I built my two storey extension for 20% of builder's quotes

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Laying a Concrete Slab with insulation


I plan to lay a concrete floor with insulation. Most commonly, builders lay the hardcore, sand, DPM and concrete slab finishing well below finished floor level. Once the house / extension is built, floor insulation and a screed layer of around 75mm (with UFH pipes if required) is then laid above the concrete slab. Tiles or floor finish of choice is then laid on the screed.

However, as Mark Brinkley suggests in his book,The House Builder's Bible, the screed layer can easily be omitted provided you are suitably organised. As most builders do not want to involve the UFH guys at this stage it is rarely done, but as a one man outfit responsible for all aspects of the build I get the luxury of going at my own pace and doing it my own way.

There are several reasons I chose to omit the screed layer.

As the overall thickness of the floor is reduced, less excavation and subsequent removal from site of the waste is needed, saving labour and cost.

There will be a further cost saving as I do not need to lay a screed. (For me this is just material costs but if you are not doing it yourself there is also a significant labour cost saving)

I have my heart set on creating an acid stained concrete floor, so I need a concrete finish. If you have never heard of it, I suggest you Google and Youtube - acid stained concrete. Done well, they are amazing, and I really fancy having a go at this finish. Acid stained concrete is more popular in the States.

The down side to this method of construction is that I need to be well organised. For example, I need to have my UFH laid prior to pouring the slab. As the slab is my finished floor, I also need to ensure I get a good quality finish and be sure to order the right amount of readymix to bring me exactly to finished floor level.

Also, the concrete into which I will lay my UFH pipes is a little thicker (100mm) than a traditional screed (75mm). This may reduce reaction time for warming up. As UFH systems are not designed to be turned on and off with fast reaction, I don't think this will be an issue. I spoke with a couple of guys that have used this system and they say that reaction times are not appreciably slower anyway. They suggest that this may be because concrete is a better conductor than traditional screed that, even well compacted, has tiny voids.

Here goes.

This is my floor slab construction detail, as approved by the BCO.

  1. 100mm Concrete slab with A142 fabric
  2. DPM / Vapour barrier
  3. 65mm Xtratherm insulation
  4. DPM
  5. 25mm sand blinding
  6. 150mm type 1 hardcore     

Type1Twelve tonnes of Type 1 scalpings were duly dumped outside the front of my house. You guessed it, it all needed barrowing round the back. Somebody needs to rake out the scalpings. Cue, beautiful assistant.

Next I needed to compact the scalpings. As I planned to do quite a bit of compaction, I bought a wacker off ebay for about £75, which I am sure will be cheaper and way more convenient than repeated hirings.

Whack, rake, whack, rake - eventually I got a pretty level and compacted surface.

Moles! No, just two and a half tons of sand for the blinding. Now it's time for more rake, whack, rake whack - you know the routine.

The first layer of DPM was laid on the levelled and compacted sand. Joints were taped with double sided tape and then further tape over the top.

rakingBecause I would have no further covering above my slab it was important that the slab ran into the doorways to prevent the edges from showing. A builder suggested I cut out some blockwork to allow for this. I will, of course, run my edge insulation around to prevent cold bridging. On that basis, the BCO was happy.

Although 65mm of insulation met the regulations, I had sufficient depth to lay 75mm. In due course, I would also run 25mm of insulation vertically around the edge to prevent cold bridging.

The BCO had asked that I lay a second DPM above the insulation to act as a vapour barrier. I'm not sure I really understood the need for this but as it wouldn't break the bank it certainly wasn't worth arguing about. I doubt is should make any difference, but I laid the second DPM in a slightly different pattern so that joins of the top and bottom DPM did not line up.


wackerI laid my A142 mesh and wired them all together as suggested in the NHBC good practice guide. I supported the mesh on small (no bigger than 50mm square) blocks to allow a space of approx 50mm below the mesh. I used my level to ensure that I had sufficient room from the top of the mesh to the final surface of the concrete.

The concrete slab will be my finished floor. It was necessary, therefore, for me to lay the UFH pipes into the concrete. This, of course, meant I had to design the layout. Having read a good deal, I came up with the proposed ground floor layout in pic 10.


sand pilesHowever, not being a UFH expert, I wanted confirmation that my design was ok, or to have it corrected if it wasn't. I contacted several UFH companies and sent them my proposal. Nearly all companies held onto the plans for around three weeks and then issued me a price for all the materials required. These prices varied hugely, and I felt I could not be sure what I would be getting for my money. I understand that they do not wish to just offer a design for free, but this way of working was not very helpful to me.



dpm 1Eventually I found UFH Supply - they work slightly differently. I sent them my drawing and they came back telling me that the layout was broadly sound but required some modification. For a very reasonable fee of £40 they would modify my design and then allow me to purchase all the necessary parts from wherever I chose. This was a far better system for the way I work.






door cutoutsBelow is the modified drawing from UFH Supply. The main change being to break the large Kitchen/Utility zone into two separate zones because the pipe length would be excessive - likewise for the Lounge area. (I don't know why the hall and snug area are not showing in the pic but the layout is unchanged)

I found UFH Supply both helpful and speedy.

Following my design, I strapped the UFH pipes to my 142 mesh. It was a real pain in the butt stepping between the mesh! 


floor insulationI connected up the pipes to a 6 port manifold, fitting short dummy loops to the zones I was not laying at this stage. I then attached hosepipe to the inlet and outlet to fill the pipes with water. By opening and closing each zone and running the pump for short periods I managed to purge the system of air.

I closed off the valves and removed the hosepipes and left the system sitting at about 2.5 bar (mains pressure) I know that pros pressurise the system to more like 6 bar to check for leaks over 24 hours. However, it would be another week before I could pour the concrete so a UFH installer suggested that if my system stayed at 2.5 bar for this period I should be ok.

142 fabricThough I was now ready to pour my concrete, the weather had other ideas. Rain, rain, rain. Despite my best endeavours to cover the area and prevent creating a swimming pool, water will get wherever it wants. It was hard to get the water out with the mesh and pipe work in place but with ladle and mop I managed to dry it out ready for the concrete.

But then it rained again before I could organise the concrete so I did it all over again. In the end I ordered and cancelled the concrete three times because of the weather. I was now getting really fed up with trying to step over the mesh. On the plus side, I now knew my pipe work held pressure for the best part of three weeks.


ufh 1ufh 2








ufh pipesConcrete mixes vary considerable so I contacted my concrete supplier and told them what I had planned. I was advised on the correct mix and decided to have nylon fibres mixed into the concrete to further reduce the risk of cracking. Because of awkward access I also planned to have the concrete pumped in. All these factors can effect the best mix, so it is a good idea to make sure your concrete supplier knows all the details.

The guy that came with the concrete pump offered to stay and help me tamp down and level the concrete whilst his mate cleaned up the pump. I was short handed so this seemed like a great offer. We tamped it and leveled it in traditional way. I had hired a Magic Screed to do the leveling and thought I would go over with this after the traditional tamping. However, I found the the concrete was setting much quicker than I had anticipated and the concrete was getting too sticky to use the Magic Screed. Still - it was well tamped down so, when it was ready, I set to with a power float.














The finished slab.

I had not used a float before and, having learnt, know I could do it better if I had another go. That said, the finish, though not perfect, is pretty good. My floor level is spot on and I ordered exactly the right amount of concrete!

I will need to clean the slab before I get around to staining it, and I could always do some light grinding / polishing at that stage if I deem it necessary.

I am sooooooooo pleased not to be stepping down into an abyss or tripping over mesh and pipe work every time I step out of the back door.