a separate base-coat screed before a finishing compound.
Also confirm the compound is compatible with your substrate — concrete slab, plywood, or existing tiles all have different requirements.
Step 2 — Prepare the Substrate Thoroughly
This is the most important step. A clean, sound, and primed substrate is non-negotiable.
- Sweep and vacuum the floor to remove all dust, grit, and debris.
- Remove any grease, adhesive residue, or paint — a floor grinder or scraper works well for this.
- Check for loose or hollow areas — tap the floor with a hammer; hollow sounds indicate delamination that must be fixed first.
- Fill large cracks or holes with a suitable patching compound and let it cure fully before levelling.
- Apply bonding primer — most manufacturers require their own primer product. Apply it evenly with a roller and allow it to become tacky (usually 20–40 minutes, but check the data sheet). Don't skip this step.
Step 3 — Plan Your Pour
Before you open a single bag, think through the whole job:
- Calculate how many bags you need and have them all on site before you start. Running out mid-pour is a common cause of ridges.
- Work out your exit route — you'll be walking backwards as you pour, so plan to finish at a doorway.
- Set up your mixing station close to the work area to minimise travel time between batches.
- Check the ambient temperature — most compounds require the air and substrate to be between 10°C and 30°C. NZ winters can be too cold in unheated spaces, so check the product data sheet.
Step 4 — Mix Correctly
Consistent mixing is critical for a lump-free, ridge-free result.
- Always add powder to water, not water to powder — though some manufacturers specify the reverse, so check the product data sheet.
- Use a paddle mixer attached to a drill — never mix by hand. A consistent, lump-free slurry is the goal.
- Stick exactly to the manufacturer's water ratio. Measure by weight or volume, don't guess.
- Mix for the full recommended time (usually 2–3 minutes), then let it slump for 30 seconds and mix briefly again.
- Mix one bag at a time and pour immediately — most compounds have a working time of only 10–20 minutes.
Step 5 — Pour and Spread
- Pour the compound in a continuous ribbon, working from the far end of the room back towards the door.
- Use a gauge rake or spreader to guide the compound to the correct depth — don't over-work it.
- For self-levelling products, the compound will find its own level; for thicker screeds, you'll need to spread more actively.
- Overlap each pour slightly into the previous one while it's still wet to avoid cold joints and ridges.
- Use a spiked roller immediately after spreading to release any trapped air bubbles, which can cause pinholes and surface irregularities.
- Work quickly and keep a consistent pace.
Step 6 — Protect and Cure
- Keep foot traffic off the compound for the time specified on the data sheet (often 2–4 hours for light foot traffic, longer before laying flooring).
- Avoid draughts and direct sunlight during curing, as rapid drying can cause surface cracking.
- Check flatness with a 2-metre straightedge once cured. For most flooring types, a tolerance of ±3 mm under a 2 m straightedge is acceptable, though your flooring manufacturer may specify tighter tolerances.
When to Call a Professional
If you're levelling a large area (over 20–30 m²), dealing with significant height variation (more than 15–20 mm), or preparing a substrate for expensive flooring like engineered timber or large-format tiles, it's worth getting a qualified flooring installer involved. Mistakes at this stage can cause flooring failures that are costly to fix. You can find experienced local flooring professionals on tradedirectory.co.nz.
Also note: if your levelling work is part of a larger building project, check whether a building consent is required with your local council. Minor floor repairs generally fall under Schedule 1 exemptions, but confirm with your council — MBIE Building Performance has useful guidance on what work requires consent.
