In most cases, yes — fitting a new internal window sill board (also called a window stool or sill lining) is a task many capable homeowners can tackle themselves. However, because you're working in the context of a reclad, there are some important building code considerations, consent implications, and sequencing issues to be aware of before you pick up a saw.
What Is an Internal Window Sill Board?
An internal window sill board is the horizontal timber or MDF board fitted on the inside face of a window opening, sitting between the window frame and the internal wall lining. It provides a neat finish, protects the substrate, and can serve as a small shelf. After a reclad, the window joinery is often repositioned or replaced, which means the internal sill board typically needs to be replaced or refitted to suit the new reveal depth.
Understanding the Reclad Context
A reclad is a significant building project — usually triggered by weathertightness failures — and almost always requires a building consent. Before you do any internal finishing work, confirm with your builder or project manager that:
- The building consent has been issued and the reclad work has passed all required inspections.
- The Code Compliance Certificate (CCC) process is underway or complete, or that your internal finishing work won't interfere with outstanding inspections.
- The window joinery installation has been signed off, as windows are a critical part of NZ Building Code Clause E2 (External Moisture) compliance.
Fitting an internal sill board is generally considered finishing work and is unlikely to require its own consent, but always check with your local council if you're unsure. Some councils may want to inspect internal linings before they're closed up.
Is It Restricted Building Work?
Internal window sill boards are not restricted building work (RBW) under the Building Act. RBW covers structural, weathertightness-critical, and certain other elements that must be carried out or supervised by a Licensed Building Practitioner (LBP). A decorative internal sill board doesn't fall into this category, so a homeowner can legally install one themselves.
That said, if the sill board installation involves making good the internal plasterboard lining around the window reveal, be aware that GIB plasterboard stopping and finishing is also DIY-legal but does require care to achieve a good result.
What You'll Need
- Timber or MDF sill board — common widths are 140 mm to 290 mm depending on your reveal depth; solid pine or finger-jointed pine are popular choices available at Mitre 10 or PlaceMakers
- Tape measure, pencil, square
- Mitre saw or hand saw
- Construction adhesive (e.g. Sika flexible adhesive) and/or brad nailer
- Sandpaper and primer
- Paint — a moisture-resistant trim paint from Resene or Dulux is ideal for window sills
- Flexible paintable sealant/caulk for the joints
Step-by-Step Overview
1. Measure the Reveal
Measure the width of the window opening between the side linings (jamb linings), and the depth from the window frame face to the face of the internal wall lining. Add a small horns overhang (typically 10–15 mm each side) for a traditional look.
2. Cut the Sill Board
Cut the board to length, allowing for the horns. If the wall isn't perfectly square, scribe the back edge to fit snugly against the plaster or lining.
3. Prime All Faces Before Fitting
This is important — prime all faces and edges, including the underside and back, before installation. This prevents moisture absorption and reduces the risk of warping, which is especially relevant in NZ's varied climate.
4. Fix the Sill Board
Apply a bead of flexible construction adhesive to the substrate and/or use a brad nailer through the top face. Ensure the board is level. Fill nail holes with a suitable filler.
5. Seal the Joints
Run a bead of flexible paintable sealant along all joints — between the sill and the window frame, and between the sill and the side linings. This prevents moisture ingress and cracking as the timber moves seasonally.
6. Paint
Apply at least two topcoats of a quality trim enamel or semi-gloss paint. Window sills cop a lot of wear and condensation, so don't skimp on paint quality.
Tips for a Professional Finish
| Step | Common Mistake | Best Practice |
|---|---|---|
| Measuring | Measuring only once | Measure twice, cut once |
| Priming | Skipping the underside | Prime all six faces |
| Sealing | Using rigid caulk | Use flexible paintable sealant |
| Painting | One coat only | Two or more topcoats |
When to Call a Professional
Consider engaging a qualified builder or joiner if:
- The window reveal is complex, out of square, or involves curved or bay windows.
- The internal linings around the window are damaged and need significant repair — this may edge into work that benefits from LBP oversight.
- You're not confident with accurate scribing and cutting.
- The reclad project still has outstanding inspections and you don't want to risk covering anything up prematurely.
A skilled local builder can knock out a window sill board quickly and ensure it integrates perfectly with the rest of the reclad finishing. You can find qualified builders in your area on tradedirectory.co.nz.
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