Open-jointed flat fibre-cement sheet

Fibre-cement sheet is available in a range of sheet sizes and thicknesses and in a range of material compositions. The durability and performance of different systems varies considerably. Designers should ensure that the specified system is fit for purpose, and designers and builders should be aware of all the system’s installation requirements. 

This is also usually a proprietary system that incorporates a range of compatible components and must always be installed in accordance with the manufacturer’s instructions.

Fibre-cement is a very absorbent material, and the sheets must be painted or coated on the front and back to be made weathertight and durable. Dark colours should be avoided to prevent distortion (although there are some compositions that are more stable). Some types of sheets come with all faces primed or sealed and with sealed edges.

The sheets are installed with an 8–10 mm gap between sheets, both vertically and horizontally, over a neoprene or butyl (or similar) backing strip behind the joints that restricts water penetration. The system is very reliant upon the installation and robustness of the joint backing system for weathertight performance.

The entire installed system is then coated with an exterior finish paint system to make it weathertight, and this requires regular maintenance to ensure that it remains weathertight. Uncoated sheet edges are very vulnerable to moisture absorption, and they can wick water from adjacent surfaces. Sheet edges need to be well coated. 

The weathertight performance of these systems is very dependent on careful detailing and construction, as horizontal joints in particular have a high failure rate. Consider the use of a specialist installer. Warranty requirements also often call for installation by a manufacturer-approved specialist.

Open-jointed flat fibre-cement sheet claddings are not covered by E2/AS1. Check the manufacturer’s specification regarding installation requirements for weathertightness risk and the requirements for a cavity.

Open-jointed flat fibre-cement sheet systems are vulnerable to:

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Flush-jointed flat fibre-cement sheet

Fibre-cement sheet is available in a range of sheet sizes and thicknesses and in a range of material compositions. The durability and performance of different systems varies considerably. Designers should ensure that the system that has been specified is fit for purpose, and designers and builders should be aware of all the system’s installation requirements.

This is also usually a proprietary system that incorporates a range of compatible components and must always be installed in accordance with the manufacturer’s instructions. The sheets are installed with a 3–5 mm gap between sheets and rebated sheet edges, and the joints are then taped over with a flush-jointed fibreglass tape and jointing compound. Under E2/AS1, these systems must be installed over a drained and vented cavity for all risk scores.

Expansion or movement control joints must always be incorporated in the finished cladding in accordance with the manufacturer’s requirements.

Fibre-cement is a very absorbent material, and the sheets must be coated on the outside to make them weathertight and durable. This is done with a texture-finish plaster that incorporates a paint exterior finish, or the textured plaster must be coated with a paint exterior finish after it has been applied. Dark paint colours must be avoided to prevent thermal movement that can stress the sheet jointing system and the coating, as this can result in cracks that allow moisture to penetrate.

Some types of sheets come with all faces primed or sealed and with sealed edges, which makes them more robust and not totally reliant on the coating system.

Texture-coated flush-jointed fibre-cement sheets are used to create a monolithic finish on a building. The paint coating exterior finish creates a weatherskin and a face seal to the cladding that is very airtight. Uncoated sheet edges are very absorbent, and they will also wick water from adjacent surfaces such as waterproof decks and roofs. All exposed sheet edges must be well coated.

As the weathertight performance of these systems is very dependent upon accurate installation, consider the use of a manufacturer-approved specialist installer. (Warranty requirements often call for installation by a manufacturer-approved specialist.) Jointing and coating systems must always be installed by a manufacturer-approved specialist. 

Check the manufacturer’s specification regarding installation requirements for weathertightness risk.

Owners should also be made aware that these systems are very high maintenance and that the face seal coating system must be well maintained. Any faults in the cladding must be repaired immediately as they are intolerant of moisture penetration.

Texture-coated flush-jointed fibre-cement sheet systems are vulnerable to:

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Plywood sheet

Plywood sheet cladding is available in a variety of timber species and in a range of sheet thicknesses and sizes. The exterior sheet face is either plain or band sawn and can incorporate shallow decorative grooves.

When not manufactured from a durable timber (such as cedar), exterior plywood needs to be treated to a minimum level of H3.1 (or H3.2 if it is to be left uncoated) and be a minimum of 5 ply and 12 mm thick. 

Uncoated plywood is very absorbent but also very durable. A paint or stain exterior coating system will increase its weathertightness and durability, and this is a requirement to meet the durability requirements of the Building Code. Edge joints are very moisture absorbent, and these must always be coated. 

Treatment requirements for plywood sheet are covered in NZS 3602.

Open-jointed plywood sheet

These sheets are installed with an 8–10 mm gap between sheets, both vertically and horizontally, over a neoprene or butyl (or similar) backing strip behind the joints that restricts water penetration. The system is very reliant upon the installation and robustness of the joint backing system for weathertight performance. It is not covered by E2/AS1.

In some cases, the horizontal joints incorporate a suitably durable metal Z flashing that underflashes the upper sheet and overflashes the lower.

Open-jointed plywood sheet is considered to be a robust cladding system with good weathertight performance and can be direct-fixed on buildings with a risk level of 6 or below, or over a drained and vented cavity for higher-risk buildings when using E2/AS1 as a means of compliance.

Open-jointed plywood systems are vulnerable to:

Vertical-lapped plywood sheet 

The plywood sheets are manufactured with a side lap rebate (including a weathergroove) on one side and an overlap tongue on the other side. Sheets are installed close butted vertically with a 2 mm expansion gap between sheets. E2/AS1 does not cover this form of installation.

Sheets must be nailed through the lap rebate, and the adjoining sheet must be nailed through the face clear of the lower sheet rebate to allow sheets to move independently. 

The horizontal joints should be 10 mm deep and be protected with a suitably durable metal Z flashing that underflashes the upper sheet and overflashes the lower.

Internal and external corners must be backflashed or covered with cover battens.

Vertical-lapped plywood sheet systems are considered to be robust cladding systems with good weathertight performance when using E2/AS1 as a means of compliance.

Vertical-lapped plywood systems are vulnerable to:

Batten-jointed or proprietary jointer plywood sheet

Standard square-edged sheets are used, and these should be installed with a 2 mm vertical expansion gap between sheets. A suitably treated timber cover batten, a minimum size of ex 75 x 25 mm and incorporating two weathergrooves, is fixed centrally over the vertical joints by nailing through the gap between the sheets into the framing.

The horizontal joints should be 10 mm deep and protected with a suitably durable metal Z flashing that underflashes the upper sheet and overflashes the lower.

Internal and external corners must be backflashed or covered with cover battens.

Batten-jointed or proprietary jointer plywood sheet systems are considered to be robust cladding systems with good weathertight performance and can be direct-fixed on buildings with a risk level of 6 or below, or over a drained and vented cavity for higher-risk buildings when using E2/AS1 as a means of compliance.

Batten-jointed plywood systems are vulnerable to: