To render over a wood substrate outside, such as plywood, you cannot apply render directly. Instead, a specialized render system must be installed onto the wood surface. The best course of action involves preparing the surface and installing several layers to create a suitable base for rendering, ensuring proper ventilation and moisture protection.
Based on the provided reference, the process involves the following key steps:
Preparing a Wood Substrate for External Rendering
Applying traditional render directly onto wood is problematic because wood expands, contracts, and is susceptible to moisture damage. A specialized system is required to provide a stable, weather-resistant surface that can accept render while protecting the underlying wood structure.
Here's a breakdown of the process:
1. Surface Preparation
- Ensure the underlying wood surface (e.g., exterior grade plywood or OSB) is clean, dry, and structurally sound.
- Address any repairs or imperfections before proceeding.
2. Installing the Breather Membrane
- Install a breather membrane onto your plywood. This layer is a crucial part of the system. It allows moisture vapour from inside the building structure to escape outwards while preventing external water (like rain or moisture from the render system) from reaching the wood.
3. Adding Timber Battens
- Install timber battens on top of the breather membrane. These battens are fixed through the membrane into the wood substrate.
- The battens create a vital ventilated cavity between the breather membrane and the next layer. This cavity allows air circulation, helping to dry out any moisture that might penetrate the outer layers or escape from the interior, preventing moisture buildup behind the render.
4. Attaching Render Carrier Boards
- Install render carrier boards on top of the timber battens. These boards are fixed to the battens.
- Render carrier boards are specifically designed sheets (often fibre cement or similar composite materials) that provide a stable, rigid surface suitable for accepting external render. They are dimensionally stable and resistant to moisture compared to wood.
5. Sealing the Carrier Boards
- Finally, the carrier boards must be sealed using carrier board tape.
- This tape is applied to the joints between the carrier boards and around openings like windows and doors. It creates a continuous, watertight layer on the surface of the carrier board, preventing water from penetrating the system through the board joints before the render is applied.
Once this entire system is correctly installed and sealed, you have created a stable, ventilated, and weather-protected base onto which an appropriate external render system (like a thin coat render or a compatible traditional render) can be applied.
Summary of Layers
Here's a simplified view of the system build-up from inside out:
- Exterior Wood Substrate (e.g., Plywood)
- Breather Membrane
- Ventilated Cavity (created by Timber Battens)
- Render Carrier Boards
- Sealing Tape (on carrier board joints)
- Applied Render System (e.g., Basecoat, Mesh, Topcoat)
This layered approach ensures the longevity and performance of the render system when applied over a wood structure outdoors.
