How to get a seamless external fibre cement corner joint?

Question

Hello,

I am trying to figure out the best way to create a seamless external fibre cement sheet corner joint using 6mm exterior cladding sheets to try and achieve an exterior rendered look without rendering. Would using Siniat 30 x 2400mm 90° External Angle Plaster Trim and a fibre cement joint compound work?

Dave-1 · Answer

Good Evening @bhawke30

I havnt tried having a seamless join on fibreboard, and was trying to think of ways that you could. What @EricL has specified adds a lot of things to consider. The differance in movement is the biggest one for me. I would stick to his recomendations. If you post up a picture of the area, we may be able to come up with other ways to deal with the corner.

Dave

EricL · Answer

Hello @bhawke30

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about fibre cement corner joints.

Using a plasterboard external angle with joint compound is generally not recommended for forming seamless external corners in 6 mm fibre-cement cladding. While it can appear acceptable initially, it falls outside the intended use of both plasterboard trims and most joint compounds, and it does not account for the movement and exposure conditions associated with exterior fibre-cement systems.

The Siniat 30 x 2400mm 90° External Angle Plaster Trim is designed for interior plasterboard applications where the bead is embedded in interior joint compound to form a crisp corner. Plasterboard systems are comparatively stable and protected from weather, whereas fibre cement expands and contracts with temperature and moisture changes. Exterior exposure also introduces UV, rain and thermal cycling, which brittle plaster-style compounds are not designed to tolerate. As a result, burying a plasterboard bead over fibre cement commonly leads to cracking along the bead edges or loss of adhesion over time.

Fibre-cement manufacturers instead specify either dedicated corner trims or reinforced base-coat systems. A typical example is a PVC extrusion such as the James Hardie 6mm PVC External Corner Angle, which is intended to be installed with the sheets to conceal edges while still allowing minor movement. These trims are not normally fully buried like plasterboard beads; rather, they form the corner profile and can be lightly skimmed to reduce visibility.

Plasterboard beads rely on brittle joint compound that is designed for stable interior plasterboard surfaces, whereas fibre-cement cladding expands, contracts and is exposed to moisture and temperature cycling outdoors. Because of this, exterior fibre-cement systems typically avoid buried plaster-style corners and instead use trims, sealant joints, or reinforced base-coat systems that can tolerate movement.

Mixing plasterboard trims with fibre-cement sheets introduces different materials with different movement and adhesion characteristics, which increases the risk of cracking or delamination over time. Staying within fibre-cement-appropriate accessories and coatings keeps the system compatible and aligned with manufacturer guidance, and is far more likely to achieve a durable rendered-look finish without long-term failures.

Let me call on our experienced members @Nailbag, @Dave-1 and @Remarka6le for their recommendations.

If you need further assistance, please let us know.

Eric