minimum depth for self levelling screed

[pilsbury]

When I lay my floor slab (100mm MOT, sand, DPM, 100mm EPS, 100mm concrete), is there a minimum depth of self levelling screed that needs to go on top or can it simply be just enough that is needed to cover the floor? No UFH.

I shall of course be doing my best to get the concrete as level as possible, but as a novice to this I think getting the concrete flat enough for tiling is unrealistic, so I think the best option is to let the self levelling screed sort that out.

 

[^woody^]

1mm

 

[tomfe]

0.1mm
To a feather edge, so what ever that is.

 

[23vc]

https://www.diynot.com/diy/threads/troweling-slab.480013/page-3#post-3880077

You might want to read that, it's not easy to get it right. I got a lot of very helpful advice here when I was planning/panicking about mine. I'd advise a screed if you can, but I guess you know that.

 

[pilsbury]

Thanks all - just what I was after - no minimum depth, just enough to level.

23vc - I've seen that thread. To get the slab roughly level in the working time will be my goal. Bull floats and the like will be a sure disaster for me - I'll just end up pushing ripples around. With pumped screed being so expensive (obvious cost in the hiring of equipment and pro), my idea is to get a group of mates round, all with big buckets and their own mixing paddles and knock up a 10 bag mix up in one go to pour as one.... you have to remember I'm a skin flint who is intent on saving every penny. A must with my lofty ideas and lean wallet!

 

[23vc]

Nice concept good luck
battening out well is key to getting it level assuming your new DPC level wall is also level, ie. batten the existing house and tamp across. if you can get it tamped properly ie. really flat and no dips then the pressure is then off and its up to you whether you attempt to improve the surface by floating and troweling it.

 

[pilsbury]

Sure thing. I have a slight issue in that the extension is not square. Slightly wedge shaped. Gonna have to think of a way of making an adjustable tamper to run across my battens and also between 2 rooms.

 

[Doggit]

If you can find the highest point in the room, then you can drill and fit rawplugs, then put in screws that you level to the highest point. Use a 6ft level, and you'll end up with a set of marker points that you level to the top of. Sweep the room, and be paranoid and vacumm it if neceesary, and then prime with an acrylic primer.

You need 3 to 4 people on this sort of job. One to mix, one to clean the bucket (2 of required), and then measure out the water for the next bag, one to carry, and one to level. And you can't just dump a bag of SLC in the bucket, and then mix it, you need someone to pour as the other person mixes

You add a little bit more water than recommended (maybe half a litre max), and that'll give a runnier mix. and you need spiked shoes, and a spiked roller. (okay, maybe one more person required for that job), but you pour the first mixed bag and then spread it with a float using the markers to work the height, and you spread it around, then you keep adding bucket fulls as you move down the room, and you've got the guy with the spiked shoes etc getting the bubbles out behind you. But be careful not to over roller as it starts to go off. Better to have bubbles in the floor rather than ruin it with the spiked roller.

And most importantly, you want at least 50% more bags than you anticipate using. I did this with a 3 person setup on a hot day, and ran out of bags, so had to add another layer, and it was bloody hard work, but in 2 goes, we got a 19sqm room levelled nicely.

 

[noseall]

you have to remember I'm a skin flint who is intent on saving every penny

Also known as 'false economy'.

You have to remember also that floor levelling compounds are not entirely risk free and are touted as some magical panacea for all rough floors by architects and the like.

 

[pilsbury]

Doggit- like the idea of screw levels. But why? Isn't the nature of SLC simply pour till the floor is covered then leave it to find its level?

Noseall- the false economy is always a consideration and I am aware SLC is no magic bullet. But it certainly has a much better chance of levelling than me trying to work a 40m sq slab totally flat before it goes off.

 

[garyo]

Isn't the nature of SLC simply pour till the floor is covered then leave it to find its level?

It's a bit of a black art, but doesn't really work like that in my experience. It's more like custard than water, so you need to vaguely push it to the right area of the floor, and in to the corners, and then it'll 'self flatten' in each localised zone.

A floor fitter once said it was cheaper compounds that are the problem: Spend more and they do tend to self level a little better. You have to do it in one pour though because they go off quickly and unless you have a (very)wet edge it'll never be right where the buckets join.

 

[^woody^]

Concrete is difficult enough to get a finish on for internal coverings, and if you are crap at levelling floors, then you will stand more chance of getting an acceptible finish if you screed.

And remember that self-levelling screed is not realy self levelling. You could get just as much uneveness and rough surface after using it as there was before.

 

[Doggit]

I did the first go at leveling the floor, and found it didn't actually level, so then tried the trick of the screws to level to, and also added about a litre extra per bag to make it runnier, and that got a better level with me working towards the screw tops, but the extra water then caused puddles where there was more water than compund, so I actually had to scrape it off, and then use a final bag with feather edge to sort out the water puddle area. I phoned the tech guys, and they admitted that SLC doesn't acutally self level, it needs help to be moved around, and it needs a guide to level to. I achieved a damned good level at the end of the project, and I'll be able to do it in one hit the next time, but it's definitely not as straightforward as they make it out to be.

It's also best not to do it when the weathers to hot, so very early in the morning if necessary. On a hot day, you might only get 12-15 minutes max from pouring it into the bucket, to it going off on the floor.

You have to do it in one pour though because they go off quickly and unless you have a (very)wet edge it'll never be right where the buckets join.

With 40sqm, you couldn't do this in one pour, hence why you need a slick method to keep the buckets coming, and that'll keep the edge wet. As a bag is often about 20kg, you can only mix and transport one bucket/bag at a time, and as it starts to go off whilst you're mixing it, you can't just pour it in the bucket, and then mix it up; you need someone to pour it in whilst you mix it, and that gets the bag mixed in half the time. Hence one buckets getting cleaned and prepped whilst the other ones getting mixed.

 

[pilsbury]

Cheers for the top advice. I'm some way off doing this but I shall certainly enlist a gang of mates with several buckets to mix up quick and keep it coming swiftly. I shall also bear in mind to make it a little runnier, but no too much so as to avoid the puddles of water. Might try the screw level trick and trowl it to a decent flatness and then let the SLC do the final settling itself.

 

[Doggit]

I think you're starting to get the idea of things, so best of luck.