Alex Carr

Correct, mostly. One concrete floor with PIR 70mm insulation fitted betwee the joists on a slimline joist grid; chipboard on top of that; visqueen; ufh pipes. Two suspended floors with PIR 100mm insulation fitted between the joists with layout as above. All three floors to be tiled. There are five rooms in total, each with individual build-ups, all planned to be level with each other at the end of the day, the other two rooms with engineered wood floors. Two tiling respondents referred to decoupling mats, which sound too good to be true: do away with the cement boards and battens in the biscuit screed; increase the screed thickness to 70mm; lay the mats; lay the tiles. I think I'd like to be told that would work, but if that isn't possible the reason why would be good.

A decoupling mat? That's a new one for me. Thanks for the tip; I'm going to be reading up the subject.

If you’re worried about expansion you can fit a decoupling mat. It’s not a cheap option though. But there a reasonable difference in finished floor level between fitting a 12mm or not so presumably you have that all in hand.

Some guidance would be really appreciated as I just can't find what I want on the web. Here we go: Wet underfloor heating pipes have just been laid. Next step: a 50mm thick biscuit screed in 3 rooms, interspersed with battens to support the floor when it's laid. This is what I'm looking for: a cement overlay panel (not a backing board) @ 12mm thick with low thermal resistance, to lay on the screed and then a ceramic tile floor on top of that. I can find all kinds of variations on this theme but not what I'm describing here. I can get 18mm thick concrete boards but I'm figuring it will take longer for the heat from the UFH pipes to permeate the screed, with the screed having to heat up the board and then the board having to heat up the tiles - I could be wrong, but it seems a big ask to expect all this. A 12mm cement board overlay would go some of the way to reducing heat trasmission time, surely? And no, I can't lay the tiles on the biscuit screed as that's asking for trouble with the screed expanding and contracting with the temperature changes and possibly causing the tiles to crack. Look forward to a bit of wisdom here. Thanks in advance.

Hmmm...

Problem is, screeders seem thin on the ground here in Edinburgh. I mean, how does one find them? The internet offers v. little in the way of people who do this kind of work except big companys. I have a feeling I might have to suck it up as he's scheduled to start Monday.

I have 80m2 of basement floor (5 x rooms + hallway), already laid with ufh pipework. The contractor in the frame for laying a biscuit screeed (sharp sand in an 8:1 mix) 50mm thick, interspesed with battens at 400mm centres is asking for a £9,000 fee, which includes materials & labour for his crew. But here's the thing: the sand (2 x tonne bags) & cement have to be crane lifted over the back wall where there will be 2 or 3 or possibly even 4 mixers on the go (so he says); from there it's a 30 mts walk with wheelbarrow(s), down 3 steps and then a couple more metres to half a dozen more steps and a turn through a conservatory into the property, where the mix is dumped to where 2 x screeders will be doing the tamping and levelling. The contractor's fee works out at £112.50 per. square metre, which seems steep, but I'm not sure whether it is or not, given the logistics of hiring mixers, delivery charges, barrow hire (possibly) moving the mix from the mixing point to where it is needed, etc. Any wise folk out there with an informed opinion on this?

That's interesting: I will insist to the contractor that he be precise in creating the mix, and as we have been working well together for the past 12 months I don't expect issues. What bothers me is my ignorance about how to assess the moisture level of the the sand - which, I presume, is determined by how it has been stored; if it's outside in a big pile then the weather becomes a factor, surely? I exited the building trade more than 30 years ago, so maybe these days sand comes already bagged up - is it or not, do you know? Buckets: good idea. I ordered a hygrometer today because half of our 200 year-old Edinburgh property is a basement that was soaking a year ago because of decades of v. poor ventilation: we stripped everything back to the bare brickwork and the stone of the external walls and lined every surface with Permaguard joined up at the base with a DPM; fingers crossed on that. Too we have PIR insulation throughout in the various build-ups. The air feels dry, but I'm very keen to get an accurately monitor ongoing relative humidity levels. The upturned glass is a delightful idea. I'd be happy to report back. Thanks for taking the time to write.

I don't get what you mean, sorry. If you have a moment, please would you explain.

Hi - Well, no, I don't have a mix recipe - I'm not sure I know what you mean. How is the water content gotten right? I mean, the contractor goes to the builder's merchants, buys his sharp sand in bags, ditto cement; 8 shovels of the sand into the mixer + 1 of cement. How is there more to it than this? I mean, how does my contractor avoid buying a load of wet sand? Thanks for the pointer on the fibres; I won't add them. Apropos drying time: I thought I would use a hygromenter to measure the humidity level of the rooms where the screed is going to be laid; this will give me some indication of how much moisture is coming off the screed, though I'm nore sure how to leverage that information.

I'm getting an ASHP & wet underfloor heating installed on Tuesday and Wednesday and an 8:1 biscuit screed laid over the top to a depth of 47 - 50mm. Solid answers to the three questions I have would be really helpful: 1) How beneficial would it be to add polypropyline fibres to the mix? 2) And in what proportions? 3) How long will it take for the mix to dry sufficiently to float 14mm engineered wood flooring over the surface?

So informative were the posts I've been reading on this forum I became a member. I'm about to embark on extensive renovation of 922metres of space extending over the lower two floors of a 4-floor, 200 yr old Grade B listed building and would really value advice on making the right choices about various aspects of the work. For example: With regard to wet underfloor heating. I'm about to have the basement tanked, on top of which will be laid a concrete slab (150mm estimated depth) and on top of this either 75mm or 100mm foil-backed insulation boards & above this a layer of visqueen + screed with all the pipework gubbins embedded in it. My preference is to use 75mm insulation boards because they cost less, but is there is a measurable advantage in using 100mm boards instead?