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Hi All So I've posted a few topics on the extension I'm having regarding UFH. So far, I've originally specced a Wunda EPS400 SYSTEM covering 52SQM across both the original part of the house and the new extension. This was on the assumption that beneath the current floor (15mm engineered oak), there was a thin ply board on top of the existing slab. The builder has removed the flooring to find there's the following: 20mm plyboard 50MM of polystyrene insulation which sits on the existing slab (picture attached) Approximately 90MM of depth to line up with new extension slab, although this can go to 120MM potentially. There are two options or more if you advise: Remove the rest of it to expose slab, add 50mm celotex xr4000, membrane then Wunda EPS400 Remove existing insulation, install 50mm celotex, membrane then UFH screed. I'm sure there will be better suggestions and thanks for your input in advance. Ultimately the outcome is to have an UFH system which now it seems the existing slab may not need to be dug out due to the level of the new slab which is yet to go in.

Hi, I’ve got a rear extension within 45sqm of floor area which will be new floor (insulated, concrete and liquid screed with ufh). The existing area at rear of house is 25sqm of timber suspended floor. I’ve looked at the void is 750mm deep. Is this too much to make up? So I just leave the existing as suspended and run ufh under floor boards onto insulation between joists then vent it through the new slab or fill it? The other advantage of doing that would be less chance of crack straight across what will be a new porcelain kitchen floor. Thanks in advance

Hi all I'm about to put in a concrete pad for an external water tank that will weigh 3 tons when full. This will be the biggest concrete job I've tackled and I'd appreciate your input please. The top soil has been dug away and the pad will sit on the surface of very heavy clay sub soil (currently also supporting the foundations of our self-build house). Dimensions of the pad are 2700mm square and 125mm thick. A C40 mix seems to be best for septic tanks so I'm assuming the same for this. I'll be mixing it myself, and I plan to add grid rebar. My questions are: 1. should I put down membrane and gravel layer first? If so, can I use a random mix of rocks and stones (we have plenty on site)? 2. there's currently a layer of soupy churned up clay on the surface, is it best to remove this first? 3. I've found variations online for a C40 mix, can anyone tell me the correct mix? 4. If necessary could I pour the slab in two sittings? TIA, any other suggestions welcome.

After much heart ache, time and tears I'm looking for some advice from you guys regarding my heating system, which is completely separate from DHW. I plan to directly heat my slab with ASHP using Willis heaters as back up. Hydraulically does this look ok? Input much appreciated.

Hello, I’m refurbishing a Victorian terrace including removing chimney to create open plan space at the rear. Existing solid floor is being dug up and replaced to include underfloor heating. Existing quote and spec was hardcore, sand binding, DPM, 150mm concrete slab, 120mm PIR insulation, 75mm sand cement screed. Builders are now suggesting hardcore, sand binding, DPM, 120mm insulation, then 150mm flow screed i.e. no concrete slab. Aside from issues this might create with the heating (less responsive due to thicker top layer), is anyone able to comment on the structure of this build-up? I've seen specs for similar build-ups (i.e. insulation directly onto hardcore instead of slab), but that's when the top slab is 150mm concrete, not flow screed. I'm not convinced in not having a concrete slab somewhere in the flooring structure, but would be good to get some thoughts... Many thanks, David

I'd like to run the slightly vague ideas I have past the forum-mind: How do you bring in mains water through your wall(s) and up through the slab? My hazy idea is that I run the blue alkathene through a suitably lintelled hole in my (solid, 215mm) wall, sleaved in 110mm soil pipe that is in turn surrounded by 'pea gravel'. Then presumably the alkathene is directed up through the slab to a stopcock (under the kitchen sink in our case, just the other side of the wall). Does the pipe run un-sleeved up through the slab concrete? How is airtightness maintained? I'm a bit clueless here, all info gratefully accepted.

I am annoyed this morning. Once again my washing-up water - the first hot water I have used in the kitchen today - is running warm then cold then hot. And the cold water is running warm then cold. This probably means that the last people, who renovated the house, did not insulate the water pipes where they pass through the zone where there is underfloor heating, and the water standing in the pipes has heated up. A small annoyance due to lack of sweat applied to the detail. But one that is noticeable and about which I can do nothing practically. Boo !

The slab team from MBC arrived on site this morning. It's like having the building version of whirling dervishes who've just dropped a few speedballs. My word, they make progress! The team is headed up by Harry and he has 3 others in his team, but this will fluctuate a little over the course of the job with Harry needing to have a look at another job for most of tomorrow then the younger lad taking some leave to go to a music festival. Tsk, the youth of today! He worked like a machine, though, apart from the bit where he nearly rolled over one of the piles as he was looking in awed astonishment as a rather attractive young lady farmer drove past on the nearby track in a JCB that was most definitely bigger than the roller machine he was on. I don't think it was the vehicle that caught his attention so much as the driver. Sniggers all round. I arrived just after 8 am this morning and the first lot of hardcore had already been delivered. In total, there were 4 loads of type 1, but I piggybacked onto this and ordered an additional load (paid for by me) which the team will then spread and roller for me in the area beyond the bucket in the above photo, which will create a nice level area for the crane when it arrives to bring in the timber frame. I've had really good luck with the weather so far and hope it continues, but if it rains between now the completion of the timber frame, the site will turn to mud PDQ and slow things down horribly. Once the hardcore was going down and getting compacted, the piles were cut off to the correct height, leaving the rebar in position, ready to be tied into the beams. I have no idea what you call the digger thingy that they are using to move the stone around the site, but it's an impressive beast. It looks a lot like the bottom of a tank with its caterpillar tracks and then something a bit more transformers-like with its swivelly cab and arm. Either way, it was mechanical poetry in motion when driven by someone who clearly knew what they were doing. The team will have been working till 7pm this evening, so they will have got all the hardcore down and compacted and were going to start on the sand, if they had the time. The first load of sand arrived about 4.30 this afternoon, more to follow on tomorrow morning. As well as working 12 hour days, Harry has already had a chat with the neighbours to let them know that they will be working over the weekend, too, on both days. The insulation is due for delivery tomorrow and they will be putting the pipes that carry the service cables into this, along with the UFH pipes. I'm not sure when the steels will arrive, but that must be also imminent as the piles will need to be tied in before the concrete is poured. The building inspector is coming on Monday to check out everything before the pour. For interested parties, THE CONCRETE POUR IS CURRENTLY SCHEDULED FOR TUESDAY. I've read a couple of horror stories and some not-quite-horror-but-not-very-nice-stories about uneven slabs, so I've told Harry that before they leave site, I need him to demonstrate with a laser that everywhere that a wall rests is absolutely level and within tolerance. Harry is a man of few words and he didn't quite bat an eyelid, but I explained that I would much rather that something like that is demonstrated rather than just verbally assured. He seemed fine with it. So, one final picture of the hardcore going down, from the garage side of the house. More blow-by-blow action to follow tomorrow.

I wasn't going to visit the site today, but we've had heavy rain showers today in Dorset and I thought that would be an ideal opportunity to see how level the slab looks after its late night power floating. My reasoning was that whilst I can't identify any high spots by eye, it would be easy to look for the low ones by where the puddles were lying. Here's a photo taken from a slightly elevated viewpoint (the top of a pile of wood chippings!), looking from the south east corner where the snug will be, over towards the north west corner, where the main living area will be. Most of the puddles that you can see are barely a couple of millimetres. I'm not sure how long it had been since the shower that caused these, but it was a breezy day and not hot. The next is taken from the other end of the right hand side of the property, as seen above. Between the brown foul waste pipe and the white UFH pipes, you can just about see that there is a hole in the slab. This is meant to be there right now, but is due to a mishap yesterday. As the concrete was being transferred in the digger and poured from the bucket, the digger rocked slightly and the bucket bounced on the exposed UFH pipes. Harry from MBC reckoned that one of the pipes has been damaged as a result, and so the area around the punctured pipe has been left uncovered. My trusty plumber/UFH person will be coming early next week to fix the damaged section of pipe and make good on the concrete floor, and MBC will be covering the cost of this. Once I have the bill for the repair, I will pay it and MBC will deduct the amount from my next stage payment. This was all agreed this morning without any arguments or quibbles. So far, all the puddles in the photos have only been a couple of mm deep. The deepest is on the far north west corner of the living area, shown below. It's not easy to guage the depth of this area, but I think it's about 4mm at the deepest. The thingies are a couple of end caps that get put on the pile rebar, but the wind was blowing them towards me. You can see how they are tilting. Here's another view of the same: I don't recall the exact tolerance that the slab needs to be within off the top of my head, and I'm not going hunting for it right now as I'm one g&t into Friday evening with a couple more to follow, so no point now. However, the figure of 5mm is scratching away in the deepest recesses of my grey cells, so I think this should be acceptable. If anyone knows otherwise, please speak up! The finish on the surface overall is very nice. I had a walk over the whole thing and couldn't see anything obvious, but then apart from squishy concrete under my boots, I wouldn't know quite what to look for anyone. To my unpracticed eye, it looks pretty good. One very good think that came about from all that excess concrete being dumped all over the place yesterday is that the team spread it all out between the hard standing and the slab, so I now have an even more level and sturdy surface for the crane when it arrives with the timber frame:

So I have had a ten ton digger pecking rock out to make way for a basement and extension. Was planning on 200mm EPS under the slab, and 200mm EPS around the exterior of the basement walls. I do have two walls against the existing house to retain the house founds, as well as creating the basement walls. SE has categorically stated he will not entertain EPS300 under the slab. We are on solid rock for almost the whole slab. It is supporting three stories above. Some pics showing existing founds etc. Was going to use Jackodur system as it also creates the former to pour the concrete too. I was initially concerned when we exposed the existing house founds and the SE had a fit and demanded all work stop, until the house was underpinned. However I elected to continue and pecker out the rock, wether through luck or judgement the house is still standing. So now I either accept the SE proposals or find another one for a second opinion. Rebar sched2 (1).pdf SE version2.pdf

MBC are currently scheduled to pour my slab next Tuesday, no idea what time yet. If anyone would like to come along, let me know. There are details on my BH blog of today's work on the slab with some photos of progress.

Spent the weekend setting out our house. Hired a theodolite. Turns out theodolites have gone digital in the 23 years since we last used one ? but a You Tube video helped and we spent a joyous 3 hours setting out all the corners of the house. Amazingly, we have dug pretty much in the right place. And we feel more comfortable setting out accurately rather than measuring from a vague boundary hedge. MBC on site Wed / Thursday (in theory). Our sub-base is going in now (MBC doing the top 150mm MOT type 1). Just wondering about ducts. for water and BT. We also need to run the electric armoured cable in. We know we should have posted this question earlier. It has been bothering us for weeks. Site plan looks roughly like this. Its a weird shape (cut off a larger house behind). Plant room is shaded in yellow. Intended route of water (blue) and electricity (black, from kiosk) shown. Both go under the garage slab so we are ducting that tomorrow. Do we run these ducts at 700mm deep to the plant room and then up into the plant room? i.e. is the bit I am filling with around 300mm subbase as I type going to have to be dug out to put these ducts in? Or do they rise slowly and then go in the final layer (which MBC are laying)? BT duct is another question but please help us sort out water and elec first. Thanks.

We are looking for some help from anyone with experience of MBC slabs. Its the order of doing things that is confusing us. We believe we need a land drain. We will also have surface water drainage. We have been advised by a drainage contractor to do the drainage (foul and surface inc soakaways) after MBC put the frame up, so probably after the roof is on and the rainwater pipes in place? Maybe before the render because of dirt? So is the best order broadly as follows? Dig foundations, compact hardcore MBC slab MBC frame Roof / windows Drainage (foul, surface, soakaways) plus other service trenches Render / cladding Where does the land drain fit into all this? It seemingly is supposed to go where the hardcore is around the slab. So do we dig that out again (when we do the other drainage) or do we put it in at the same time as compacting the hardcore. Opinions please. And thank you.

We're extending rearwards to create a large open plan kitchen / dining / living area. The existing kitchen / diner at the back of the house used to be two rooms (kitchen and back half of the lounge. I built a stud wall to divide the lounge and then took down the wall between the new room and the existing kitchen. The floor is very much less than flat, particularly in the middle where the wall used to be, but also around the edges of the lounge side of the room. The intention was to dig up the existing slab and relay with insulation and UFH as per the new slab in the extension. I have taken up some of the screed to expose the slab. The screed varies between 10mm and 55mm thick. The slab is concrete approx 50mm thick, poured directly onto approx 200mm 20-30mm stones/pebbles. First thought was to take all of this up and replace. This will probably be OK in the original kitchen as there is almost certainly a footing under the doorway, and there is a footing where the old dividing wall used to be. However, in the other side, (what used to be the back of the lounge), the slab runs the whole length of the house, so there is unlikely to be a footing underneath where I have put the stud wall. Any ideas how to prevent the stones under the front of the slab from being disturbed whilst I remove the stones from the back? I was thinking of some kind of shuttering, but not really sure how to go about installing it with 200mm of stones trying to slide around.

OK, so I know my slab isn't level, you only need your eyes to see that. As an example; on the large sliding door I've got 20mm difference from one end to another (over 4m). I've got 5 sets of doors with level thresholds. Huge amounts of pre-planning detail was put into ensuring the doors would sit correctly into the slab so that the slab floor was 18mm below the FFL to allow for the tiling that is running through the entire house. I was lucky enough to be lent one of these: https://www.powertoolsuk.co.uk/bosch-gsl2set-floor-surface-lasers.html?gclid=EAIaIQobChMIuZPQ3tiI1gIV6rDtCh3uSwV-EAQYAyABEgJ3z_D_BwE so in a spare hour yesterday (whilst nursing my severe man flu, sympathy please) thought I'd have a look at "how the land lies" (quite literally! ). Its an incredible peice of kit, very clever. But alas all it did was make by brain hurt . The floor levels run all over the place and I'm failing to see how a tiler is ever going to create the beautifully flat level access around the house which was so important to us. But im no pro tiler and so perhaps I'm just worrying too much (as we all know a self builders biggest affliction). So what now? MBC have offered to come back and level the slab but from the measurements I've taken where they will level one door they'll throw the levels out elesewhere. I was was wanting to use the laser above to gauge out the various level changes so that I got what I wanted (I'm not entirely convinced MBC have got the skill set to put this right). But with so many conflicting levels I just got myself lost. So so some specific questions: 1. How do you choose a rooms datum point. I assume it's normally the highest point and work out from there. Or do you just go with the middle of the room? 2. Can a pro tiler (like our resident @Nickfromwales) "loose" these level changes? Presumably by feathering out across as big a distance as possible? 3. Is it sometimes better just to grind off an area of high spot? Presumably there are tools for the job that are designed to do this and get into corners? (Without trashing my expensive frames!). Many thanks in advance.

Pouring slab soon and how have people terminated their soil pipes pre pour? Trying to make power floating easier so do not want loads of pipes sticking up but also may want to make connection for toilets etc at low level. Have people just put on a coupling with temporary socket on the end of the pipe or something more elaborate? TIA

Hi All, I've managed to get a contractor quote me to pour and level our slab. It's a 'renoextension' jobbie (Extensions are up and old slab removed from current house) so all internal walls are up and I won't have any other brick/block walls going up on the new slab. Only major load will the the stairs. Buildup: Elevated A142 mesh with UFH cable tied 300mm EPS Insulation Compacted MOT T1 I therefore don't regard this as a structural slab. With the mesh in the slab, the mix will be a GEN1. I know others here like @JSHarris had C35 mix but that's only as they are true structural slabs, so I don't think I need to go that way. Would you agree? Some people around here keep on ranting on about not forgetting to add fibres.... Did someone forget??? looking at--> @Onoff Also, do I still need to add fibres? £15 extra/m3 seems to be worth adding I guess. I called the concrete company the aforementioned contractor uses and I was quoted £2130 (inc) for 15m3 so not too bad seeing that includes the pump. Nice bit of profit for the contractor who says the job will take 2 days with the two of them with the total cost being £4000 (excluding fibres)! So labour charge comes to about £400each per day! Is that realistic? Said he'd come rounf the day before to set up lasers and mirrors etc, but I bet he'll end up doing that 1/2hr before concrete arrives! I'm asking for a breakdown of his figures, but that does seem a bit high. I've called another national concrete firm and their price with fibres is £300 cheaper but then does not include the cost of a pump (which is likely to cost at least £300-£400 around here. So it's just his labour cost that seems excessive, imho. Guy seems to know his stuff at least and came recommended by a local architect and having found it difficult to find someone to pour the slab I may just have to bite the bullet. OB PS special offer at waitrose: https://www.mysupermarket.co.uk/tesco-price-comparison/Breakfast_Cereals/Kelloggs_Fruit_n_Fibre_500g.html

I've just about completed my slab insulation for my extensions (EPS + Celotex) and old bungalow (EPS). I've used 25mm of EPS perimeter insulation and will probably stick on the 8mm roll stuff that I will get from UFH supplier. So I'm just about ready to get my 2.4x4.8m mesh sheets into place (will have to cut most of the buggers in order to get through door ways. Luckily I should at least get them through the larger sliding door at the rear. The plan is to have UFH cable tied to mesh and then have a wet mix concrete pour. No final screed! Before I start cutting and laying them, i have some questions: 1) How much overlap should I have? Is one 'square' overlap enough? The mesh has 200mm squares. 2) How far from the walls should I have the mesh? I know there should be some distance and I think I read somewhere 150mm? 3) Tying wire. As it's just a case of keeping the mesh in situ whilst concrete is being poured, would cheapo galv wired be sufficient rather than defacto tying wire? e.g. https://www.toolstation.com/shop/Landscaping/d130/Wire+Fencing+%26+Tools/sd3333/Galvanised+Straining+Wire/p40640 4) Should I continue the mesh through doorways? 5) Not mesh related, but if I am supposed to cover the celotex top layer (in the extensions only) with a poly sheet (vapour barrier) or should I use something more heavy duty like a DPM? (I have the DPM/radon barrier in already but further down the layers. My only concern with the lighter vapour sheets is that it is more prone to being punctured / ripped with the mesh (I will be using chairs) and people (i.e. me!) repeatedly tripping over the mesh. 6) I intend to tape up the celotex joins (not the EPS sheets). Can I use any tape to do this or need the foil type tape? Thanks in advance.

Here's our floor construction. I'd appreciate any opinions on it and in particular on a couple of details currently in decision limbo: 1) I'd assumed the DPM would go as pictured in this diagram - under the insulation - as it seems to make sense to have it contained within the waterproof envelope. A couple of people have suggested that the membrane goes OVER the insulation though. 2) slab reinforcement & UFH... am I overkilling by adding reinforcement mesh mainly to give me something to attach the UFH pipes to? Would it be easier to clip the UFH to the insulation (I saw a handy clip gun thing) and leave out the steel? By the way, the EWI isn't shown on the diagram above but is (probably) 200mm of EPS extending down 200mm below the slab insulation.

Okay, big iceberg delivery yesterday, so now I need to start laying the insulation for my slab: Bungalow has two extensions which are piled slabs – both extension slabs will have a mix of PIR and EPS. The bungalow floor has been dug up, hardcore is down and I am ready to fill that with EPS. The extension slabs both have SIPS sitting on blockwork, with cold-bridging around the sole plate best mitigated with marmox block beneath and external insulation on outside SIP wall (see pic). No feedback required on that please! : ) I just need a sanity check on the slab insulation please: Bamboo flooring 80mm meshed concrete with UFH (with 25mm EPS upstands) DPM 100mm PIR 160mm EPS Radon barrier (lapping with DPC under sole plate) 225mm Existing piled slab A mix of PIR and EPS to attain desired u-value / get correct FFL height. My thoughts were that it’s safest to get PIR on top of the EPS away from piled slab in terms of moisture. I was also thinking the PIR is better at withstanding point loads from the mesh chairs/spacers that will keep the mesh elevated above the insulation. Should I wrap it entirely in polythene/DPM to prevent moisture travelling up into the PIR? I was just going to cover the top with DPM as it is foil faced. Or should I have EPS on top and PIR at the bottom? Or does it matter?? Feedback appreciated. OB