backyard retaining wall and raised garden bed render

robertye112 · May 21, 2024 9:24 pm

Hi there,

I have a retaining wall in my backyard which ends up turning into a garden bed as transitions from below grade to above grade and I want to render the wall. It is built by blocks, core filled with steel and has solid footings. Unfortunately, the wall has not been built with any control joints / expansion joints and is just one solid piece. It is 12 meters long in it's longest side. Sections of the backside of the wall also doesn't have access for waterproofing.

Knowing this, what is the most suitable rendering option? Sand/cement or acrylic render? While I like the idea of acrylic render being flexible and hate cracking, I am even more worried about paint bubbling within a few years.

While I'd love everyones opinion, I'm particularly interested in Pedro4137 's opinion
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Thanks

alexp79 · May 21, 2024 10:07 pm

Sand cement and lime in 1:1:6 proportion (approximately) should work really well, thickness around 10 mm.

I would purchase sand cement and hydrated lime from Bunnings:

https://www.bunnings.com.au/australian- ... g_p0760612
https://www.bunnings.com.au/australian- ... e_p0086148

For every 3 bags of sand cement you would need to mix in half a bag of lime and approx. 11 litres of water.

To prevent cracks and improve strength, you can add polypropelene fibers:

https://spf.net.au/products/masterfiber ... cement-5kg

Dosage is approx. 0.8 kg per cubic meter of render.

After base coat dries up, you might want nice finish, so I would recommend skim coating, sanding, sealing/priming and painting with some nice exterior paint or alternatively you can spray on or trowel textured coat.

robertye112 · May 22, 2024 7:26 am

Thank you so much for the time to reply, greatly appreciated! Would you mind please telling some of the reasoning behind sand/cement/limestone render in this situation over acrylic render ?

Thank you!

alexp79

Sand cement and lime in 1:1:6 proportion (approximately) should work really well, thickness around 10 mm.

I would purchase sand cement and hydrated lime from Bunnings:

https://www.bunnings.com.au/australian- ... g_p0760612
https://www.bunnings.com.au/australian- ... e_p0086148

For every 3 bags of sand cement you would need to mix in half a bag of lime and approx. 11 litres of water.

To prevent cracks and improve strength, you can add polypropelene fibers:

https://spf.net.au/products/masterfiber ... cement-5kg

Dosage is approx. 0.8 kg per cubic meter of render.

After base coat dries up, you might want nice finish, so I would recommend skim coating, sanding, sealing/priming and painting with some nice exterior paint or alternatively you can spray on or trowel textured coat.

chippy · May 22, 2024 10:16 am

What Alex has recommended is what is called hard plaster and that is the float coat that is designed to get a nice flat straight surface. If it was internal it would then have a set coat over the top which gives the nice hard smooth finish that you paint.
Used externally it gives a textured finish particularly if it is sponge finished as it is setting off. It can also have acrylic render applied over the top and gives a very nice flat finish.
Unfortunately as soon as you paint it or even if you just went straight with an acrylic render on your block work you will end up with issues if you haven't waterproofed the back of the wall. The moisture will cause efflorescence and a build up of hydrostatic pressure will cause the paint or acrylic render to start to bubble off.
It's the waterproofing of the back of the wall and appropriate drainage that is most important to making sure whatever you do on the front face stays intact for the life of the product.
Can you get behind before it's back filled and waterproof? Ideally you would want to paint waterproof membrane then builders plastic and then coreflute the back of the wall and if you aren't on free draining soils then some form of ag drain behind that.

alexp79 · May 22, 2024 2:22 pm

Yes, waterproofing and drainage is a must. I am not very familiar with using acrylics as a base coat, generally it is applied as a top coat.

robertye112 · May 22, 2024 5:32 pm

thanks chippy!

Here is the back of the wall which has not been backfilled yet.

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Here is the same back of the wall taken from the other end, behind the steps. As you can see there isn't much space down there and only 10cm of space behind the stairs there. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅
Even if I waterproof behind the stairs really well, I imagine it's not going to be water tight because I can't waterproof behind this wall with the red arrow shown below:
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I'm wondering if there are options to waterproof the front of this wall and then render over that?

Ashington Homes · Dec 17, 2024 4:09 am

robertye112

thanks chippy!

Here is the back of the wall which has not been backfilled yet.

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Here is the same back of the wall taken from the other end, behind the steps. As you can see there isn't much space down there and only 10cm of space behind the stairs there. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅
Even if I waterproof behind the stairs really well, I imagine it's not going to be water tight because I can't waterproof behind this wall with the red arrow shown below:
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I'm wondering if there are options to waterproof the front of this wall and then render over that?

There is a range of products called "negative side waterproofing" which are designed for this exact application.

We use these all the time when access to the rear is not possible ie we waterproof drop edge beams in slabs on the internal side and then render over the top.

So I would waterproof, core drill a few holes to relieve any water pressure build up in a few locations and then acrylic render it.

robertye112 · Dec 17, 2024 8:46 am

thank you Ashington Homes, that is helpful!

Ashington Homes · Dec 17, 2024 2:04 pm

robertye112

thank you Ashington Homes, that is helpful!

No problem at all.

For anybody else reading this thread there is a great waterproofing shop in Sydney called "The waterproofing shop" they are in Roselands....massive operation ( for a waterproofing shop).

You can look them up and get advice. I deal wth Christian but they have a number of staff and they love giving advice especially on an quirky waterproofing issues.

https://thewaterproofingshop.com.au

Also, if its easier there is generally some old dude in the trade section of Bunnings who has knowledge. I know our local Bunnings at Pymble now has a really big range of waterproofing products in the trade section and that team seems to be well trained.