Gypsum core (plasterboard)
The gypsum core is the compressed calcium sulfate dihydrate panel between two paper faces in plasterboard. Fire-rated and impact-resistant cores explained.
Ask Chalkline about this →The gypsum core is the compressed calcium sulfate dihydrate panel sandwiched between two paper faces in a sheet of plasterboard. It is the load-bearing and fire-protective element of the sheet; the paper faces protect the gypsum from handling damage and provide the finish substrate for paint and tile.
The base ingredient is gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O), milled, calcined to dehydrate it partially, then re-hydrated and rolled between two paper layers as it cures into a rigid panel. The chemical bound water in the cured gypsum is what gives plasterboard its fire-resistance behaviour: under heat, the bound water boils off and absorbs energy before the core fails.
The four common core compositions:
| Core type | Marker | Where to use |
|---|---|---|
| Standard | ”Standard” or no marker on the back | General internal walls and ceilings, dry areas. |
| Fire-rated | ”F” or “Fire-rated” stamp (often pink paper face) | Boundary walls, garage-to-dwelling walls, intertenancy walls, ceilings over fire-rated zones. Adds glass fibres for cohesion at high temperature. |
| Impact-resistant | ”I” or brand-specific marker | High-traffic areas (commercial, garages, schools). Denser core composition. |
| Wet-area / water-resistant | ”W” or green paper face | Bathrooms, laundries, toilets behind tile (not as a tanking layer; the membrane does that). Core contains additives reducing water absorption. |
Most manufacturers (Knauf, Gyprock, USG Boral) produce a fire-and-impact dual-rated variant for use in walls that need both properties. Specialist sound-rated boards use a denser core or laminated multi-core construction for higher Rw values.
What core composition affects on site:
- Weight. Fire-rated and impact-resistant cores are heavier than standard by 10 to 20% per sheet. Lifting and handling become more demanding; a panel lifter becomes essential.
- Cut speed. Standard core scores and snaps easily; fire-rated and impact cores need a sharper score and sometimes a second pass on the back.
- Fixing torque. Over-driven screws crush impact-resistant and water-resistant cores less than standard, but the head still must be flush, not dimpled below paper.
- Tape and joint compound bond. All core types take the same paper or fibre-mesh tape and bedding compound; the paper face is what matters for adhesion.
For builders.
- Match the spec. A wall called up as 90 minutes FRL with 13 mm Fire-rated plasterboard cannot be substituted with standard 13 mm even if it looks identical.
- Sight the manufacturer stamp on the back before fixing. A jobsite mix-up of standard and fire-rated sheets (similar appearance from the front) is a common defect.
- Off-cut storage. Fire-rated off-cuts go into a separate stack from standard. Cutting from the wrong stack on a feature-rated wall is the most common BCA-non-compliance defect for plasterer-finished work.
Also known as: plasterboard core, drywall core, calcium sulfate core.
Category: Materials / linings / composition.
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Last updated: 2026-05-14. Verified: 2026-05-14.