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How to anchor cinder blocks to exisiting slab for a bench seat?

Just Starting Out

How to anchor cinder blocks to exisiting slab for a bench seat?

Hi There, 

I am using 190x190x390 cinder blocks to build a bench seat that will be topped off with inset 18mm ply and a piece of custom foam. I am only building it 2 blocks high and then capping it off and putting a foam cushion on top.  Total cinder block + cap Height is 420mm (not including cushion).


I will be laying the blocks on an existing concrete slab. How do i anchor the blocks to the slab?  Is morter alone enough? I was also going to construct a internal frame using 900 x 45 on the inside for added stability and so there is a platform to put the cushion on. This will prevent the blocks from falling inwards but not outwards.  Can i bolt the timber frame to the blocks from inside?  I have a tendancy to over engineer things because i have 3 small children, is this overkill? 


Also, I was considering adding another section that would be 4 blocks height for the BBQ and some bench seating, for this add on the blocks will be much higher, perhaps 800mm high including caps. How should i ensure it is stable? 


Please see my design images, they are images of the blocks roughly arranged WITHOUT caps. This is not the final arrangement but it gives you an idea of the basic shape.  Its a very wide daybed, bench seating and then staggers up to become an L shaped bench top height for BBQ and outdoor cooking space. 


Lastly, I wanted to render is white to have that Spanish look (I have included lots of inspiration shots), can you please assist me with what products i might need to achive that look and can i render on to wood or ply or does that need a primer first?


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Bunnings Team Member
Bunnings Team Member

Re: How to anchor cinder blocks to exisiting slab for a bench seat?

Hello @MamaCass 


Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your cinder block seating project.


I propose drilling into the concrete base and anchoring threaded rods using Sika 300ml Grey Anchorfix 1 Adhesive. You then place the cinder blocks into place and filling the holes with builder's mix. The threaded rods will serve as the anchor to the base. 


Building a timber frame inside the cinder block structure is a plus as it adds structural support. Anchoring the cinder blocks to the frame is perfectly fine. For the higher sections make sure that each section of the cinder block is supported with a Reo bar anchored to the concrete base. 


Make sure that all the holes of the cinder block are filled with builder's mix. In order to get that Spanish look, I propose rendering the entire surface to a fine finish and painting the surface in an off-white colour.


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


If you have any other questions we can help with, please let us know.




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Home Improvement Guru

Re: How to anchor cinder blocks to exisiting slab for a bench seat?

Evening @MamaCass 

@EricL took the words/suggestions straight out of my mouth. I was thinking first that mortar would be fine until I realkise dthat some of the structure would be 800mm high. Like you I tend to over engineer stuff, better to be safe then have to worry is usually the way I think.


Attaching the timber to the inside of the blocks - I never know if I trust the blocks to hold together while drilling, however if they are filled with a builders mix of concrete it does sound like it would work. I would use masonary screws to fasten the timber for sure in this instance.


I havnt rendered anything .... yet :smile: I do have an retaining wall that has a long horizontal crack on it that I will need to repair but thats the closest I have come to answering that question


Looking forward to seeing your project, It sounds like it will be an interesting and well worked out one :smile:



Making a Splash

Re: How to anchor cinder blocks to exisiting slab for a bench seat?

Hi @MamaCass Sounds like you very well planned out and it looks like it's going to be w terrific entertainment space once completed. I have nothing to add over and above both @EricL & @Dave-1 suggestions 👍



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