Min screed thickness to level over hollow core slabs

[cwr]

Hi folks

Our first floor is pre cast hollow core which will have a screen over it, no UFH. My understanding is the purpose of the screed is purely to provide a level floor surface, particularly as the slabs are supplied slightly convex. Is there any recommended minimum thickness other than enough to creat a flat surface? Builder wants to put down nominal 50mm, but I think we can get away with much less. Want to keep it thin to scrape back a few mil of ceiling height.

Thanks

[George]

https://www.concrete.org.uk/fingertips-document.asp?id=823#:~:text=The optimum thickness of a,for more heavily loaded floors.

 

Is it bonded or unbonded? 

 

If bonded, then highest point of the slab + 25mm. 

Although if you're taking about arguing over 25mm on the height of a ceiling, it really, really isn't worth worrying about. It's a ... 1% difference. 

Edited September 22, 2021 by George

[Conor]

You won't get away with less than ~50mm. Setup a laser and go round with a tape and you'll see the variations in the slabs. You don't want less than 35mm at any point for standard screeds, so 50mm depth gives you a bit of tolerance for high points.

[Nickfromwales]

3 hours ago, George said:

https://www.concrete.org.uk/fingertips-document.asp?id=823#:~:text=The optimum thickness of a,for more heavily loaded floors.

 

Is it bonded or unbonded? 

 

If bonded, then highest point of the slab + 25mm. 

Although if you're taking about arguing over 25mm on the height of a ceiling, it really, really isn't worth worrying about. It's a ... 1% difference. 

My screed guy will only go less than the nominal if bonding. Then he will only go down to 25mm minimum. From there I use SLC to feather down to nothing where slopes diminished. 
25mm on last job, with UFH, down over 10 years, tiled, one tile cracked ( but tiler was shit and prob didn’t use a mat ( or a flexible adhesive )) I’ll bet. 

[cwr]

On 22/09/2021 at 10:57, George said:

Although if you're taking about arguing over 25mm on the height of a ceiling, it really, really isn't worth worrying about. It's a ... 1% difference. 

 

Yes I get that, however... the other half wants a built in wardrobe that's 2400mm tall, and wants it raised up 100mm on a plinth and set down form the ceiling by 100mm. At the mo the ceiling' height is about 2500mm, so every mm is a significant % of what I have to play with. Note that 2400 + 100 + 100 ≠ 2500 ..........= I'm already in going to be in trouble.

 

One local supplier has said they have a liquid screed that is ok at 30mm. The floor varies within 20mm range, so 30mm to 50mm think is looking like the way to go.

[saveasteading]

1 hour ago, cwr said:

a liquid screed that is ok at 30mm.

So the screed is 'OK' at 30mm but what does the plank need to perform as specified?

The screed on a hollow-core floor is an important part of the strength, unless yours has been specified conservatively. Check.

[cwr]

I hadn't thought of that. Will check with the supplier.

[Conor]

1 hour ago, cwr said:

I hadn't thought of that. Will check with the supplier.

If you need structural screed the SE would have specified it on the drawings. It's unlikely unless you have high loadings on top (e.g. garage and green flat roof was only place it was specified for us)

[saveasteading]

And yet, such slabs are usually specified only for large spans or heavy loads. I can't think I ever specified them for other than that as they need to be craned.

 

Also note that there is almost certainly a crown, so there will be much ticker screed at the ends