Alex Carr Posted April 18 Share Posted April 18 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. Link to comment Share on other sites More sharing options...
Kelvin Posted April 18 Share Posted April 18 (edited) What problem are you trying to fix? Your aim with UFH in a screed is to get the house at the desired temperature and then more or less keep it there rather than using it like a radiator. Therefore while it might take longer to heat initially it’ll be fine thereafter. 50mm screed isn’t that thick, it’s almost close to the minimum depth so UFH will react more quickly even with something else on top. Some people on here have their UFH pipes in 100mm concrete slab so it performs like a heat storage radiator. 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. Edited April 18 by Kelvin Link to comment Share on other sites More sharing options...
Thorfun Posted April 18 Share Posted April 18 17 minutes ago, Alex Carr said: 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. isn't that what ditra matting is for? (other decoupling mats are available) our tiles (the entire ground floor pretty much) was laid on a cement based liquid screed with a decoupling mat. tiler had no issues doing this. Link to comment Share on other sites More sharing options...
nod Posted April 18 Share Posted April 18 Why would you lay cement boards under the tiles Your doubling up on likely issues Link to comment Share on other sites More sharing options...
Russell griffiths Posted April 18 Share Posted April 18 I don’t think we are getting the full story, I believe this could be a non structural biscuit screed on a suspended floor. Link to comment Share on other sites More sharing options...
Alex Carr Posted April 18 Author Share Posted April 18 2 hours ago, Kelvin said: What problem are you trying to fix? Your aim with UFH in a screed is to get the house at the desired temperature and then more or less keep it there rather than using it like a radiator. Therefore while it might take longer to heat initially it’ll be fine thereafter. 50mm screed isn’t that thick, it’s almost close to the minimum depth so UFH will react more quickly even with something else on top. Some people on here have their UFH pipes in 100mm concrete slab so it performs like a heat storage radiator. 2 hours ago, Kelvin said: What problem are you trying to fix? Your aim with UFH in a screed is to get the house at the desired temperature and then more or less keep it there rather than using it like a radiator. Therefore while it might take longer to heat initially it’ll be fine thereafter. 50mm screed isn’t that thick, it’s almost close to the minimum depth so UFH will react more quickly even with something else on top. Some people on here have their UFH pipes in 100mm concrete slab so it performs like a heat storage radiator. 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. 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. 2 hours ago, Kelvin said: What problem are you trying to fix? Your aim with UFH in a screed is to get the house at the desired temperature and then more or less keep it there rather than using it like a radiator. Therefore while it might take longer to heat initially it’ll be fine thereafter. 50mm screed isn’t that thick, it’s almost close to the minimum depth so UFH will react more quickly even with something else on top. Some people on here have their UFH pipes in 100mm concrete slab so it performs like a heat storage radiator. 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. Link to comment Share on other sites More sharing options...
Alex Carr Posted April 18 Author Share Posted April 18 A decoupling mat? That's a new one for me. Thanks for the tip; I'm going to be reading up the subject. Link to comment Share on other sites More sharing options...
garrymartin Posted April 18 Share Posted April 18 +1 for the decoupling mat - exactly the situation they're intended for. Link to comment Share on other sites More sharing options...
Moonshine Posted April 18 Share Posted April 18 23 minutes ago, Alex Carr said: A decoupling mat? That's a new one for me. Thanks for the tip; I'm going to be reading up the subject. Link to comment Share on other sites More sharing options...
Alex Carr Posted April 18 Author Share Posted April 18 6 minutes ago, Russell griffiths said: I don’t think we are getting the full story, I believe this could be a non structural biscuit screed on a suspended floor. 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. Link to comment Share on other sites More sharing options...
Alex Carr Posted April 18 Author Share Posted April 18 57 minutes ago, Alex Carr said: The problem is that the work is being done in a 200 year-old basement that was in a horrendous mess when we took possession of it. We tanked the whole space (80 mts2); laid new suspended floors in 3 rooms, with the other 2 rooms already having a concrete floor which we had to build up to a particular level - I won't go into this further as it has no direct relevance to the question at hand, which is: Can I have a 70mm biscuit screed overlaid with a decoupling mat and tiles laid on the mat? I'm beginning to think a 4:1 cement mix with builder's sand would be a better option for the screed in order to give it some strength. Any comments? Link to comment Share on other sites More sharing options...
Russell griffiths Posted April 18 Share Posted April 18 Is there a mistake using some of the description here. as far as I’m concerned a biscuit screed is a non structural very weak mix sand and cement lean mix designed to go around ufh pipes to spread the heat and hold the heat in a controlled manner. it has no structural strength, cannot be tiled on or walked on. the flooring is then laid across this on joists carrying the load, the screed is just a heat carrier. the other thing is a structural screed which could either be liquid flow screed or a traditional sand and cement mixture, normally fairly strong mix which can have added fibres for strength and anti cracking. this drys very hard and can support multiple floor finishes. what are we actually talking about because I’m confused as to what’s trying to be accomplished. Link to comment Share on other sites More sharing options...
Alex Carr Posted April 18 Author Share Posted April 18 35 minutes ago, Russell griffiths said: Is there a mistake using some of the description here. as far as I’m concerned a biscuit screed is a non structural very weak mix sand and cement lean mix designed to go around ufh pipes to spread the heat and hold the heat in a controlled manner. it has no structural strength, cannot be tiled on or walked on. the flooring is then laid across this on joists carrying the load, the screed is just a heat carrier. the other thing is a structural screed which could either be liquid flow screed or a traditional sand and cement mixture, normally fairly strong mix which can have added fibres for strength and anti cracking. this drys very hard and can support multiple floor finishes. what are we actually talking about because I’m confused as to what’s trying to be accomplished. 35 minutes ago, Russell griffiths said: Is there a mistake using some of the description here. as far as I’m concerned a biscuit screed is a non structural very weak mix sand and cement lean mix designed to go around ufh pipes to spread the heat and hold the heat in a controlled manner. it has no structural strength, cannot be tiled on or walked on. the flooring is then laid across this on joists carrying the load, the screed is just a heat carrier. the other thing is a structural screed which could either be liquid flow screed or a traditional sand and cement mixture, normally fairly strong mix which can have added fibres for strength and anti cracking. this drys very hard and can support multiple floor finishes. what are we actually talking about because I’m confused as to what’s trying to be accomplished. You're description of a biscuit screed is correct, which is why I've been looking for 12mm cement boards to cover it to carry the tiling. The floor joists have 70mm PIR insulation fitted between them and sit under 12mm OSB boards ; Visqueen covers the OSB; the UFH pipes are sitting on the visqueen and are waiting for the screed to cover them. My question was really about whether a 65mm semi-dry screed (4:1) would have sufficient structural integrity to allow the tiles to be laid on a decoupling mat glued to the 4:1 screed, and if it does could I do away with cement boards I had originally planned to lay over the 8:1 dry biscuit screed? If it does not, what ratio of sand to cement (builder's sand or sharp sand) + polypropolene fibres would do the trick? And how thick? However, the thickness could be tricky - as I have limited space to work with. Link to comment Share on other sites More sharing options...
Alex Carr Posted April 18 Author Share Posted April 18 8 hours ago, nod said: Why would you lay cement boards under the tiles Your doubling up on likely issues Because the biscuit screed is simply thermal mass, it has no structural integrity. The cement boards are there to convey heat from the screed to the tiles and also to ensure the floor can be walked on without the biscuit screed coming under pressure. Link to comment Share on other sites More sharing options...
Russell griffiths Posted April 18 Share Posted April 18 In this case I think we all need to re address this question. a few diagrams would be good. im saying your biscuit mix is no good to support your tiles and you will need a load spreading board of some description. 2 Link to comment Share on other sites More sharing options...
nod Posted April 18 Share Posted April 18 20 minutes ago, Alex Carr said: Because the biscuit screed is simply thermal mass, it has no structural integrity. The cement boards are there to convey heat from the screed to the tiles and also to ensure the floor can be walked on without the biscuit screed coming under pressure. Your not giving any thought to your tiles Why not go with a traditional or flo screed Or use a different floor covering if you must use such a thin biscuit screed I lay thousands of m2 of tiles per year and wouldn’t entertain what you are proposing Link to comment Share on other sites More sharing options...
Alex Carr Posted April 19 Author Share Posted April 19 7 hours ago, nod said: Your not giving any thought to your tiles Why not go with a traditional or flo screed Or use a different floor covering if you must use such a thin biscuit screed I lay thousands of m2 of tiles per year and wouldn’t entertain what you are proposing I have limitations on build-up space. But what would be the traditional screed consist of? Link to comment Share on other sites More sharing options...
Alex Carr Posted April 19 Author Share Posted April 19 7 hours ago, Russell griffiths said: In this case I think we all need to re address this question. a few diagrams would be good. im saying your biscuit mix is no good to support your tiles and you will need a load spreading board of some description. What would be the minimum thickness of a traditional screed to support the tiles with the addition of a decoupling mat? Link to comment Share on other sites More sharing options...
Kelvin Posted April 19 Share Posted April 19 If it’s sand/cement over insulation then 65mm-75mm typically. Flowed screeds can be less than this. https://www.concrete.org.uk/fingertips-document.asp?id=823#:~:text=The optimum thickness of a,for more heavily loaded floors. Link to comment Share on other sites More sharing options...
Alex Carr Posted April 19 Author Share Posted April 19 Hello Kelvin - Thanks for the info Link to comment Share on other sites More sharing options...
Temp Posted April 19 Share Posted April 19 13 hours ago, nod said: Your not giving any thought to your tiles Why not go with a traditional or flo screed Or use a different floor covering if you must use such a thin biscuit screed I lay thousands of m2 of tiles per year and wouldn’t entertain what you are proposing But can you put a heavy traditional 60-75mm thick screed on a suspended wood floor? Link to comment Share on other sites More sharing options...
Russell griffiths Posted April 19 Share Posted April 19 This post is going nowhere without a diagram traditional screed can go on a suspended floor, if the floor was designed for it. flow screed can go on a suspended floor, if it was designed for it. but you cannot put anything on top of unsupported insulation and think it will make a suitable structure. Link to comment Share on other sites More sharing options...
nod Posted April 19 Share Posted April 19 6 minutes ago, Temp said: But can you put a heavy traditional 60-75mm thick screed on a suspended wood floor? If not I’d go down a very different rout Perhaps insulation and ply Link to comment Share on other sites More sharing options...
Alex Carr Posted April 19 Author Share Posted April 19 Per sq.mt, what is the weight of 65mm screed?| Link to comment Share on other sites More sharing options...
Mr Punter Posted April 19 Share Posted April 19 55 minutes ago, Alex Carr said: Per sq.mt, what is the weight of 65mm screed?| I guess about 130kg Link to comment Share on other sites More sharing options...
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