nav[aria-label="Primary Navigation"] { padding: 0; & ul { list-style: none; width: 100%; display: flex; flex-direction: row; justify-content: start; align-items: start; gap: 30px; padding: 0; & li { margin: 0; } & ul li { list-style: none; } } }

How to protect fence plinth from rot?

PatDIY_Dad

What can I put against the timber fence bottom sleeper to protect it from wood rot

We are putting a garden bed here with plants and it will come up about 250mm-300mm just before the paving

Was going to put builders plastic (vapour barrier) as I already got it but just googled and Mr AI is telling me not to use plastic as it traps moisture and makes wood rot worse?

Would prefer not to make the width of the garden bed a y shallower as it's already at min shallow depth for the plants we want to put in

The railing is going to be a cladding wall. Cladding panels won't go all the way down to The bottom so soil will be in contact with bottom fence sleeper

Find more posts tagged with

Tools and Materials

Repairing

Building and Construction

Fencing and Screening

Accepted answers

All comments

MitchellMc

Hi @PatDIY_Dad,

As that bottom member is a timber sleeper, there’s a good chance it is H4 treated and therefore designed for ground contact. It’s the same treatment level commonly used for timber sleepers in retaining walls, so I wouldn’t be overly concerned about soil contacting it.

That said, there are a couple of ways you could provide additional protection. You could coat the garden-bed side of the sleeper with a suitable bitumen waterproofing membrane before adding the soil. This provides an additional barrier between the damp soil and the timber.

Another option would be to install an additional H4 sleeper against the existing fence sleeper, effectively creating a sacrificial retaining edge for the garden bed. The soil would then be retained by this sleeper rather than sitting directly against the structural components of the fence. Given you only need to retain around 250–300mm of soil, this could provide a neat and relatively straightforward solution.

I’d favour either of these approaches over simply placing loose builder’s plastic against the fence.

Please let me know if you have any questions.

Mitchell

PatDIY_Dad

Thanks for the reply

You'd think the fencing company would use h4 but I am concerned it isn't because there is another section nearby where we had a higher raised garden bed and we've removed it for pool renovation install

It looks like it's already rotting. Check the video I uploaded to YouTube for you to see ( cannot attach video here).

I can stab it with a screw driver and some of it can poke in. Can also dig it a little with my finger nail

I can probably call and ask them but those sleepers in the video don't give me confidence it's h4

Fence and house was built in 2019

I got bitumen paint also

Small tub . Would this better than plastic and can I paint over the bad section in the video? The soil has been removed for at least about 2 months so it should be dried out

I just want the quickest solution without loosing any more garden bed space as we're already on the minimum side for type of plants we are going to put in

Video:

https://youtube.com/shorts/9z-pI7LcB7s?si=L3o3TfoKjS2rZRUw

EricL

Hello @PatDIY_Dad

Thank you for sharing that video of your sleeper. It does appear that it has bit of surface softness, but the main core of the wood is still intact despite being buried in 2019. From what I've seen it appears that you can still paint over this piece with bitumen paint. But if you wish to change the old sleeper for a new one, I'll leave that choice up to you.

If you use a small tub such as the Trough Plastic Northcote 60x16cm Rec Graphite Villa or the Eden 60cm Charcoal Roman Self Watering Trough Planter - Charcoal, it does reduce root space and limits the size of the plant you can grow in there.

As an alternative have you considered filling in the gap with soil or gravel and building a timber raised garden bed that goes from the back of the fence to the first 100mm of the pool walkway. A small sacrifice that will allow you to plant larger plants and you won't have to worry about the sleeper on the fence line.

I've placed a sample rendered image below using AI to show you what it might look like by planting above. In this manner you are not limited to the size of the gap.

If you need further assistance, please let us know.

Eric

ChatGPT Image Sep 10, 2026, 07_22_50 PM.png

PatDIY_Dad

No worries

We will maybe put a slightly raised bed if we get dirt and rocks spillage into the paving area but for but for now we'll keep it lower

I don't want to take out and replace the wood so I might use earls wood hardener first the paint it with timber Bitumen if you think that'll make it rot alot less and last longer?

I have about 18 metres worth of 200mm sleeper to paint

I've already got the small tub of Bondal wood bitumen ( just under 1L)

How much you think I'll need for 2 coats

EricL

Hello @PatDIY_Dad

If for example the sleeper is 18 meters long x 200mm high and you are only painting the front of it you would have an area of 3.6m2 to paint. The Bondall 1L Inground Timber Protector provides a coverage rate of approximately 6 square meters per litre. You'll need an extra 1 litre if you are putting on two coats.

Eric

PatDIY_Dad

Thanks so this stuff would make the timber last alot longer and is better than using plastic?

MitchellMc

Hi @PatDIY_Dad,

I just wanted to check the thickness of this sleeper. If it is less than 50mm, say 30mm, it might actually be a treated plinth rather than a sleeper. If it is a plinth, there is a chance that it is only H3 treated, as they are designed to be above the soil level.

If that is the case, I would not recommend building the soil up against it, as its thinner profile means it is not designed to retain soil and having the soil in contact with it will cause it to decay. You really need to install sleepers in front of it.

Regarding the bitumen paint, it would be better than plastic as it doesn't create a void that can trap moisture and raise humidity, accelerating decay. However, I would only use it on H4-treated timber.

Mitchell

PatDIY_Dad

Thanks for checking that @MitchellMc

I just measured one and it's 25mm

So it would probably just be h3

Why on earth do these fencing contractors don't use h4 on the bottom is beyond me. Literally every likes to put garden beds at their fence perimeter

We can't afford to go any shallower in depth of the narrow garden bed

I've painted it with the Bondalll bitumen wood protector

Will do 2nd coat tomorrow

PatDIY_Dad

Unless you know any other alternatives to h4 or if painting the h3 plinth with the wood projecter will make it last alot longer

Or if maybe cement sheet might be better option as it's very thi

MitchellMc

Unfortunately, @PatDIY_Dad, I wouldn't rely on coating an H3 plinth to effectively turn it into something suitable for permanent soil contact. The Bondall coating will certainly provide additional protection, but if the timber is only H3 treated, it's still not something I'd recommend burying behind 250–300mm of damp soil long-term.

Cement sheet also wouldn't be my preferred alternative. Although it's thin, you'd still need to adequately support it against the pressure of the soil, and you'd essentially be asking the existing 25mm plinth and fence to provide that support.

Given you don't want to lose any more garden-bed width, I think an H4-treated timber sleeper installed immediately against the existing plinth remains the simplest and most reliable option. You'd only lose the thickness of the sleeper, and it would give you something that's actually designed for ground contact rather than relying on the fence plinth to retain the garden bed.

Mitchell

PatDIY_Dad

Consulted with the project manager ( wife ) she's ok with loosing 50mm

Would you fix directly to the existing plinth with screws or screw to the fence posts?

JacobZ

Hi @PatDIY_Dad,

I wouldn't rely on the plinth for any kind of structural support. It could lean up against it, but it won't be adequate to support the sleeper and the weight pushing on it.

It would be better to screw to the posts than the plinths. Is there any way you could remove and replace the plinths from the opposite side of the fence? Or is this a no go?

If you couldn't access the other side of the fence, I'd drive a couple of 75mm batten screws through the sleepers into the posts on an angle. I'd then also suggest you drive a hardwood stake into the soil at the middle of the span, then screw it to the sleeper. It will just help to add a bit of additional strength and if you drive it down far enough, it will get buried and won't be visible.

Let me know what you think.

Jacob

PatDIY_Dad

Hi Jacob. Cant remove the plinths as I would start messing with neighbour side of fence.

I did it all today. Put bitumen wood protector on allm the cuts

Im building a cladding fence around the pool which I want to be fully independant of the timber fence. So when I am old and not fit. I dont have to undo my cladding fence when the timber fence requires replacement.

I was able to use galv angle brackets to hold the sleeper on the galv steel posts I previously concreted and on the other side I screwed into the fence posts

But then at the end when I finished I looked at it all and relised. Dam if they replace the fence they will have to remove the sleepers to and it could mess with my cladding fence. Being one side attached to it and one side attached to the fence

Before we put soil in for plants I was thinking it might be better ( to future proof the timber fence indepenance) To drive 600mm star pickets and fix one side to those instead of the timeber fence. That was the fence can be removed without touching the sleeper

The plants will be non invasive Heliconia 'Hot Rio Nights' so it wont push the fence or sleeper

Unless you can think of another way. I could not run the sleepers from steel posts to steel post as it would have to then go in front of the steel post. Reducing width of the garden bed by like 100mm

Wife Project Manager would fire me

Also how would you deel with empty screw holes with h4 timber. Bitumen paint over or fill hole with waterproofing like silicone

JacobZ

Hi @PatDIY_Dad,

If you want the sleepers to be entirely independent of the fence, then driving stakes in and screwing the sleeper to them would be about the best you can do with the fence already in place. I recently helped a mate landscaping his backyard and we did more or less the same thing, although we did drive a couple of screws into the posts as well. It worked well. Assuming the posts are spaced at 2.4m centres, I'd be using at least 3 stakes per sleeper. Keep in mind, you'd have to bring the one on either end in a bit to avoid the concrete foundation of the post.

For the holes in the sleepers, I'd fill them with https://www.bunnings.com.au/turbo-500ml-builder-s-bog_p1212026 and then paint over them with your bitumen membrane. The sleepers are H4 treated, so they will be fine with a bit of water, but long term, it is still in your best interests to keep water from getting inside the timber. Bog and bitumen membrane will give you the most protection I see possible.

Let me know if you have any further questions.

Jacob

PatDIY_Dad

Ths thanks @JacobZ

With that bog, how would I inject it inside the hole. Looks like it's more of a spreading type application like your hole filler for painting

Or is that the same method I'd use in this situation?

JacobZ

Hi @PatDIY_Dad,

Just take a small spatula like this https://www.bunnings.com.au/uni-pro-25mm-real-good-paint-scraper-25mm_p1674074 and push a little bit into the hole repeatedly until it is full, then clean up once it seems full.

As long as the hole is relatively full and there is no vertical lip for water to sit on, it should be all good. Once the bitumen has been painted over it, it should never really come into direct contact with water anyway.

Jacob

PatDIY_Dad

Thanks

Is that builder's bog good for I ground?

JacobZ

Hi @PatDIY_Dad,

If it was uncovered it wouldn't work particularly well, but as long as it is covered with your bitumen membrane, it should be fine. In saying that, if water gets into it, it can degrade over time, so you might feel more comfortable using something else.

https://www.bunnings.com.au/sika-300ml-black-sikaflex-11fc-purform_p0628964 would be another option that is far more water resistant on its own. You can also paint over it, so in hindsight, it is likely a better option.

Jacob

PatDIY_Dad

Thanks so much @JacobZ

I actually think I got some off that stuff in the shed

Anyway finished with the h4 sleepers and started the cladding

Will get some pickets and do that later