Technical guide · Pandaboard

Gypsum Cornice: Installation, Splicing and Storage

Gypsum cornice is supplied in 2000, 2400 and 3000 mm lengths, 60–200 mm wide, at roughly 1.5–3 kg per meter (confirm on order). It is fixed with gypsum adhesive or fast-setting plaster spread over the back, plus screws or brads at 300–400 mm — the combination of adhesive and mechanical fixing is what keeps the joint from cracking later. Corners are cut at 45 degrees, and the miter is filled and sanded. Gypsum softens when it absorbs water and has low bending strength, so keep site humidity at or below 70% and store flat, no higher than 1 m.

Gypsum cornice is used at ceiling angles, light coves, wall panel divisions and shaped terminations. Installation looks simple, but success lies entirely in the joints: whether the 45° cut is accurate, whether the pattern lines up and whether the profile is loaded before the adhesive sets decide whether it stays sound or cracks three years later.

01Key specifications and suitability

Suitable

• Living rooms and bedrooms in housing

• Hotel guest rooms and corridors

• Junction between a drywall ceiling and a masonry wall

• Pelmet, architrave and cove transitions

• Refurbishment where the wall and ceiling line is out of true

Not suitable

• Shower enclosures and steam rooms

• Directly above a cooking range

• External walls, balconies and semi-exterior areas

• Damp basements and garages

• Blown or flaking paint and plaster, which must be removed first

02Preparation Before Installation

2.1 Materials and tools

Table 2-1 Main materials and tools

Parts list and reference quantities：Measured by length (per 100 m of cornice), for standard 80–120 mm gypsum profiles.

2.2 Site conditions

• Ceiling and wall substrates accepted, flatness ≤3 mm/2 m, no hollow spots, no dusting;

• Wall skim coat completed and dry (the cornice is usually fixed to the skim or to the substrate, as specified);

• Indoor temperature ≥5 ℃, humidity ≤80 %;

• Bring profiles on site 24 h in advance and store them flat indoors to acclimatize;

• Snap the ceiling level line and the wall horizontal line as control lines for the top and bottom edges of the profile.

2.3 Quantity estimate

Calculate from the room perimeter and add 8 %–10 % waste (corner cuts waste the most). Cove cornice comes in 2.5 m lengths; a 4×5 m room with an 18 m perimeter needs about 8 lengths.

2.4 Delivery and site conditioning

• Bring cornice indoors 24 h ahead and store flat on a level platform — never upright or leaning, because the thin section cracks under its own weight

• One batch and one mould per room; section dimension tolerance 1 mm

• Spot-check 5 %: sharp arrises, no chipped ends, intact paper face

• Carry lengths over 2.4 m with two people supporting both ends, never from a single point

03Method 1: Adhesive and Pin (recommended)

Figure 1 Adhesive and pin fixing Adhesive dabs on the back, pins from the face; adhesive and pins share the load. Pin spacing 300~400.

1. Snap lines - Snap the upper and lower control lines for the profile on the ceiling and on the wall, closing the loop on both faces as the setting-out datum.

2. Measure and miter - Measure each length and cut 45° at the corners with a miter box and fine-tooth saw; the cut must be straight with no chipped edges.

3. Dry fit - Dry-fit the corners on the floor, check that the pattern aligns and the joint is tight, then fix to the wall.

4. Mix adhesive - Add water to the adhesive per the instructions until it is like toothpaste, let it stand 2 minutes, and mix only what can be used within 5 minutes.

5. Apply adhesive - Apply dabs to the back at 200–300 spacing, 8–10 mm thick; coat the ends and corners fully.

6. Set in place - Support both ends with both hands, press to the ceiling first and then against the wall, squeeze out excess adhesive and wipe it off immediately.

7. Pin - While the adhesive is still wet, fix with a drill driver or pin nailer at 300–400 spacing; sink the heads 1 mm without breaking the face.

8. Treat joints - Fill joints and screw holes with jointing compound, press joint tape in place, then level with a second coat.

9. Sand and paint - Once dry, sand lightly with 240# paper and apply two coats of emulsion paint.

Do not let go before the adhesive setsThe profile slides before the adhesive reaches initial set. After setting it in place you must hold it by hand or prop it for 3–5 minutes while pinning; only then release. This is the most common cause of cornice falling off right after installation.

04Method 2: Adhesive Only

For small-section profiles (≤80×80) on flat substrates; no pins, no visible holes.

• The substrate must be clean, free of dust and dusting; prime with bonding agent where absorption is high;

• Fully coat the back with adhesive (not dabs), 5–8 mm thick;

• Press firmly in place and prop for ≥30 minutes;

• Treat joints the same way: fill, tape, sand;

• Not for large sections, or ceilings where the gypsum board substrate is unreinforced.

When adhesive-only fixing is prohibitedWhere the section is >100×100, the length is >3 m, the substrate is insulation board or the hollow part of a lightweight partition, or the ceiling carries a vibration source (fans, adjacent elevator shaft), adhesive-only fixing will drop off because the bond area is insufficient or the substrate moves. Switch to adhesive and pin, or substrate fixing.

05Method 3: Substrate Fixing (large sections, features)

Roman columns, ceiling roses, large cove cornice (>150×150) and features that need support require a timber or light-gauge steel substrate:

1. Build the substrateUse 18 mm plywood or light-gauge steel framing matched to the profile section, fixed to the structure or to the wall studs.

2. Embed fixingsSet screws or hangers in the substrate at the profile position.

3. Set the profileApply adhesive to the back, seat the profile on the substrate, fix with the embedded items and add splice plates at the joints.

4. Fill and finishFill the joints, tape, sand and touch up the paint.

06Joints and Details

Figure 2 45° joints Internal corners: 45° on both faces; external corners and turns: 45° pair with the pattern continuous.

• Straight joints: cut a 45° scarf or a square end with a splice plate; joint width ≤1 mm; fill and tape;

• Internal corner: 45° on both faces, apply adhesive to the joint face and press; wipe off squeezed adhesive at once;

• External corner: 45° pair, pattern continuous, no offset;

• Terminations: use the matching end cap or corner ornament; never leave a cut section exposed;

• Light fittings and curtain boxes: install the body first, then the profile, overlapping the body edge by 3–5 mm.

07Quality Acceptance

Table 7-1 Acceptance requirements

08Common Problems

Table 8-1 Typical defects

09Product Storage and Handling

Figure 3 Stacking Gypsum cornice must be stored flat with paper between layers; upright or leaning storage breaks it under its own weight, and the break cannot be repaired.

9.1 Environment

Table 9-1 Storage conditions

9.2 Handling

• Carry at mid-span with both hands; never hold one end only — its own weight breaks it;

• Lengths over 2.5 m require two people;

• Do not throw, drag or lean against a wall;

• Stack cut-offs separately and wrap the cut ends in paper.

9.3 Shelf life

Recommended stock age ≤12 months. Damp profiles look darker, feel soft and sound dull when tapped; do not install them. Apply first-in-first-out (FIFO).

10Transport and Safety

• Load flat with paper between layers, stack height ≤1.0 m, corner protectors and strapping;

• Cover with a waterproof tarpaulin; no rain exposure;

• Handle gently when loading and unloading; no throwing;

• Wear goggles and a dust mask when cutting and sanding (gypsum dust is irritating);

• Use a trestle for overhead work; never stand on window sills or furniture.

11Appendix: Checklists

11.1 Goods receiving

• □Section size and pattern match the approved sample

• □Same batch number, uniform color

• □No breaks, no chipped corners, no soft damp sections

• □Length tolerance ≤3 mm, bow ≤2 mm/m

• □Packaging intact, interleaving paper between layers

11.2 Before installation

• □Ceiling and wall substrates accepted

• □Skim coat dry

• □Upper and lower control lines snapped and closed

• □Miter box, fine-tooth saw and adhesive on site

• □Dry fit on the floor completed

12Frequently asked questions

Can I fix cornice with adhesive only?

On short runs in a dry interior, yes, but adhesive plus screws is recommended. An adhesive-only joint cracks along the bond line when humidity or the structure moves; mechanical fixings spread that stress.

Do I need a miter box for 45 degree cuts?

Yes in practice. A hand saw rarely holds a 1 mm joint, while a miter box or chop saw keeps the two angles complementary. Measure the diagonals before cutting.

How soon can it be painted?

Only when the joint compound and filler are fully dry, usually 24 hours and longer in humid weather. Painting too early traps moisture and causes yellowing and blistering.

Can gypsum cornice go in a bathroom?

Not recommended. Gypsum loses strength as it absorbs water and supports mold, worst in a shower enclosure. Use moisture-resistant cornice or PU molding in wet areas.

Why does the joint keep cracking?

Usually too thin an adhesive bed, no mechanical fixing, substrate movement, or an unfilled miter. Spread full coverage, screw at 300–400 mm, and fill the joint with compound before sanding.

Can it be stuck onto an old painted wall?

No. Old filler, wallpaper paste or paint films will not hold the weight, and the cornice comes down with it. Strip back to plaster or masonry, prime, then fix.

About Pandaboard

Guangzhou Panda Commercial Development Co., Ltd. · Pandaboard / PEIDA

We manufacture and export gypsum board, mineral fiber and fiberglass ceiling tiles, calcium silicate and magnesium oxide boards, fiber cement board, aluminum ceilings, WPC and SPC wall panels, exterior decking, fencing and handrails, together with the matching grid and framing systems. The methods on this page are compiled from product behaviour and standard trade practice; for any project, follow the approved drawings, local codes and the specification confirmed on your order.

Website: www.pandamanufacturer.com Contact sales: contactus

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Guangzhou Panda Commercial Development Co., Ltd. · Pandaboard / PEIDA
This guide is technical information for specifiers and installers; it does not replace project drawings or local codes.