If your new home ends up with a hairline crack or a raised ridge running along a ceiling joint a year after handover, there's a good chance the joint wasn't backblocked. It's a step that's invisible once the ceiling is up, which is exactly why it gets skipped.

C12 - Fixing Stage Inspection Checklist

$119.00 AUD

Last updated 28 Oct 2026 · Revision 5 · Free updates for life

Checklist Type: Major (Fixing Stage)

Form Factor: Fillable, print-ready PDF (A4, 40 pages)

When to use: Fixing stage: plasterboard, eaves, joinery, fixout carpentry and tiling

A 159-point fixing stage checklist. Inspect plasterboard, eaves, joinery, doors, skirtings and tiling over two visits, before the stage is signed off.

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What is backblocking?

Backblocking means gluing strips of plasterboard to the back of the sheets across a joint that falls between framing members. The backblock bridges the joint so the two sheet edges can't move independently.

That independent movement is what causes:

  • Joint cracking, a fine line along the joint
  • Peaking, a visible ridge along the joint line, most obvious under side light from a window or downlight

Backblocking isn't about structural strength. Plasterboard isn't load-bearing. It's about crack and peaking control, which is what you'll actually notice living in the house.

Where should backblocking be used?

The Australian Standard for plasterboard installation, AS/NZS 2589:2017, sets out where backblocking applies:

  • Butt (end) joints between framing members in walls and ceilings must be backblocked. These are the joints where the short, square ends of two sheets meet.
  • Recessed edge joints in ceilings are strongly recommended to be backblocked. These are the long factory edges of the sheet, which are tapered to take the tape and compound.
  • Mid-span butt joints used to cut down plasterboard waste must also be backblocked.

For fire-rated or acoustic-rated walls, follow the manufacturer's tested system exactly. That system may or may not include backblocking, and you can't swap details around without losing the rating.

How backblocking is installed

  1. Make sure the back of the sheets is clean and free of dust.
  2. Cut backblocks from plasterboard offcuts, sized to fit between the framing (joists, trusses or battens).
  3. Using a notched trowel, apply backblocking compound to the backblock.
  4. Press the backblock firmly onto the back of the joint, centred across it.
  5. Hold the joint in a slight depression with a temporary timber batten and packers (around 3mm) on the face side while the compound sets. This leaves a shallow hollow for the plasterer to fill, so the finished joint sits flat instead of proud.
  6. Let the backblocks fully set before taping and stopping the joints.

Two ways to do ceilings

  • After hanging. Fix all the ceiling sheets, then climb into the roof space and backblock the joints. It works, but it's hard going in a low-pitched roof.
  • As you go. Hang the first sheet, apply the backblocks to its edge so they overhang, then lift the next sheet into place so it beds onto them. Most plasterers find this quicker.

Video

This video shows part of the process. It leaves out the temporary batten and packer, which are what create the depression in the sheet while the backblocked joint dries. Don't skip that step.

What to ask your builder

  • Are the ceilings backblocked, and at which joints?
  • Which level of plasterboard finish is in the contract?
  • Can I see the ceilings, or photos of the roof space, before the insulation goes in?

Once the insulation is laid, backblocks are hard to check. The fixing stage is the time to look, and our C12 Fixing Stage Inspection Checklist covers it.

Backblocking costs a bit of extra time and compound. Compared with patching cracks and repainting ceilings during the defects period, it's cheap insurance.

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