Replace Plinth and Drainage Upgrade

[sylviawang1113]

Hi there, I am looking for some advice on a multi-step repair project for my weatherboard house.

The Situation:

• Rotted Plinth & Cavity: A section of the plinth board has failed, revealing a cavity where I can see the Bottom Plate and a Concrete Stump. All the ends of the boards/plinth are covered with lawn now.
• Low Clearance & Ventilation: The gap between the internal floor level and the external lawn is very tight. This has caused poor sub-floor ventilation and is a major concern for termite ingress and ongoing moisture damage.

To seal the cavity, fix the drainage, and prevent future moisture ingress to protect the foundations, my Proposed Step-by-Step Plan:

• Phase 1: Plinth Repair & Sealing
• Dig out a 200mm wide trench along the wall to expose the damage.
• Remove rot and treat remaining timber with a wood preserver.
• Insert H4 Treated Pine blocks to support any compromised sections.
• Use Expanding Foam to seal the internal cavity (air/water seal) and install a new sheet as the external plinth.
• Phase 2: Waterproofing
• Apply a Bitumen-based waterproofing membrane to the new plinth and any timber-to-ground contact points.
• Ensure a proper "drip edge" logic so water doesn't wick behind the new plinth.
• Phase 3: French Drain & Landscaping
• Lay a Socked Agi Pipe at the base of the trench, sloped away from the house
• Use Weed Mat and 20mm Blue Metal (Gravel). I will ensure the gravel level remains at least 50-75mm below the bottom weatherboard to maintain airflow to the sub-floor.

My Questions for the Community:

1. Does this workflow seem correct for a long-term fix?
2. Is Expanding Foam the best way to seal such a large cavity in a weatherboard wall, or should I consider other materials?
3. Are there any specific "flashings" I should install between the new plinth and the existing weatherboard to ensure water sheds correctly?
4. Does the Agi-pipe need to be connected to the stormwater main, or can it discharge to a lower point on the lawn?

I've attached some photos of the rotted area and a sketch of my proposed drainage plan. Any advice or product recommendations would be greatly appreciated!

Thanks in advance!

[MitchellMc]

Welcome to the Bunnings Workshop community @sylviawang1113. It's sensational to have you join us, and many thanks for your question about sealing up your home.

You’ve clearly put a lot of thought into this, and overall, the approach you’ve outlined is quite reasonable and follows a logical sequence for addressing moisture, drainage and long-term durability. The steps you’re proposing are broadly in line with how this type of issue is typically tackled.

Based on what I can see and the way the wall is constructed, I’m assuming the house was built prior to the mid 1990s. In your first couple of images, particularly on the lower left-hand side, it appears that there was originally a cement sheet installed in front of the plinth. This sheet would have tucked up behind the weatherboards and extended down towards the ground, effectively acting as a sacrificial barrier so that water dripping off the weatherboards hit the cement sheet rather than the timber plinth itself before draining away.

When you reinstall the plinth, waterproofing it with a bitumen-based coating is a good step, but you may also want to consider reinstating a strip of cement sheet in front of it, similar to what was there originally. That layer does a very good job of keeping moisture off the timber and takes the brunt of weather exposure over time.

Before anything else, though, and this is really important, given the age of the house, that cement sheet is highly likely to contain asbestos. You should not break it up or remove it without having it tested first. If it does contain asbestos, it should be removed by professionals before you proceed any further. There may also be fragments of it already in the soil around that area, so caution is needed when digging and cleaning up.

In terms of sealing the cavity, expanding foam can be useful, but for a cavity of that size, it’s generally better to pack out as much as possible with solid timber first. If you can fit treated timber infills so they block most of the void, that gives you a far more stable and durable base. Expanding foam can then be used to seal smaller gaps and irregular spaces, provided it is fully enclosed and protected by the external cement sheeting so it’s not exposed to weather or UV.

There isn’t really a separate flashing as such in this type of construction. Historically, the cement sheet performed that role by ensuring water shedding off the weatherboards never contacted the timber plinth. Reinstating that detail achieves the same outcome and avoids relying solely on sealants or coatings.

Your drainage plan with the socked agi pipe and gravel trench makes sense, and keeping finished levels well below the bottom weatherboard is important for ventilation and termite risk management. As for where the agi pipe discharges, that’s something you should confirm with your local council. Connecting it to stormwater is generally the preferred solution, as it removes the water entirely from the area. Discharging to a lower point in the yard can sometimes be acceptable, but councils often have specific requirements around this, so it’s worth checking before committing.

Overall, you’re on the right track. Just make asbestos identification and management your first priority, then focus on reinstating the original water shedding logic of the wall, combined with improved drainage and clearances. That combination will give you the best chance of a durable, long-term fix.

Please let me know if you have any questions.

Mitchell