patio extension

I Built A Riven Patio Extension For My House Extension

Post Description

How I extended an existing patio in front of a new ground floor house extension. I talk about matching the the new slabs with the originals and keeping the pattern random. It also tells how I put a curved wave in the edge to match other curves and create a pleasing shape to the patio.


After extending my house with a single story extension I extended my patio in front of it with a riven patio extension. This was all part of the plan since the extension had a patio door. Before extending the patio this door opened onto an area of rough ground.

This area had previously been covered by slabs which were part of my original patio. These had been removed to make way for the building work, leaving behind their hardcore base and bedding mortar.

The new riven patio extension connected my original patio, onto which the back door of the house opened, to the path between my shed and the front garden. That path is laid in a passage between the street fence and the house extension.

Fortunately I was able to match the original riven patio style used when it was built in 2001. Exactly the same type of slabs were still available. I ordered slabs from the same manufacturer, using the same moulds, via my original builder’s merchant. The builder’s merchant had been taken over in the meantime but without any damaging changes.

My Riven Patio Extension Paving

When I built my original patio I used riven slabs (slabs with an undulating surface). This makes them look like they were chipped out of real stone. I believe they are cast from concrete poured into rubber moulds formed around real stone slabs.

They came in three rectangular sizes from which I made a random pattern. Here is an example of those sizes:

riven patio extension

They also came in three curved sizes for making a 2,400mm diameter circle similar to this one below available from B&Q.

riven patio extension

The circle’s three slab sizes are:

  1. Central Circle – made of two semi-circles with a radius of 300mm.
  2. Inner Circle – 16 slabs to make a circle extending from 30mm to 750mm radius.
  3. Outer Circle – 16 slabs to make a circle extending from 750mm to 1,200mm radius.

The semi-circles have a grouting groove moulded at the 90° point where they can be cut in half if they are to be used as quadrants. When used to make a semicircle or full circle the groove is grouted to look like two quadrant slabs.

Diameters declared above are approximate since the slabs are under size to allow for grout between them. As they are laid they have to be spaced apart until they look right.

I used all three sizes to make rounded corners instead of square corners. fortunately I was able to buy individual slabs sufficient for my needs.

NOTE: Buying individual slabs is not possible for every style. With some styles a full circle has to be bought.

The Design of My Riven Patio Extension

Below is my back garden plan. It shows the slabs of the original riven patio and the riven patio extension in detail.

riven patio extension
The Plan of My Back Garden

Rounding Corners On My Original Patio

The original riven patio has a 1,200mm wide path leading from it to a garden gate. I wanted to round off the corners where the path joins the patio to give a softer style than square corners. I did it by using the moulded curved edge of the circle slabs to form concave quadrants. This was achieved by cutting the convex ends off at the required angles so they fitted into the patio. You can see the path-patio junction here:

riven patio extension
A 1.2m wide path connecting the patio to a garden gate.

Choosing A Random Pattern of Slabs

When designing the original patio I tried hard to create a random pattern of slabs using only the regular sizes. But, I had to concede that it wasn’t possible to round off corners without using irregular sized slabs somewhere in the pattern. In the end I had to concede and cut a slab down to 300mm x 300mm for each concave corner. This discovery came about when I originally built the patio in 2001. I have clearly marked two such slabs in the picture above.

In the end I had to concede and cut a slab down to 300mm x 300mm for each concave corner. This discovery came about when I originally built the patio in 2001.

How Big Does This Patio Need To Be?

The original patio gave me enough space for the garden furniture but If I’d maintained the full width throughout the extension I’d have made the lawn look too small by comparison. So I made a sweeping curve using a concave quadrant and a convex quadrant in the riven patio extension. This halved the width (projection from the house) from 3m to 1.5m (ten to five feet).

Rounding Corners On My Riven Patio Extension

I used the same pattern of slabs for the concave quadrant as I’d used before. By doing this I had a consistent style at all corners. Again I had to cut one square slab 300mm x 300mm to fit with the random pattern I wanted. You can see it in the garden plan and in the picture below.

riven patio extension

Below you can see a central semi-circle cut in half to make a central quadrant surrounded by four inner circle slabs. Together, when grouted, they make the convex quadrant with a 750mm radius.

Spacing Between Slabs To Allow For Grout

riven patio extension

The circle slabs have to be moulded to a size which allows for grout when the circle has a diameter of 2.4m. This produces practical issues when butting up parts of a circle to the rectangular slabs. The circle slabs need more grout between them than the rectangular ones. Rectangular slabs don’t make the same allowance for grout. This caused me a problem when I first made the patio. As a consequence my rectangular slabs tend to be too close together.

If the rectangular slabs were laid with a regular pattern in rows then the space between them could be increased or decreased without issue. But with random patterns some areas may have a row of five slabs and be adjacent to an area with six or seven slabs filling the same width. It is then difficult to maintain a constant grouting width when the total grouting width has to be divided by five on the one hand and seven on the other.

I noticed this grouting issue when I started the random design on my computer. I had to, for example, keep the patio seven slabs wide however I randomised them.

In the next picture you can see the actual outer circle slabs I cut for the riven patio extension. These were used to make the concave quadrant with a 750mm radius.

riven patio extension

When laying a patio with curved and straight slabs in it the outer edge of all the slabs have to follow a smooth line. Then any slack caused by the, “not quite correct,” sizes of the slabs has to be taken up by adjusting the gaps between slabs until they look OK.

Cutting The Concave Corner Slabs

If slabs were thin like cardboard it would be easy to lay them down in an overlapping fashion and draw pencil lines on them showing where they need to be cut. Even though they’re thick, heavy and don’t lend themselves to easy manoeuvrability that is what I did, initially. I propped them up on other slabs to keep them level while I aligned them in various ways to look for the best pattern.

Some designs didn’t quite fit together. I had to be careful not to leave very small peculiar shaped gaps at the corners of the curved slabs. I did it without rushing and eventually produced the design I used.

Once I had cut one corner to my satisfaction I took measurements so I could draw lines with pencil on the slabs of other corners. The approximate dimensions I ended up with are shown below.

There needs to be 10mm gap between slabs. So when cut at an angle adjacent slabs spaced 10mm apart don’t have quite the same length. They may be different by 5mm, i.e. one might have a 21cm long edge and the adjacent slab may have a 21.5cm long edge.

riven patio extension

Marking Out Slabs & Preparing To Cut

There was always much consideration given to where I would cut a slab. It requires thought, measurement and an understanding of where the saw blade will go. When I had decided where a cut would go I would mark the slab with a straight edge and pencil.

Remember, “It’s easier to cut 50mm off a slab than to cut 5mm off it. So be exact with marking out.”

One approach to marking out is: Lay the slaps to be cut on a flat surface in the desired pattern with appropriate gaps between them. Then mark across all the slabs at once (in this case two) using a straight edge.

It’s only after they are cut and fitted together satisfactorily that their actual dimensions can be measured. Taking those measurements over to another group of similar slabs, and cutting them, may not produce such a good result as drawing straight lines across them when laid out.

Cutting Slabs With A Diamond Saw

Most people cutting slabs use a diamond saw they have bought or rented specifically for cutting concrete. In my case I came across two diamond studded blades in a closing down sale at “Do-It-All”. These had 22mm holes in them. This made them suitable for my SKILLSAW®. Their original purpose was for cutting interlocking flooring boards made by EDGE. This was back in 2001 when I initially laid the patio.

They cost £7 each in the sale. A bargain I thought since I expected diamond blades to cost more. I presumed they would be suitable for concrete simply because they were diamond blades. It was a good bet. one blade carefully used lasted throughout the construction of the original patio and beyond. I only started on the second blade in 2018 when I laid this riven patio extension.


If I had to buy a new blade I would try a similar style blade from BOSH available from BOSCH an Amazon reseller.

Using These Diamond Saw Blades

The blades I bought fit onto the boss of the saw just like a wood blade. However, unlike a wood blade, it doesn’t matter which way they are mounted.

I learnt to keep the blade cool by only cutting to a depth between 5mm & 10mm. Then I repeated the cut several times after increasing the depth by 5mm to 10mm each time. This took a while but It was important to preserve the blade. If I felt it and it was hot I would take a break.

EDGE Flooring blade Model #801.
The diamond cutting edge of the EDGE blade.

In the past I experimented with wetting the blade to keep it cool. The blade and the concrete were happy to be wet but the wet swarf produced was not good for the saw it filled and clogged the guard around the blade. I had to give the machine a good clean and learnt to cut dry and gently instead. The dry dust can be blown and brushed aside.

Cutting The Slabs On A WorkMate

To cut the slabs I rested them on top of a WorkMate and clamped a straight piece of timber to them for the sole plate of the saw to run against. The example below shows how this is done.

riven patio extension

To get the timber in the right place the distance between the inside of the blade and the edge of the sole plate has to be measured. In my case it is 38mm. This is then used to set the distance between the pencil line and the guide timber. Another pencil line can be drawn for the guide timber to align with.

NOTE: The inside of the saw blade, in my case, is the side nearest to the motor.

riven patio extension
From the inside of the blade to the edge of the sole plate is 38mm.

Check Measurements That Position The Saw

Clamp the timber in place then check and rechecked the measurement along the length of the pencil line. Clamps may have to be slackened and the timber nudged into place before retightening.

NOTE: The riven nature of the slabs makes clamping the timber awkward. It rocks about the peaks it rests on.

In the above procedure it’s quite acceptable to clamp the timber, slab and bench together. Let the saw cut the slabs on the waste side of the pencil line.

riven patio extension
The 38mm mark is where the pencil line would have been before this slab was cut.

This is how to recheck the position of the timber guide along which the saw runs. The settings on my saw give a pencil line to guide distance of 38mm when it’s on the waste side of the line. The blade is about 2mm thick. When the guide is placed on the ‘finished slab side’ of the line reduce the distance to 36mm to keep the blade within the waste material.

Snapping The Waste Material Off The Cut Slab

With about 5mm left to cut I just snapped the waste piece of slab off by giving it a downward strike with a rubber mallet. It was more or less resting on the bench already so it didn’t move but the shock cracked it. You can see the result of that action below:

riven patio extension

Cutting all the way through the slab without also cutting into the WorkMate would require the complication of raising the slab up on blocks of wood or letting it overhang the end of the WorkMate. In the latter case the way the waste material broke off and fell to the ground would have to be managed too. It might take the slab corner with it.

Cracking off the waste leaves a small ridge on the edge of the slab. There’s no problem when left in place because of the grouting gap between slabs being about 1cm. The ridge can be removed with a carborundum stone or a file with silicon carbide sintered in the steel. These files are made for working on tiles and do a good job on a slab as you can see.

riven patio extension

Laying The Riven Patio Extension

riven patio extension
The Patio Extension with two thirds completed.

The Riven Patio Extension Foundation Was Already Laid

Immediately after my house extension was completed I had a path laid between the fence and the extension. This was built using 900mm x 600mm x 50mm grey pressed slabs. They form a straight run between my shed and the front garden. I had the same person build that path who erected the fence.

Knowing how much effort is required to lay slabs of this size I considered it to be a job for the professionals. I needed it to be in service very quickly to give good access to my shed and prevent there being a muddy mess when winter came.

When the stone foundation was laid for that path I had the foundation laid for the riven patio extension too. They were both dug out, laid and whacked down together. So where the patio would eventually be I had at least stone to walk on. Temporarily I put some old slabs on the stone to make stepping stones and raise the level outside the patio door.

Eventually, all I had to do was lay an inline drain in front of the extension and then follow on with the riven patio slabs.

Laying The Inline Drain For My Riven Patio Extension

At the time of building the house extension the builder determined that the patio drain could be plumbed into the sewer. That isn’t always acceptable to water companies (they prefer it to drain into a soakaway), but my back garden isn’t large enough to build a soakaway. It has sufficient area but a soakaway must be a certain distance from boundaries, and I have three. It was that distance which couldn’t be achieved. The only part of my garden I could use for a soakaway had an area about 600mm x 450mm which is much too small.

How The Drains Are Connected To The Sewer

The original patio drain used a buried 100mm pipe which went around the side of the house to the sewer. Ultimately the builder buried the old pipe under the extension floor and ran a new 100mm pipe near to it for me to provide a new section of inline drain. He connected the two pipes and a gulley into the sewer at the same point using a multi-way connection point.

Both 100mm pipes were bedded in pea gravel to allow movement over time. The extension concrete floor was then laid over the top. A concrete lintel was placed over the pipes in the wall foundation to support the wall above.

This new pipe was turned up at a convenient place to take water from the guttering’s downpipe. The builder knew my intentions regarding the inline drain. He tried to accommodate it while temporarily feeding the downpipe directly into the underground drainpipe. This had to be done to clear the water from the roof prior to me installing the inline drain.

inline drain
On the left the new drainpipe is fitted to the black drain.
On the right the original drainpipe is fitted to the grey drain.
Note: The new drain has a leaf catcher installed.

After the new drain pipe comes through the wall foundation it immediately bends left as we look here and then up to the surface using two push fit bends. I was able to alter the drain’s location slightly by flexing the bend and pipe in the loose pea gravel. That enabled me to align it with the black plastic inline drain section outlet hole I cut. This brought it up to the correct height.

Connecting The Drain To The Underground Pipe

To obtain pipe alignment with the inline drain I had to pull two coupled bends apart by about an inch. I did it by placing a large block of wood in the upturned bend and striking it sideways with a lump hammer. Each strike moved it by a small fraction in the gravel it was laid in. So with a bit of effort I got it where I wanted it.

inline drain
The downpipe feeds into the inline drain cover through a hole cut for the purpose.

Some of the foundation stone had to be removed to deepen the bed area for the inline drain to sit on and then I bedded the plastic sections on a 4:1 mix of sharp sand & cement. I was able to lay it level with the left-hand end surface flush with the grey slab path and the right-hand end surface flush with the old inline drain surface. The drain was laid tight up to the house wall and has proved very satisfactory.

I used four 1m inline drain sections in total which are all interlocked as they are laid. The total length was just under 4m so I just had to cut about 50mm off the last section to make it fit the available space

Fixing The Ends To The Inline Drains

Both old and new inline drains have an end piece to keep the water in. I suspect builders rely on a close fit to keep most of the water within the drain. I glued them on the new drain with PVC filler/adhesive (STELMAX Clear). The ends for the existing inline drain are fixed with polyester resin.

I later cut a drainpipe size hole in the inline drain’s moulded grid using a junior hacksaw blade held using a leather glove. It allowed the builder’s roof downpipe to reach into the inline drain. Now it can discharge rainwater inline with the outlet to the sewer without splashing all over the patio.

I particularly chose the plastic inline drain because it was exactly the same width as the old drain and it had slots with rounded ends. So although the old one was grey colour and the new one is black I like the visible similarities in style.

The two inline drains are different in many ways:

  • the old one being made of resin bonded aggregate with a galvanized grid/cover,
  • and the new one being moulded from black plastic.

The Bedding Mix Used For The Slabs

I used a dry 4:1 bedding mix of sharp sand & cement mixed in a wheelbarrow. The mix wasn’t 100% dry. That’s not really what a dry mix is. It’s usually just a bit damp from the sand which is usually kept outdoors in damp conditions. If the mix is too dry or dries out before the job is finished the cement will blow away in the breeze. You’ll be left with a beech (just sand) which will move and maybe wash away. This will eventually cause the patio to need re-laying. My original patio may need re-grouting but the slabs haven’t moved about even though they had a thinner bed and weaker mix.

The foundation stone, already whacked down, was a little low for the patio but I managed that by laying a deep bedding layer in excess of 75mm in places. So a barrow load, like that shown below, was only enough for a couple of slabs.

riven patio extension
A barrow of partly mixed dry bedding mortar.
riven patio extension
A barrow of well mixed dry bedding mortar.

In the end the extension bedding layer was rather deep. I should maybe have got the path layer to create a deeper foundation. This hasn’t been a problem I just used more sand and cement than anticipated. A year later and it’s still nice and firm with no rocking slabs because I used a 4:1 mix.

Tools For Bedding The Slabs Down

Below are the tools I used for most of the bedding work. I also used a board about 600mm x 100mm x 20mm. Listed they are:

  • 230mm long Spirit Level,
  • 1,200mm long Spirit Level,
  • Rubber Mallet,
  • Lump Hammer,
  • Bolster Chisel (or cold chisel),
  • Pointing Trowel,
  • Paint Scraper,
  • Plasterers’ Float.
tools used to lay slabs
The tools I used to lay patio slabs.

Working Away From The Existing Patio

I worked from the right where the existing patio ended over to the left. The existing patio had an uneven edge left after the building of the house extension. Previously the patio curved around the corner of the house with a 1,200mm quadrant. The new design reused those slabs from that quadrant to make the new curved design. The bedding of those slabs was still in place and hard. I chipped it away with a bolster chisel and lump hammer to allow at least an inch of new bedding mix.

Keeping The Bedding Mix Damp

It was hot at the time of doing the work, and in full summer sun. It was, after all, the summer of 2018. So I only laid enough bedding for one or two slabs at any time. I also had to keep the unused mix in the barrow in a shady place and spray some water on it to keep it damp.

Laying and Levelling The Bed

I shovelled the mix into place and levelled it with a plastering float and the aid of a short board to drag the mix around. A spirit level was used to check the slope of the bed prior to laying. It was then used to check the slope of each slab and the patio extension in general. I didn’t want it actually level I needed it to go uphill slightly as it went away from the house. This was to give a fall for rainwater and to meet the height of the existing lawn. The grey slab path to the shed also had a slope to it which had to be flush with the patio extension.

Checking The Slope of Slabs After Laying Them

Using a spirit level to check the slope on riven slabs has its problems. You have to check in several places and get a feel for how the leveling of each slab is going. It’s important to take a good look after they’re down. Use a long level to check the average of several slabs.

Continue by laying the next slab and tap it down with a rubber mallet. The mix starts off loose and you can see the slab sink down when it’s hit. This action compacts and firms up the bed. Then further striking doesn’t make a slab sink much more. Lift slabs that don’t bed down well and re-lay them to get a better slope or to stop them wobbling.

Slabs can be gripped by the fingertips and slid around on the bed to roll the sand about and improve there slope or flatness. I’ve found that if there is a side of a slab that is too high it’s possible to grip it by the edges and slide it towards that edge. The slab’s own weight drags the surfeit of mix off the bed under the slab and onto an adjacent area. The slab is then slid back in place with a light touch without disturbing the bed so much.

The bed will keep its position once it sets with moisture from the ground. That usually happens overnight when everywhere damps down.

Removing Excess Bedding Mortar

Excess bedding mortar moved towards a laid slab sometimes has to be scooped off the bed with a pointing trowel or paint scraper while there is a gap between the slabs.

The bolster chisel and paint scraper can be used to prise slabs apart when they get pushed too close together.

I cut all four slabs that met the grey slab path down in size to fit in the available space.

I can say that although this has never been my line of regular work I have laid many slabs from time to time. These patio slabs are not hard work compared to 900mm x 600mm x 50mm pressed slabs which are really a two-person job.

Some of the early riven slabs I got in 2001 were misshapen and they weren’t all the same thickness. That’s why I lay them one at a time. The latest ones were of a better quality so It may now be possible to lay them on a bed leveled over a large area.

While laying slabs I Learned These Skills

  • How to hit slabs in the right places to level them,
  • How to hit them hard enough to level them,
  • How to hit gently so that slabs don’t break,
  • How to slide them about a bit to reposition the mix,
  • How to get the desired slope with the spirit level bubble off centre,
  • How to cut slabs with a diamond blade in a normal electric saw.


The SKILSAW® Used Here

From the rating plate below you can see that my SKILSAW® takes blades up to 190mm (7½”) in diameter. It’s not shown on the plate but the motor shaft is 22mm diameter.

My SKILSAW® Rating Plate



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