Underfloor heating screed too thin?

[Dicky Heart]

Hello, my first post proper. I have a large extension to tile. The floorscreed has been poured over the wet underfloor heating loops, and has set nicely. My problem is that the finished screed level is just under 40mm lower than it should have been. I now have a choice of either somehow adding to the screed level, or lowering the adjacent timber floors, so avaoinding any excessive steps. The screed as poured is about 65mm thick at the moment. Is there a recognised way or raising the screed level without risking cracking and affecting the efficiency of the underfloor heating? Or is lowering the timber a better suggestion. Tiles will be 10mm porcelain, already bought and paid for, so I can't use a thicker alternative. Any advice would be welcome.

Thanks, DH

 

[Dan]

I'd lower the timber else you'll end up with a fair slab to heat. Having said that, your tiles and adhesive will be 15-20mm if the remaining 40mm. So perhaps not too bad.

Let the professional have a say they'll sort you out.

 

[DHTiling]

How much timber is to remove to do it that way.?

 

[Dicky Heart]

Thanks for the replies. I think I'l take out the bricks the joists are sitting on and then pack up with slate, probably 20mm overall. Hopefully, the tops of the floorboards (which will be carpeted) will then be level with the top of the tiles.

 

[White Room]

You say the screed is poured, which one is it....

- - - Updated - - -

You say the screed is poured, which one is it....

- - - Updated - - -

You say the screed is poured, which one is it....

 

[Dicky Heart]

Things have moved on a little. I have established the screed is a GYVLON screed, i think by Lafarge. We're inclined to forget lowering the timber floors to meet the poured screed, as there's just too much work with this, and are leaning towards pouring another 35 screed on top of the existing 65. The contactor who originally poured the first screed and Lafarge say that this is possible, but give different suggestions on how to do it. The first screed has been down four weeks, and there's no heating yet.

One says to thoroughly wet the first screed and pour the next directly on top, the other says to put another layer of visqueen down and then pour the new 35mm layer. Both say that the underfloor heating will still work, it'll just take longer to heat up.

Decisions decisions...which way do we go?

Cheers,

DH

 

[Dan]

Go with whatever Lafarge said to do as that'll be the guaranteed way.

 

[Dicky Heart]

Thanks Dan.

I still have a few concerns after speaking to two different lafarge reps. One is saying use a visqueen, the other says don't.

Anyway, I have a few more questions if I may?

1. Is a floor screed of (what will be) 115mm too thick to enable the underfloor heating to work efficiently, or will it just be that the warm up and cool down times will be longer?

2. Once the underfloor heating is working, how much quicker will the screed dry out to allow the floor to be tiled? At 1mm per day for the first 40, then 1/2mm per day after that, it could be another 4 months before the kitchen could be fitted.

In the meantime I'm now looking at using stone flags instead to make up the difference, so if anyone knows of a good supplier in North Yorkshire for 55 square meters please let me know.

My only other option is timber flooring. Now this may sound like a wacky idea, but I have some 600 square plastic grids that lock together and are used for car parks. They are laid on a sub base, locked together then filled with gravel. The depth of these grids is exactly 50mm, just what I need to make up. If I covered the existing screed with this grid (it is made up of 50mm squares) and then fixed an underlay and put engineered timber on top, would the underfloor heating still heat the room, or would all the air pockets act as insulation and stop the room heating up?

Thanks for all your help!

DH

 

[Mark S]

Dickie,
just caught up on the thread from the last couple of weeks.
As the original screed is 65mm, which ideally is 10mm thicker than we would normally recommend adding extra screed thickness, which you may have to, to make up the height, will slow the heating performance down.
By adding timber panels (timber being a poor conductor of heat) will also have the effect of reducing the effectivness of the heating system.
If do add screed do not put a Viqueen type membrane down, but dry the original screed, prime the surface then apply the extra screed, There is no easy solution, but it can be done.
Mark S