How to build a raised block garden bed near house slab?

I am currently trying to achieve a flatter gradient in the backyard, and my only solution is to create a "raised" block garden bed near the house, which will go down past the house slab level, allowing the turf to be lowered.

I have done some reading and am aware of the drainage & termite barriers required, I'm just having trouble grasping exactly what needs to be done.

I am envisioning a 100-300mm gap off the house with an agi-pipe and loose gravel filled to the top, followed by the block garden bed, approximately 800mm high. This will allow the house weepholes to drain, but I am not sure how to connect the drainage from the garden bed to the agi-pipe?

There is a downpipe located in this area, which could help connect the pipe to the stormwater line I think

Will I need to tie the blocks into my house slab, as that area is on uncompacted fill and slopes away from the house?

Is compacted gravel or strip footing best for the blocks?

I also have Newport cladding in a portion of it, which I was going to waterproof & leave as gravel to avoid moisture & rot.

We also have a timber frame for reference, so I am trying to be verrrry cautious.

(The house is currently under construction)

Screenshot 2025-10-22 094821.png Screenshot 2025-10-22 094843.png Screenshot 2025-10-22 094803.png

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Comments

MitchellMc

Welcome to the Bunnings Workshop community @AB-DIY. It's a pleasure to have you join us, and many thanks for your question on building a raised garden bed.

For the raised garden bed you’re planning, it’s important to reconsider having gravel directly up against your house above the foundation line. The foundation acts as the cutoff for moisture, and you cannot have a water drainage section up against it, especially above or blocking the weep holes. The weep holes must remain completely unobstructed. Covering them or building a gravel drainage section right against the wall could lead to serious moisture issues.

Given this, it’s usually better to move the garden bed forward away from the house and make it a standalone, self-contained structure. In your image, it looks like the proposed bed is quite high and about one and a half courses above the slab. You could instead build the bed up to the height of the slab, which would allow the lawn to sit at the intended level without needing an excessively tall wall. A single course of blocks with proper waterproofing would be sufficient to hold soil and plants while keeping the structure independent of the house. Is there a reason the planter is at that height? I wouldn't have thought it would need to be any higher than your patio area as marked below.

Regarding your questions about construction: the blocks should be built on well-compacted soil or gravel, especially if you’re going three courses high. You do not need to tie the blocks into the house slab, particularly since it’s a timber-framed wall. Tying them could create long-term moisture or structural issues. Use blocks on the front, sides, and back of the garden bed to make it freestanding and stable.

Let me tag @Nailbag to see if he has any thoughts.

Please let me know if you have any questions.

Mitchell

AB-DIY

https://www.workshop.bunnings.com.au/discussion/comment/58457#Comment_58457

Thank you Mitchell, that was super helpful!

I guess I only had it raised to that height because we have an exposed agg pad outside the laundry door, which is behind the clad wall, and I thought it would be simpler for the blocks to finish taller than both sides. Idk

Should I just make it 100mm off the house since I won't need the pipe etc?

How would you address the gap between the house and the garden bed? Should I do some type of moisture protection to the slab & landscape mesh to prevent weeds maybe?

How would you finish the bottom of the garden bed? I'm hesitant to let the plants grow into the soil and affect the house slab in the future.

Also, since a portion of the house is clad, do you think I can just block off along the face of the wall up to the brick rebate / clad junction since there will only be a garden bed in the middle area? Screenshot 2025-10-22 145609.png Screenshot 2025-10-22 145700.png Screenshot 2025-10-22 145550.png

EricL

https://www.workshop.bunnings.com.au/discussion/comment/91591#Comment_91591

Hello @AB-DIY

The space of 100mm should be ok, I would suggest perhaps building a Merbau timber cover over the gap to prevent anything from falling in.

I propose covering the bottom of the garden bed with pavers so that water will still be able to pass through. But if you seal the bottom with concrete its important that you install several holes at the bottom wall of the garden bed for water to drain out.

In regards to the section that is being blocked off in front of the cladding, I suggest speaking to your builder and asking them what the appropriate gap or distance would be for that section. I would suggest a minimum of 100mm so that condensation will not form on the cladding. But it will produce a domino effect where the gap increases to 200mm behind your garden and the entire block garden goes out of alignment and moves forward by another 100mm.

As a possible alternative, I propose building a larger block garden and uniformly spaced at the back by 100mm for the entire build. The only drawback of course would be that the block garden will not be in alignment with the patio base.

Let me call on our experienced members @Dave-1 and @AlanM52 for their recommendations.

If you need further assistance, please let us know.

Eric

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/111786#Comment_111786

Good Evening @AB-DIY

I prefer an air gap between the raise dgarden bed and house, I also wouldnt really worry about sealing the base of the garden bed as you would effectivly be making a bathtub with soil and planst that potentially not handle that enviroment too well. The height, even reduced down from the origimal 800mm you suggested is likely be enough room for the roots of whatever plants you put there and the base being soil will allow resonable drainage.

The step away that @EricL mentions is a good alternative I think. There is no point covering weep holes/damp course as you will have issue sdown the track. That air gap will keep everything working the way its supposed to.

You mention about blocking up to the clad section, Does that mean you will be covering the air vents or stopping free flow of air? I am thinking long term effects of no air flow more then a fresh clean look 🙂 WOuld you be able to widen the garden bed a little more so you could still have that straight run of blocking but not hard against the house/clad area?

Dave

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