a moron's question: Do we really need a screed?

Screeds, what are they for? They provide: A nice and flat surface Impermeable (not that much) hard-wearing So? There are boards used for UFH (and similar products) which offer just that...only, they are 10mm thick. Having removed the screed from a partially screeded bathroom I'm just wondering if it would be better to use the limited height available for insulation rather than a screed - let's say, I'd lay a 50mm slab of Jablite or similar on top of the concrete slab, then an hard wearing 10mm boards to accommodate the UFH and then I'm left with about 12mm for the tiles. The lot would cover the available thickess of 70mm. Any problem?
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On Thursday, October 25, 2012 11:54:24 AM UTC+1, snipped-for-privacy@yahoo.co.uk wrote:

+ a stiff, non-flexible, surface

If you're starting with a concrete slab, I think that should be OK. (An expert will be along shortly to explain why that is wrong)

Can't see any.
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On Thursday, October 25, 2012 11:54:24 AM UTC+1, snipped-for-privacy@yahoo.co.uk wrote:

If the surface is rough, you've got concentrated point loads, resulting in the insulation sinking much more easily. I don't know if its worth putting a little self levelling down first just to stop that
NT
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snipped-for-privacy@yahoo.co.uk wrote:

You can tile over wood, provided it is securely fastened down, EG, plywood screwed over floorboards in an upstairs bathroom, that is to say, you can't just tile over the floorboards because they move individually, hence the plywood sheet. Your proposed floor will essentially be floating, and if it's tiled, it will crack up in no time
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On Thursday, 25 October 2012 20:06:18 UTC+1, Phil L wrote:

I don't get this...you are talking of floorboards while my floor is basically a concrete slab (at this stage) laid on top of some hardcore over ground (so more stable than a suspended floor).
Just as I'm getting more info about screeding...I'm puzzled by the many variants available; more strikingly, some recommend a watery kind of mix while others opt for a near dry mixture?! Can anyone shed some light on this contrasting views? Were those comments addressing leveling compound v proper screed without realizing the difference? Then, there is another question related to the thickness needed for electric heating cables (7mm thick) - some recommend a fairly thick screed (at least 65mm) to be laid on top of the cables while others say that the screed should be thin (25mm above cables so 35mm should do nice) so that the heating time is shortened - to me the latter seems the obvious choice (and quite handy too as there is no much height available).
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snipped-for-privacy@yahoo.co.uk wrote:

But you are talking about laying tiles on 10mm boards over jablite, and I said they won't last five minutes.

I think you're getting bogged down in nonsense.
Screed is a top coat of concrete or mortar or any other hard setting substance used for flooring, it can be liquid, dry or anywhere inbetween, it's not relevant what consistency or method is used in laying, only the finish

it's entirely up to you, if you're putting UFH cables in and want to screed over them, then feel free, but different people have different ideas and a thicker screed over the top will take longer to heat up, but a thinner one isn't very strong and may crack with the heat.
If you've got 70mm to play with and bearing in mind you want 15 of those for the tiles, you'll have to decide how much (if any) insulation you want under there.
If your house is less than 25 years old the floor's already insulated, if it's not you'll need at least 25mm of jablite which will give you 30mm of screed then tiles over this.
And your original question as to 'what is screed for?' it's so that the floor can be poured in one, usually while the building is only at DPC level, without paying too much attention to detail, then when the rest of the building work is completed, the screed is laid
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I don't know the difference between the Jablite boards and these: http://www.bathroom2u.com/varme-underfloor-heating-insulation-boards-50mm-p-515.html
but that sort of boards (there are many similar in the market) might represent just what I needed...at least from the way the product is described - in fact it would make it much easier as I can just laid them on top of the concrete slab, fix the heating cable on top and then tile over (in the original idea I would had to use a slab of Jablite and then a 10mm board + UFH cables and tiles on top).
How about that?
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On Friday, 26 October 2012 21:20:52 UTC+1, snipped-for-privacy@yahoo.co.uk wrote:

Well...another link to reinforce my point: http://www.warmfloorsonline.com/underfloor-heating/underfloor-heating-insulation-faq.htm
The final part of the second paragraph (FAQ 2) states that: "The 50mm boards are very popular in new conservatories & extensions where they effectively replace the top screed."
Indeed, who need a bxxxxy screed then?
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