Undercutting a door frame (also called jamb cutting) is one of the most important finishing steps when laying new flooring. Instead of awkwardly notching your flooring around the door stop and architrave, you cut the bottom of the casing so the flooring slides cleanly underneath — giving a result that looks like it was done by a professional. With the right tool and a bit of patience, most confident DIYers can tackle this job safely.
Why Undercutting Matters
When you install new flooring — whether it's timber, laminate, vinyl plank, or tile — the finished floor level rises. If you don't undercut the door frame, you'll be left with an ugly gap or you'll have to cut awkward shapes into your flooring to fit around the casing. Undercutting gives the flooring a clean, tucked-in look and is considered standard trade practice in NZ for all floating and glue-down floor installations.
Tools You'll Need
- Jamb saw (also called an undercut saw or oscillating multi-tool with a flush-cut blade) — the most accurate option
- Hand saw — works fine if you're careful and patient
- Oscillating multi-tool — very handy in tight spots
- Offcut of your new flooring — used as a depth guide
- Pencil or marking knife
- Chisel and mallet — for cleaning out waste
- Vacuum or brush — to clear debris
- Safety glasses and dust mask — essential; older NZ homes may have lead paint on door frames (see safety note below)
You can find jamb saws, oscillating multi-tools, and related accessories at Bunnings or Mitre 10.
Safety Note — Lead Paint
Many NZ homes built before 1980 may have lead-based paint on door frames and architraves. Before cutting, check with a lead paint test kit (available at hardware stores). If lead paint is present, wear a P2 respirator, contain dust carefully, and follow WorkSafe NZ guidelines for working with lead paint. Do not sand or grind painted surfaces without appropriate protection.
Step-by-Step: How to Undercut a Door Frame
1. Prepare the Area
Remove any existing skirting boards or door stops that are in the way. Sweep or vacuum the subfloor so it's clean and flat. Make sure your subfloor is ready before you start cutting — you don't want to undercut at the wrong height.
2. Set Your Cutting Height
Place an offcut of your new flooring flat on the subfloor right next to the door frame. If you're using underlay beneath the flooring, stack the underlay offcut underneath the flooring offcut as well. This stack becomes your depth guide — it tells you exactly how high to cut.
3. Mark the Cut Line
Rest your saw flat on top of the flooring offcut and mark a pencil line across the face of the door casing at that height. Do this on both sides of the door frame (the two vertical jambs) and across the door stop if needed.
4. Make the Cut
Using a jamb saw: Rest the saw's flat base on the flooring offcut and cut horizontally across the casing. The offcut acts as a guide, keeping your cut perfectly level. Work slowly and let the saw do the work.
Using an oscillating multi-tool: Fit a flush-cut blade, rest it on the offcut, and carefully cut along your marked line. This tool is excellent for getting into tight corners.
Using a hand saw: Lay the saw flat on the offcut and cut carefully along your pencil line. Keep the saw level — this is the trickiest method but works well with practice.
5. Remove the Waste
Use a chisel and mallet to gently knock out the cut section of casing. Work carefully so you don't split the wood above the cut line. Clean out any remaining debris with a chisel or vacuum.
6. Test the Fit
Slide a piece of your actual flooring into the gap. It should slide in smoothly with a snug but not tight fit — you still need a small expansion gap (typically 8–10 mm) between the flooring end and the wall behind the casing, as required for floating floors. Check your flooring manufacturer's instructions for the exact expansion gap required.
7. Repeat for All Door Frames
Work your way around the room, undercutting every door frame, wardrobe frame, and any other vertical casing the flooring will run under.
Cutting Depth Reference Table
| Flooring Type | Typical Thickness | Underlay | Approx. Total Cut Height |
|---|---|---|---|
| Laminate | 8–12 mm | 2–3 mm | 10–15 mm |
| Engineered timber | 12–15 mm | 2–3 mm | 14–18 mm |
| Solid timber | 18–21 mm | None (glue/nail) | 18–21 mm |
| Vinyl plank (LVP) | 4–8 mm | 1–2 mm | 5–10 mm |
| Ceramic/porcelain tile | 8–12 mm + adhesive | None | 12–18 mm |
Always use your actual flooring and underlay offcuts as the guide — these figures are approximate only.
Common Mistakes to Avoid
- Cutting too high — leaves a visible gap above the flooring
- Cutting too low — flooring won't slide under and you'll need to recut
- Forgetting the underlay in your depth stack
- Not allowing for expansion gaps — floating floors must have room to move
- Rushing the cut — a slow, steady pass gives a much cleaner result
When to Call a Professional
Undercutting door frames is generally a straightforward DIY task, but consider calling a qualified flooring installer if:
- You're working with expensive hardwood or engineered timber where mistakes are costly
- The door frame is structural or load-bearing and you're unsure what's inside the wall (check with a Licensed Building Practitioner)
- You have lead paint present and aren't confident managing the hazard safely
- You're tiling and need to ensure the substrate and waterproofing (relevant to NZ Building Code clause E3) are correctly handled in wet areas
- The overall flooring installation is complex or large-scale
A skilled flooring installer will undercut frames quickly and accurately as part of the job. You can find qualified local flooring professionals on tradedirectory.co.nz.
