Packing and shimming are two of the most fundamental — yet often overlooked — techniques in carpentry and construction. Whether you're hanging a door, installing a window, or levelling a deck post, getting things perfectly level, plumb, and square almost always requires a little fine-tuning. That's exactly what packing and shimming are for.
What Is Packing?
Packing refers to placing a solid material behind or beneath a structural or finishing element to bring it out to the correct position. In NZ construction, packing is commonly used to:
- Build out a framing member that is slightly undersized or bowed
- Sit a door or window frame at the correct depth in a wall
- Level a post base or bearer on uneven ground
- Fill gaps behind linings or cladding
Packing is typically a flat, consistent-thickness piece of material — often timber, plywood, or a purpose-made plastic packer.
What Is Shimming?
Shimming is closely related but usually refers to using tapered or thin wedge-shaped pieces to make fine adjustments. Shims are slid into a gap and pushed in or out until the correct alignment is achieved. You'll see shimming used when:
- Hanging a door so it sits square in its frame with even gaps all around
- Installing a window so it's perfectly plumb before fixing
- Levelling kitchen cabinetry on an uneven floor
- Aligning a staircase stringer or balustrade post
The terms are often used interchangeably on the tools, but in practice, packing = consistent thickness and shimming = tapered adjustment.
Why Does It Matter?
New Zealand homes — particularly older villas, bungalows, and even newer timber-framed houses built to NZS 3604 — are rarely perfectly square, level, or plumb. Timber moves with moisture and temperature. Foundations settle. Framing shrinks as it dries.
If you skip proper packing and shimming:
- Doors and windows will bind, stick, or fail to latch correctly
- Gaps will appear unevenly around frames, letting in draughts and moisture
- Structural loads may not transfer correctly through posts and bearers
- Linings and cladding can crack or bow over time
- Cabinetry will look crooked and drawers won't run smoothly
Getting this right at installation stage saves a lot of frustration and remedial work later.
Common Packing and Shimming Materials
| Material | Best Use | Notes |
|---|---|---|
| Hardwood timber packers | Structural and framing use | Durable, load-bearing |
| Plywood offcuts | General packing behind frames | Easy to cut to size |
| Plastic packers | Window and door frames, wet areas | Won't rot or compress |
| Composite shims | Fine adjustments, cabinetry | Pre-tapered, consistent |
| Folded galvanised steel | Heavy structural packing | Used under posts, bearers |
For anything in a wet area or exterior location, always use a non-compressible, moisture-resistant packer — timber will rot and compress over time, causing frames to drop or twist. BRANZ guidance on weathertightness highlights how poorly packed window and door frames are a common source of moisture ingress in NZ homes.
How to Shim a Door Frame: Basic Steps
This is a task many confident DIYers can tackle for interior doors. Here's the general process:
- Check the rough opening is close to square using a tape measure diagonally corner to corner — both measurements should match.
- Sit the door frame (or pre-hung door unit) in the opening.
- Use a spirit level to check plumb on both hinge and latch sides.
- Slide shims in pairs from opposite sides at hinge locations and mid-points, adjusting until the frame is plumb and the door gap is even (typically around 3 mm all round).
- Fix through the shims into the framing with appropriate nails or screws.
- Score and snap timber shims flush, or trim plastic shims with a utility knife.
- Check the door swings freely and latches without force before finishing.
Tools You'll Need
- Spirit level (at least 600 mm)
- Tape measure
- Hammer or nail gun
- Utility knife or handsaw
- Pencil
Packing for Structural Work
When packing is used in a structural context — such as under bearers, posts, or within a bracing system — it becomes part of the load path of the building. This work must comply with the NZ Building Code and, depending on the scope, may require a building consent. The MBIE Building Performance website is the best place to check whether your project needs consent.
Structural packing must be:
- Non-compressible under load
- Durable for the expected service life
- Correctly sized so loads are transferred evenly
When to Call a Professional
While shimming an interior door or levelling a cabinet is well within reach of a capable DIYer, there are situations where you should bring in a qualified carpenter:
- Exterior door and window installation — weathertightness is critical and errors can lead to costly moisture damage
- Structural packing under posts, bearers, or within bracing walls
- Consent-required work — a Licensed Building Practitioner (LBP) must carry out or supervise restricted building work
- When frames are badly out of square — this may indicate a deeper foundation or framing issue that needs professional assessment
- Staircase work — stairs have specific NZ Building Code requirements around rise, going, and balustrade heights
A skilled local carpenter will have the right packers, tools, and know-how to get the job done properly the first time. You can find qualified carpenters in your area at tradedirectory.co.nz.