What screed to use

[Russell griffiths]

I’m just sorting out screeding the downstairs floor over the top of our ufh, so a few choices on finish. 

 

1. Traditional sand and cement screed with fibres, thicker than liquid screeds at 75mm 

 

2. Gypsum based liquid screed

 

3. Cement based liquid screed. 

 

4. Flowable concrete. 

 

 

I dont need anything to dry quickly, the speed I’m going it won’t be tiled for ages??

 

what i I want is a solid feeling substantial floor, floor finish will be tiles. 

 

I have only had one quote for the flowable cement based stuff and I nearly fell over, it was double what I could do the job in standard concrete. 

 

What have you all had and  are you happy with them @nod do you have an opinion bud. 

[nod]

I laid a traditional 75 screed in our 

158m2 20 ton I could hardly get out of bed The following morning 
Still thought I was 25

Originally I was going to let someone else do it and go with the flow screed 

Great for tilling You can tile on it the following week 
Like you mine would be stood so not a lot of gain 

At least a third more expensive than getting someone in To do a traditional screed

 

We have had a couple of problems with flow screed when tiling 

UFH pipes on the surface 

or clips showing 

I also worked on one job recently and they had had difficulties with the flow burying through the insulation and finished with a 9 mtr ridge down the centre of the room 

I think the flow can be a bit hit and miss 

[CC45]

I've done both but for our build we had a screed pumped in - at ~ 75mm thick they can use a lot less additives so we got V little lattice formation but just to be safe it was all sanded down.  Our floors took 10t and cost about 2.5k.  1 morning and its done.  I could never have done it for enough less to make me want to find enough mates & time to do it.  I walked on it the next day & did other jobs and felt great about the choice.  I was very careful with the choice of tile primer and tile adhesive - stuck to what was recommended by the company.  They have some info on their website https://www.ryflow.co.uk/.

I also put a plastic sheet on top of the floor insulation to stop the leaks and made sure the ufh pipe was well pinned down.  The screed won't take weather (it sort of breaks down) so the house does need to have a roof on it.

[dpmiller]

gypsum pumped screed for us too. quick, clean, level.

[Oz07]

That gypsum stuff is good but not sure it's right if you want solid feeling whenever I've worked ontop of it always seems to have a tiny bounce/vibration. I think you want to go as thick as possible if solid feeling is your key criteria so maybe traditional screed best bet?

[Russell griffiths]

46 minutes ago, Oz07 said:

That gypsum stuff is good but not sure it's right if you want solid feeling whenever I've worked ontop of it always seems to have a tiny bounce/vibration. I think you want to go as thick as possible if solid feeling is your key criteria so maybe traditional screed best bet?

This is exactly what I’m thinking, all the companies I have spoken to want to put it in at 50mm, if I look at my tape and look at 50mm it just looks like this false topping. 

A friend has it in his house and I’m sure as I walk across it I can feel a bounce. 

 

As you you know I wanted to go with a self compacting concrete, but I am struggling to find a supplier anywhere near enough to deliver. 

 

I had always planned on 75mm. 

[bassanclan]

If you are comparing structural liquid concrete (such as agilia) to sand and cement screed it will seem expensive, but if you are putting in a concrete subbase and then screed on top and accounting for labour then the agilia looks more reasonable.

[dpmiller]

the gypsum stuff is extremely stiff even at 50mm. Sure thicker would be stiffer, but you're looking at something akin to the "temper" of a vibrated Council paving slab.

[PeterW]

1 hour ago, Russell griffiths said:

 

As you you know I wanted to go with a self compacting concrete, but I am struggling to find a supplier anywhere near enough to deliver. 

 

Just ask for pump grade concrete - smaller aggregate (10mm) and more cement so you should be fine if you run it wet and have rails to tamp or screed to

[Mike]

Gypsum screeds aren't water-resistant, so take care if you're planning a wet room.

[Olly P]

We are putting the concrete slab in tomorrow morning 8am.. based 100mm insulation  and 75mm screed on top with UFH.  Can anyone reassure me that the UFH will work well in 75mm screed? It seems alot to me but too late to change unless I pour more concrete in tomorrow morning. 

[Declan52]

2 minutes ago, Olly P said:

We are putting the concrete slab in tomorrow morning 8am.. based 100mm insulation  and 75mm screed on top with UFH.  Can anyone reassure me that the UFH will work well in 75mm screed? It seems alot to me but too late to change unless I pour more concrete in tomorrow morning. 

Will work fine. It will all depend on how long it takes for you to figure out the delay in the heat coming through due to having 100mm of insulation. Mine takes roughly 90 mins before you would feel in on your feet and I have 150mm. But as it takes long to heat up it will also stay warm for longer. Don't forget that the house will take a fair while to dry out so it won't be till the winter that you get a better idea of your timings.

[JFDIY]

 

8 minutes ago, Olly P said:

Can anyone reassure me that the UFH will work well in 75mm screed?

You can always put more insulation in if you change your mind as you go, either in layers or one layer, you can make almost any thickness within reason to fine tune things. I haven't checked recently, but my BCO stated insulation is cheaper than concrete...... 

 

[JamesP]

Hi Russell,

I had Standard Gyvlon Anhydrite Liquid Screed, 50mm. I gather it would need sealing before tiling.

Very solid, level floor on 150mm celotex with UFH. 

[Mr Punter]

Normal screed - compo sand and cement - when laid properly 75mm gives an excellent solid level and flat floor with or without UFH.  It does not need sanding or priming and you can lay any floor finish.  The reason it is not more widely used is because it takes more skill than the liquid stuff so you cannot get untrained monkeys to install 1000m2 in a day.

 

You can mix on site or have it mixed and delivered and with or without fibres - I would choose with.

[Russell griffiths]

I’m leaning towards traditional sand and cement screed at the moment as the quotes I’m getting back for any type of flow are 30% more than I was expecting. 

[Conor]

9 hours ago, Olly P said:

We are putting the concrete slab in tomorrow morning 8am.. based 100mm insulation  and 75mm screed on top with UFH.  Can anyone reassure me that the UFH will work well in 75mm screed? It seems alot to me but too late to change unless I pour more concrete in tomorrow morning. 

I'd put in another 25mm of insulation and go for 50mm liquid screed. You'll have less heat loss and will have faster heat transmission thorough the liquid screed. 100mm insulation really is the bare minimum, more is better.

 

Personally, I'm going for 175mm insulation and 75mm sand and cement screed. That's mostly a cost based decision, it's a basement that will have limited ufh. We have option of going to 200mm insultion and 50mm liquid screed if there is budget for it!

[MarkyP]

We had an anhydridte liquid screed on the extension (over 150mm of PIR).

 

My anhydrite is 60mm with ufh. It doesn't feel as solid as I would have liked. Stomping teenagers cause a noticeable tremour on the kitchen island. If i had my time again I would have gone with insulation under slab and either polished concrete or a thin smoothing screed over slab.

 

Anhydrite is super fast to install. The pair of guys who did mine were good and prepared laser set levels thoroughly, it's flat as a pancake and there can't be more than a couple of mm variation across the 80m2. I laid a large slate hearth and it just sat perfectly flat, not so much as fag paper crack between it and the floor anywhere. We topped the screed with microtopping as a wearing surface.

 

The anhydrite surface laitance is easier to remove in the first days after install, it scrapes off. If left longer it then bakes on and needs machine sanding. Guess which option I naively took?!

 

Sand and cement will feel a bit more solid at 70mm but compare the cured dry density, I can't recall there was much in it. Finding good sand and cement screeder isn't easy, its a dying art and back breaking work. I've seen several local jobs which I scouted when trying to find a screeder (before plumping for anhydrite) and the quality wasn't great, curling up at edges and not very flat, maybe OK to tile over but not good enough for microtopping without first sorting with a generous coating of smoothing compound. There are a few screeding crews still around but check carefully any jobbing builders or plasterers who offer it as it's a real art and a good flat finish is hard won through lots of experience. Fibre reinforced screed ready mix delivered to site would be my pref over a pile of sand and a shovel.

 

Both anhydrite and s+c take ages to dry! L , I left mine a full 8 weeks and even then gave the ufh a tickle for a few days to sure. 

 

 

 

[Vijay]

From what Marky P said, is it an idea to plan to have a S&C screed and levelling compound? The house next to me had "professional screeders" and commented that when the tiler went in, all the high and low spots gave them a nice surprise. So is it worth planning ahead?

[CC45]

23 hours ago, Olly P said:

We are putting the concrete slab in tomorrow morning 8am.. based 100mm insulation  and 75mm screed on top with UFH.  Can anyone reassure me that the UFH will work well in 75mm screed? It seems alot to me but too late to change unless I pour more concrete in tomorrow morning. 

yes it will, our screed is 75mm and I'm amazed how quickly it responds. rest easy

[gc100]

21 hours ago, MarkyP said:

anhydrite surface laitance

I’m  having mine poured tomorrow. My builders say I’ll need to get it all sanded in a couple of weeks to remove the laitec . However another build mate builder says he just goes in a couple of days later after pour with a stiff deck brush and removes it. 
 

anyone know if that’s good enough to do when wanting to tile after?  My mate reckons it is but he’s a bit of the cheap side with his builds ?

Edited May 21, 2020 by gc100

[MarkyP]

13 hours ago, Vijay said:

From what Marky P said, is it an idea to plan to have a S&C screed and levelling compound? The house next to me had "professional screeders" and commented that when the tiler went in, all the high and low spots gave them a nice surprise. So is it worth planning ahead?

the screeders should be working to a specified tolerance (SR2 is normal), ideally you want that in the contract. Self level would be something your finish flooring contractor might use if they were really not happy with the screed, but I wouldnt mention a plan to do that to the screeder, might be licence to do a sloppy job because you said you'd flatten it out later!

 

We used microtopping over the screed, it's only a few mm thick and needs a nice flat sub-base. The installers said that over anhydrite it's fine because the screed is so flat, but over s&c they nearly always have to apply a smoothing compound first. 

[MarkyP]

16 minutes ago, gc100 said:

I’m  having mine poured tomorrow. My builders days I’ll need to get it all sanded ina couple of weeks to remove the laitec . However a another build mate says he just goes in a couple of days later with a stiff deck brush and removes it. 
 

anyone know if that’s good enough to do when wanting to tile after?  My mate reckons it is but he’s a bit of the cheap side with his builds ?

My installers suggested a long pole scraper after a day or two. They said otherwise your finish floor installer will need to sand it. I tried a hand scraper after a day or so and the laitence came off as a very ovious crumbly skin on the top. Laitence wasnt present everywhere but it was fairly obvious where it formed. I thought I'd save myself some money by not having the whole lot sanded and I'd scrape it off myself. I left the screed to dry and then realised the laitance had set rock hard and wouldnt scrape off so I had to pay extra to have it sanded. Some anhydrite firms will offer a service to come back and remove laitance for you with a floor sanding machine.

 

[CC45]

cost of screed £2.5K, cost of tiles £2.5k, cost of tiler £2.5k. Cost for removing lattice £120.

[gc100]

8 hours ago, CC45 said:

cost of screed £2.5K, cost of tiles £2.5k, cost of tiler £2.5k. Cost for removing lattice £120.

Yes I take your point  :-)