A GIB ceiling that sags or bows between its fixings is a common problem in New Zealand homes, particularly in older houses or areas exposed to moisture. In most cases the fix involves adding more screws and re-stopping the surface — a job a confident DIYer can tackle, though more serious sag may need a qualified plasterer.
Why Does GIB Sag Between Fixings?
Before reaching for the drill, it pays to understand what's causing the problem. The most common reasons include:
- Insufficient fixings – GIB recommends specific screw spacings depending on sheet thickness and joist centres. If the original installation used too few screws or nails, the sheet will gradually deflect under its own weight.
- Moisture damage – Leaks from plumbing, roofing, or condensation can soften the plasterboard core, causing it to lose rigidity. Check for water staining before doing any repair work.
- Joist spacing too wide – Thinner GIB board is only rated for closer joist centres — if joists are widely spaced, a thicker board is required. Check the GIB installation guide at gib.co.nz for the specific thickness and span requirements. Incorrect board thickness for the span is a frequent culprit.
- Original nails instead of screws – Older ceilings fixed with clouts (nails) are prone to nail-pop and gradual sag as the timber dries and shrinks.
- Structural movement – In some cases, deflection in the framing itself is the root cause.
Assess the Damage First
Gently press the sagging area with your hand. If the board feels firm and springs back, it's likely just under-fixed. If it feels soft, crumbly, or damp, you have moisture damage and need to fix the source of the leak before any plasterboard repair — otherwise the problem will return.
Also check whether the sag is isolated to one bay between joists, or whether it runs across multiple bays. Widespread sag may indicate a more serious structural or moisture issue worth getting a professional eye on.
What You'll Need
| Item | Notes |
|---|---|
| GIB drywall screws (25–32 mm) | Coarse thread, bugle head |
| Cordless drill/driver | With a drywall screw setter bit |
| GIB Basecoat or Finishing Compound | Available at Bunnings or Mitre 10 |
| Broad knife / plasterer's trowel | 150–200 mm |
| Fine sandpaper (120–180 grit) | For feathering |
| Primer and ceiling paint | Resene or Dulux ceiling paint |
| Safety glasses and dust mask | Essential |
Step-by-Step: Re-Fixing and Stopping
1. Locate the Joists
Use a stud finder or tap along the ceiling to find the joists. Mark their positions lightly with a pencil. Joists typically run at 400–600 mm centres.
2. Add Screws Across the Sagging Area
Drive GIB drywall screws through the plasterboard into the joists at no more than 200–230 mm intervals along each joist line. The screw head should sit just below the surface — dimpled in — without breaking through the paper face. If you're working on a ceiling, use a drywall screw gun or a drill with a depth-stop bit to make this easier and safer.
If the sag is mid-span between two joists and there's no timber to fix into, you may need to install noggings (short timber blocks between joists) from above, or use a GIB adhesive in conjunction with screws. Access from above (through the roof space) makes nogging installation straightforward.
3. Allow the Board to Pull Up
Once screwed, the board should draw back up toward the framing. For a significant sag, you can use a deadman prop (a T-shaped brace made from timber) to hold the sheet up while the screws are driven — this prevents the board from cracking under its own weight.
4. Stop the Screw Dimples
Using GIB Basecoat or a pre-mixed jointing compound, fill each screw dimple with a broad knife. Apply two or three thin coats, allowing each to dry fully (typically 4–6 hours between coats depending on humidity). Feather the edges out 150–200 mm from the dimple so the repair blends smoothly.
5. Sand, Prime, and Paint
Once the final coat is dry, sand lightly with 120–180 grit paper. Wipe away dust, then apply a coat of plasterboard primer before repainting. Skipping the primer will cause the repaired spots to flash (show through the paint finish).
Dealing With Moisture-Damaged GIB
If the board is soft or crumbling, it needs to be cut out and replaced — stopping over damaged board won't hold. Cut back to the nearest joists on either side, fix new GIB sheet, tape and stop all joints, then prime and paint. This is more involved work; if you're not confident, a plasterer will make short work of it.
When to Call a Professional
Call a qualified plasterer or builder if:
- The sag covers a large area or multiple bays
- The board is moisture-damaged and needs replacing
- You suspect the framing itself is deflecting or damaged
- You need to access the roof space to install noggings and aren't comfortable working at height
- The ceiling is textured (e.g. stipple or spray finish) and you need to match the existing texture
For structural concerns, you may also need a Licensed Building Practitioner (LBP). You can find qualified local plasterers and builders on tradedirectory.co.nz.
Safety note: Always wear safety glasses and a dust mask when drilling overhead and sanding plasterboard. Work from a stable platform — never a chair or an unstable ladder.
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