HOW TO LAY AN OUTDOOR FLOOR USING HAND MADE CONCRETE COBBLES

We’ve spent around 18 months’ worth of weekends building a gazebo in our back garden and yesterday I finally finished the last part – the floor. The floor is made up of individual hand made concrete cobbles shaped like hexagons. In this post, I’m going to cover from start to finish how we made the cobbles, laid the floor with them and how I’ve just finished the floor by sanding and sealing it.

Unfortunately, this is one of those projects that I didn’t document too much as I didn’t know I was going to blog about it later but I will do my best to explain each part clearly. 

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THE PLAN

When we decided to build a gazebo, my initial thought was to have a wooden floor inside – like a decking. However, the issue with decking is that it’s spiders’ paradise underneath all those wooden planks. I really didn’t want to have the buggers crawling up my leg every time I stayed out in the gazebo after dark.

My next choices were either patio slabs or pavers but both of those options were extremely expensive because the gazebo measures 4x4m and I just couldn’t justify spending that sort of money.

Around the time of pricing these up, I saw one of those viral videos where the guy was laying a garden floor using cement and a plastic mould. That’s when I got the idea to do the same – it’s cheap and looked really easy on the video. My plan was to pick a nice looking concrete mould and do the same as the video – pop the mould down, scoop some concrete into it, lift it and move on to the next bit. Little did I know how hard the mould I picked would be to use. Long story short, the stamping didn’t work and we ended up using the mould to make individual cobbles to lay as pavers.

This is the mould I picked – it makes hexagon shaped cobbles. I absolutely love the honeycomb look it gives and I thought it would match really well as our gazebo has hexagon shaped bitumen roof tiles. I did want to edge the floor so that the tiles didn’t go all the way to the edges so I thought just having a poured concrete edge all the way around the floor would be nice.

Needless to say, this floor was going to be a big job.

MATERIALS & TOOLS REQUIRED

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STEP 1: PREPARING THE AREA

The first thing we did was prepare the area for the concrete floor. We were building a gazebo so we marked out where our posts will go. Then we measured out the size of the floor we wanted. Ours is roughly 4x4m.

We dug a 20cm deep hole where the floor will be. We then filled it with hardcore about halfway, avoiding filling the parts where the gazebo posts will go. We had loads of random rubble and old bricks that we dug out around the garden so we didn’t actually have to buy hardcore. We did borrow a big heavy plate compactor (like this one) from a friend and used this over the top of the rubble along with a breaker bar to break the big stones into smaller ones. When we finished it looked roughly like the sub-base gravel you would buy from a DIY store. If you’re planning on building a concrete floor, make sure that you compact the hardcore very well to stop it from moving in the future. Hardcore movement can cause cracks in your finished concrete.

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STEP 2: FIRST CONCRETE POUR

The next thing we wanted to do was to just pour a level concrete floor so that we could work on the gazebo before finishing the floor as the last step.

We cemented the gazebo support posts in and edged the floor by screwing some decking boards around where we wanted the floor to end. As there was so much concrete to mix, we borrowed a small cement mixer from a friend to do the job. As a rule, always follow the mixing ratios on the bag of your cement to get a good mix. For concrete, this will be some proportion of sand, cement and aggregate. Once the concrete was mixed, we poured it into the gravel patch. Tip on your edging boards – to be able to remove them easily, oil them up so that the concrete doesn’t stick to them. We didn’t do this and while the decking boards came off easily when I used them on another project later, I had to sand them to death to get the concrete off. I think you can buy special ‘releasing oil’ for this.

One mixer full of concrete was nowhere near enough so we kept mixing and pouring until we reached the height we wanted – this was about 5cm off the top of our dug hole. We left 5cm unfilled as we were planning on laying our moulded concrete on top of here and the mould was 5cm high.

Once we reached the height required, which was quick because we needed to do it before the concrete went off, we used another long decking board to level the concrete floor. Using a long level placed on top of this plank helped us check that the floor was not wonky. When moving the concrete around to level it, make sure to sort of shimmy the long plank from side to side instead of just dragging it across the concrete. This helps distribute the concrete evenly.

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STEP 3: POURING THE FLOOR BORDER

After pouring the first level of concrete we carried on with the gazebo and didn’t come back to the floor for another few months. Once we were ready to carry on, we had to build a frame around the border that we wanted to sit around the hexagon cobbles. We once again used decking boards that were cut to the required lengths and screwed together.   

The mixing, pouring and levelling process was the same.

I don’t have any pictures of this stage but this is roughly how the floor looked after it was poured and we removed the decking boards. 

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STEP 4: MAKING THE COBBLES

Once the border dried it was time for the best bit – moulding the cobbles. Originally, we were planning to pop the mould down in place, scoop some cement into it and lift it up leaving the perfectly moulded cobbles to dry. To make the cement, we used a mix of 4:1 to start with and then added more sand/ cement/ water to make the mixture more pliable. We tried a variety of mixes and just could not get the mould to work. When we lifted the mould, either the cobbles melted into misshaped slops or they lifted up with the mould and refused to come out. We even tried soaping and oiling the mould. It just wouldn’t work. We did also try making concrete with aggregates but the mould is so small that the aggregates just stuck out in completely awkward positions and did not look good at all.

After loads of wasted time, we decided that the issue was that the mould was just too high and each individual hexagon shaped cobble was quite small so it just wasn’t heavy enough to stay down while the mould lifted off.

To solve this issue we cut the mould in half to make it 2.5cm high instead of 5cm. This worked much better and we were able to lift the mould off successfully. However, this is when we discovered the next issue. When you lift the mould, you have to place it right next to the cobbles you just laid to lay the next batch. This was way too fiddly and we kept breaking the existing cobbles so to carry on like this, we would have to leave each mould to dry before doing the next one. Additionally, our cobbles were now only 2.5cm high which meant they were below the border we already poured. To solve this we decided to do something completely different.

We got a huge sheet of plywood and laid a plastic sheet on top of it. Then we went to town laying moulds all over the sheet without getting them too close together. Once the cement dried, we just lifted each individual hexagon shaped cobble off and put it to the side to store. We kept doing this until we thought we had enough cobbles to cover our floor. This took many weekends and many beers.

STEP 5: LAYING THE COBBLES

Finally, after we thought we had enough cobbles, it was time to lay them. Since we had 5cm to fill but our hexagons were 2.5cm high, we had two options: lay them on a bed of sand as pavers or a bed of cement. We decided to go with cement because the cobbles were quite thin at 2.5cm and we thought they may crack when walked on if they were on sand.

We mixed cement with a ratio of 4:1 in a wheelbarrow as we didn’t want to mix too much in one go. We tried to add aggregate to start with but this was causing problems when we were trying to push the cobbles into the cement so we left it out in the end. To mix it, we put sand and cement in the wheelbarrow and mixed it with a rake. This helps cement get properly distributed. Then we added water to bring it together.

We then scooped some cement onto the floor, spread it out to 3cm thick, and levelled it a bit using a trowel. Into the cement, we pressed the cobbles around 0.5cm deep to bring them level with the border. To lay the cobbles in an orderly fashion, we worked in rows trying to leave a fairly even gap between them.

Around where the cobbles are near the border, full cobbles didn’t fit. We left these small gaps for the end as we would need to cut individual cobbles down to the correct size. 

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STEP 6: FILLING IN GAPS BETWEEN THE BORDER AND CENTER WITH HALF COBBLES

For the half cobbles, we initially tried to plop some cement into the gaps and use a small screwdriver to make a gap in between the plop and existing cobbles/ edge to create a sort of fake half tile. This didn’t work at all as the screwdriver was just flicking out too much cement creating a right mess. In the end, we had to use a disc grinder to cut full blocks down to the right size. I think you could also achieve the same effect with a chisel and hammer, similarly to how you would cut bricks. We just happened to have borrowed a grinder from a friend for another job and it was easier to use that than cut each cobble by hand using a chisel.   

These are the steps we followed to cut the cobbles:

• Lay a full cobble over the gap where it will need to go
• Mark where it needs to be cut to fit into the gap
• Use a grinder to cut it
• Plop a bit of cement into the hole
• Push the newly cut half cobble into the gap to set it

We repeated these steps for all the gaps filling the floor completely. This did take a long time and was quite fiddly. Unfortunately, because the cobbles were quite thin, some did snap during the grinder cutting stage.

STEP 7: FILLING IN GAPS BETWEEN COBBLES

Once the whole floor was finished, we left it alone for a few months because winter hit and the floor sealer we had wouldn’t dry in temperatures less than 10 degrees c. Unfortunately, this meant that the dogs managed to get quite a few muddy paw prints all over the lovely porous concrete before we came back to finish it. I also had to vacuum in between the cobbles to get out all the dust and dry leaves that had blown over there during the winter. And I had to scrub the cobbles with warm soapy water to try and get the muddy prints out. Some of them came out but some of them are destined to stay permanently engraved into the floor. That’s ok though, those bits are hidden by the dining table we have in there anyways. If you’re making a floor like this, don’t leave it too long. Unsealed concrete is really porous and just sucks all the dirt in – seal it as soon as it dries!

After all this cleaning, I got some kiln dried paving sand and just piled it into one corner of the floor. I then had my (not so little anymore) helper sweep the sand into each crevice in between cobbles to completely fill the gaps as high as possible. Because our gaps weren’t that deep (only about 2cm), we only needed 1 and a half of the large bags of sand.

Once all the gaps were filled, we used a softer handheld brush (the type that comes in a dustpan and brush set) to gently sweep off all the loose sand from the cobbles, just leaving it in the gaps.   

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STEP 8: SEALING THE PATIO

Now is the sealing part. This is really important to do with a paver/ cobble floor that has been sanded. If you don’t seal the floor, the sand will just get blown out of the gaps over time. Applying a good quality sealer makes the sand harden around the pavers/ cobbles which stops them from moving and stops the sand from getting blown away.

The sealer I bought can be applied by brush or roller according to the bottle back. However, when I tried to use either one of those the sand just kept getting stuck to them and was getting lifted out as a result. To stop the sticking I had to completely saturate my brush so that it was almost pouring the solution on. I used a 5^th of the bottle to do less than 1m2 of the floor. The whole bottle is supposed to do 40m2 so this obviously was not working.

Because the solution was very watery, I decided to have a go at using a sprayer. I used a small pressure garden sprayer and this worked perfectly. It was way quicker and way more economical. By the time I finished, I still had half of my bottle left which I can use in a couple of years when the sand needs topping up. In terms of the application, I first applied it under the seating (which I couldn’t move out of the gazebo because it was too heavy to lift alone even after I unscrewed the seating planks). Then I applied it in the centre working in lines to make sure I covered everything.

It says on the back of the bottle that you shouldn’t let the solution pool but I found that it wasn’t much of a problem – the cobbles were so porous any pooling soaked in after a couple of minutes.

I only applied one coat as this is supposed to be enough and once it dried, it gave a nice pearly sheen to the cobbles without changing their colour. It also turned the sand very hard.   

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THE RESULT

The result is a really ‘old’ looking cobbled floor which I love. Because each cobble was made by hand and then laid into place by hand, the floor is far from perfectly symmetrical or even. The gaps are slightly bigger between some cobbles and some cobbles themselves aren’t perfectly symmetrical. Because of this, it reminds me of walking through a small old English village town centre where the cobbled floor has gotten unsettled over time. I also love the natural colour of the cement, each cobble is slightly different and yet it somehow matches together perfectly. This is definitely one of those projects where the imperfections actually make the final result better.