Installing a new hard floor, such as timber or laminate, is an exciting way to transform a space. However, the success and longevity of your flooring project depend heavily on one critical step: proper subfloor prep. A well-prepped subfloor ensures your new floor looks great, feels solid, and performs as expected over time. In this guide, we’ll walk you through the key steps and standards for preparing your subfloor, so your hard floor installation goes off without a hitch.
Why Subfloor Prep Matters
Subfloors need to be relatively flat to accept floor coverings like timber or laminate. A poorly prepared subfloor can lead to serious issues down the line. For floating floors (those not glued down), an uneven base can make the floor feel spongy underfoot. For glued floors, imperfections can result in a hollow sound when walked on, indicating poor adhesion. The flatter you can get the subfloor, the better the end result—both in terms of aesthetics and functionality.
One important distinction to understand is that floors need to be flat, not necessarily level. Flatness refers to a smooth, even surface without significant highs or hollows, while level refers to a surface that’s perfectly horizontal. In older homes especially, floors may slope slightly but still be flat enough for a successful installation. With that in mind, let’s dive into the specifics of subfloor prep.
Step 1: Check for Flatness
Before you begin, lift all existing floor coverings—carpet, tiles, or anything else—to expose the subfloor. Carpets, in particular, can hide humps and hollows, so don’t assume the subfloor is ready just because it feels okay underfoot. Once exposed, use a straight edge (a long, rigid tool like a level or a metal ruler) to check the subfloor for flatness. Here’s what to look for:
- Hollows:
- If there are any hollows (dips) greater than 2mm over a 1-meter span, they need to be filled.
- If there are hollows greater than 1mm over a 20cm span, these also require filling.
Filling these dips ensures the subfloor provides consistent support for your new flooring. - Highs:
- If there are any highs (bumps) greater than 2mm over a 1-meter span, they need to be ground flat.
- If there are highs greater than 1mm over a 20cm span, these must also be leveled by grinding.
Removing these protrusions prevents rocking or uneven wear in your hard floor.
To fill hollows, use a high-quality leveling compound designed for the purpose. Cheap or low-quality levelers can crack or fail under the weight of your new floor, so invest in a product that’s up to the task. For grinding down highs, a floor grinder or sander may be necessary, depending on the subfloor material (e.g., concrete or timber).
Step 2: Clean the Subfloor
A clean subfloor is essential, but the level of cleanliness depends on your installation method. Here’s the breakdown:
- For floating floors: Since these use an underlay between the subfloor and the new flooring, some residue (like minor dust or small patches of old adhesive) can be left as long as the surface is flat. The underlay helps cushion and smooth out minor imperfections.
- For glued floors: The subfloor needs to be perfectly clean. Remove all old adhesives, building rubble, dust, and debris. Any leftover material can prevent the glue from bonding properly, leading to lifting or hollow spots.
Sweep, vacuum, and, if needed, scrape the subfloor to achieve the required cleanliness. For glued installations, a pristine surface also ensures leveling compounds adhere well when filling hollows.
Step 3: Ensure Structural Stability
Your subfloor must be sound and structurally stable to support the new hard floor. Inspect it carefully for:
- Loose boards, tiles, or sections of concrete.
- Cracks, rot, or other damage.
Any loose or unstable parts need to be secured, repaired, or replaced. For example:
- In a timber subfloor, nail or screw down creaky boards, and replace any that are rotted or broken.
- In a concrete subfloor, patch cracks with a suitable filler and remove any crumbling sections.
If the subfloor isn’t solid, your new floor won’t be either—no matter how flat or clean it is. Take the time to address these issues before moving forward.
Tools and Materials You’ll Need
- Straight edge: To check for flatness.
- Leveling compound: For filling hollows (choose a high-quality product).
- Grinder or sander: For smoothing highs.
- Broom/vacuum: For cleaning.
- Repair materials: Screws, nails, or concrete filler, depending on your subfloor type.
Final Thoughts
Proper subfloor prep might not be the most glamorous part of installing timber or laminate flooring, but it’s the foundation of a great result. By addressing hollows and highs, cleaning appropriately for your installation type, and ensuring structural stability, you’ll set the stage for a hard floor that looks fantastic and performs reliably for years to come. Remember: the goal is a flat, stable surface—not a perfectly level one. Floating floors offer a bit of forgiveness thanks to underlay, while glued floors demand perfection in cleanliness and prep.