A knee wall is a short partition wall built beneath a sloped or raked ceiling — common in attic conversions, upstairs bedrooms, and dormer rooms. With basic carpentry skills, the right materials, and a clear plan, most homeowners can tackle this project themselves. Read on for a practical, NZ-focused guide.
What Is a Knee Wall and Do You Need Consent?
A knee wall is typically 900–1200 mm tall and runs parallel to the roof slope, creating a vertical face that hides the awkward triangular void behind it. The space behind is often used for storage.
In New Zealand, building consent may or may not be required depending on your council and the scope of work. Under Schedule 1 of the Building Act 2004, some minor internal work is exempt — but this depends on whether the wall is structural, affects fire egress, or involves electrical or plumbing work. Always confirm with your local council before starting. You can find general guidance at MBIE Building Performance.
Tools and Materials You'll Need
Tools:
- Tape measure, pencil, chalk line
- Spirit level and plumb bob
- Circular saw or drop saw
- Framing nail gun or hammer and nails
- Drill/driver
- Speed square or sliding bevel
Materials:
- 90×45 mm H1.2 treated timber framing (for bottom plate, top plate, and studs)
- 75 mm or 90 mm framing nails
- 10 mm or 13 mm GIB plasterboard for lining (if finishing)
- GIB stopping compounds and tape
- Insulation batts (if required — see below)
- Skew nails or joist hangers for fixing to rafters
Step 1 — Plan Your Layout
Measure the length of the wall and decide on the height. A common knee wall height is 900–1200 mm from the finished floor. Mark the bottom plate position on the floor with a chalk line, ensuring it is square to the room. Use a plumb bob or level to transfer the line up to the rafter or ceiling structure above — this is where your top plate will fix.
Because the ceiling is sloped, your top plate will need to be cut at an angle to sit flush against the underside of the rafters. Measure the roof pitch angle with a sliding bevel or digital angle finder and transfer that angle to your saw.
Step 2 — Fix the Bottom Plate
Cut your bottom plate to length from 90×45 mm framing timber. Fix it to the floor joists using 90 mm nails or 75 mm structural screws. If you're fixing into a concrete slab, use a powder-actuated nailer or masonry anchors. Make sure the plate is straight and level.
Step 3 — Fix the Top Plate
Cut the top plate with the roof pitch angle on the top edge so it sits snugly against the rafters. Skew-nail or screw it up through the rafters from above, or use metal framing connectors for a stronger connection. Check it is directly above the bottom plate using a plumb bob.
Step 4 — Cut and Install the Studs
Knee wall studs are typically spaced at 600 mm centres (or 400 mm if you want a stiffer wall or are using heavier lining). Because the top plate is angled, each stud will be the same length if your floor is level — measure carefully and cut a test stud first.
The top of each stud needs a bevel cut matching the roof pitch so it sits flush under the angled top plate. Nail through the plates into the studs using two nails at each end (skew-nailed or end-nailed depending on access).
Step 5 — Insulation
If the void behind the knee wall is part of your thermal envelope, you should insulate both the knee wall itself and the sloped ceiling above. New Zealand's Energywise guidance and the NZ Building Code (Clause H1 — Energy Efficiency) set minimum insulation requirements that vary by climate zone. Check what R-value is required in your region. Fit insulation batts between studs before lining.
Step 6 — Line and Finish the Wall
Fix GIB plasterboard to the stud face using the manufacturer's recommended fastener spacing. Stop and tape all joints, apply two or three coats of stopping compound, sand smooth, then prime and paint. Resene and Dulux both offer good interior paint ranges suited to bedroom walls.
If you want a timber or panel finish instead of plasterboard, tongue-and-groove lining boards or MDF panels are popular alternatives.
Step 7 — Access Door (Optional)
If you want to use the void behind the knee wall for storage, frame a rough opening for a small access hatch or door before lining. A standard hatch is around 600×600 mm, but you can make it larger if the space allows. Frame the opening with a header and trimmer studs, then fit a pre-hung door or a simple plywood hatch.
When to Call a Professional
While framing a knee wall is within reach of a competent DIYer, there are situations where you should bring in a qualified tradesperson:
- Structural concerns: If rafters or joists need to be cut or modified, this is likely restricted building work requiring a Licensed Building Practitioner (LBP).
- Electrical work: Relocating or adding power points or lighting circuits is restricted electrical work and must be done by a registered electrician — see EWRB.
- Plumbing: Any changes to pipes or drainage must be carried out by a licensed plumber — see PGDB.
- Consent uncertainty: If you're unsure whether consent is needed, consult your council or an LBP before starting.
You can find qualified local carpenters and builders on tradedirectory.co.nz.
