Floor Screeding for numpties?

[FresnoBob]

Hello everyone, if you've read any of my other posts you'll know I have a very uneven Block and Beam type floor to deal with. In an ideal world, I'd love to just be able to chuck a bucket of some sort of self levelling stuff over it (I appreciate that's not as simple as I've made it sound lol) and the whole thing to sort itself out with minimal input from me....after much much research and a very sore head, I'm starting to think that I'm going to have to learn how to screed a floor....even if I decide to use a builder it would be nice to have an idea how to do it.

So far, all I seem to have picked up is use a very dry mix of 3 to 4 parts 'sharp' sand to 1 part cement. My head is struggling to cope with the concept that this mixture will dry into a solid walkable surface if it's that 'dry' to start with. Does some water have to be added to the mix to allow it to be spread more easily, or will there be enough moisture in a bag of sharp sand by itself?

Do you guys think it would be worthwhile nailing some batons to some plywood or a paving slab or something and having a go at this on a smaller scale, before I'm let loose to potentially make a mess on the real floor?

Sorry if theres a tutorial kicking about that I've missed, or I'm just being a git in general .

 

[RedHerring2]

You will need to add some water. A good tip I read recently, can't remember where.
Take a ball of the mix in the palm of the hand, about a cricket ball size.
If it won't hold it's shape, it's too wet. Or if when you try to break it in half and it just makes holes, it's too wet.
If when you try to break it in half and it crumbles it's too dry.
If, when you break it in half, you get two halves, that's about right.

The way I did it recently, in France, lay several lathes/battens on the floor at about the correct depth of screed, levelling them all up together, with about 1.8 metres gaps/bays.
Using these battens as guides, screed your floor.
When that bay is finished, lift the batten(s) and fill the gap. You'll have to repair your foot steps a bit also.

Alternatively, which I'll try next time.
Lay the battens, screed a couple of inches either side, then lift the battens, then infill where the battens were and the rest of the bay, using the already completed "ridges" as guides.

Like painting try working to a wet edge all the time.
Try to complete the floor in one day.

 

[FresnoBob]

Thanks red, I'll use that cricket ball test , I'll maybe have a go on a practice go on a small scale area, and try and get used to instinctively knowing how much water to add.

Next thing I've got to work out is which insulation is the best to use, I like the look of those marmox board thingies, so if a DMP on top of the Block and Beam, then Screed, then Marmox was the right way to do it, I'd be tempted to go that way then hopefully tile straight over (someones bound to correct me here as that sounds too easy lol). I've also seen videos using Kingspan and such, where the DMP is laid, then the blocks of insulation, then the screed on top of that....is there a standard/accepted/best way to do it?

 

[jeds]

What sort of area are you talking about? If it's more than a few metres I would strongly recommend getting in a premix with fibres. That will come the correct moisture content and swill stay workable all day. Small extra cost but believe me you will be so glad you spent the extra dosh when you've finished. If it's a very large area I'd get it done. Good screeders make it look easy but it's hard work keeping it flat and consistent if you're not used to it.

 

[FresnoBob]

....well I'm ripping out and adjusting some stud walls, if it was done as one area it would be appro 5m x 3.75m (plus about 2m x 1m for an area of hall/cupboard), but with the adjusted stud walls in (which I believe should go in first before the screed?) the biggest part would be about 3m x 3.75m, with the rest being split between a downstairs toilet and small hall area.

This premixed stuff is completely alien to me, I'll have to research it, unless you have some details of how it arrives, minimum quanties, example prices etc?

 

[wavetrain]

FAO jeds:

If a person mixed their own screed WITH FIBERS and plastersiser (in a cement mixer of course), would they find it more workable/easier because of the fibers?

 

[RedHerring2]

Anyone pro's on here able to give an opinion of the use of Retarders in the mix?

At 5m X 3.5 m and about 75mm thick it will only be about 1.5M³, not really cost-effective using readymix for that volume, and with a couple of people, easily achievable in a good day.

Have your materials close and mix them close to where you will use them, in order to reduce the barrow journey length.

 

[FresnoBob]

I suppose I could do more than twice that area if I decided to do the living room at the same time....but between the missus and the kids I can't see that working out lol....maybe pack them off to the in laws for the weekend lol . It's probably best I start off on a smaller area anyway, build my confidence up.

Is the correct way to lay everything (on top of the Blocks and Beams), DMP sheet, then insulation then screed onto the insulation? As this guy has done here (nice pictures, well I think so)....

http://www.myhouseextension.com/screed.htm

Richard C mentioned Marmox boards on another thread, and I like the look of them, could I screed directly onto the blocks and beams with no DPM and just lay the waterproof Marmox tile backer boards on top of the screed? Or am I missing something really obviousl? lol (btw I believe there's a dpm under the blocks and beams, which are bone dry....also the original blue plastic sheeting I pulled up didn't cover the entire floor - how odd??!?)

Currently we have (or had lol) 50mm white crumbly polystyrene insulation, so I'm guessing any of the branded sheet insulation boards would be better than this, have you lot any recommendations/thicknesses depending on which way I do the job?

Sorry to ask so many questions, I'm getting really enthusiastic about this, but don't want to jump in unprepared an make a cock up lol.

 

[mointainwalker]

IMO using Marmox as you propose is just a waste of money. It is a specialist board for tile-laying and you will not be using it for this. The insulation used is either PIR or extruded ( not expanded ) polystyrene so you will save money by buying a straightforward insulation-board.

You can get floor-grade expanded polystyrene ( which I don't really understand as it still crumbles ) and it is, I feel, more cost-effective when comparing the extra cost of the two alternatives against their higher insulation values and saved energy costs.

 

[FresnoBob]

Sorry, I thought I'd already mentioned it on this thread when I hadn't lol

The finished floor surface would indeed be tiles. The missus has her eyes on some slate effect porcelain jobbies.

 

[RedHerring2]

I always use expanded polystyrene (100mm), but under the concrete.
I suppose, if I was going to use the insulation under the screed, I'd consider using PIR boards.

 

[FresnoBob]

Celotex perhaps? Would 25mm do, the polystyrene was 50mm? Orrrr will Building Control likely have something to say? lol. (I'm gonna have another word with them on Monday)

 

[meadowhog]

Hi Fresnobob

I posted the cricket ball screed test last month because I know it works and I was taught by the guy who layed my screed. Im glad its been noticed as its the best description I have come across. try to crack it open like a chocolate orange. Screeding is the one job I didnt do on my extension as you really do need to do each room in one hit but I did knock up the screed. I worked solid for 9 hours feeding 2 mixers and 3 of us managed 65m at 65mm deep. Add half a bag of cement to your mixer then four times the volume of sharp sand then add enough water in one go get close to right consistency. A bit more water or sand can be added to get it just right. Plasticisers are not normally added to screeds as they can weeken it. I must remind you doing the larger room will be hard work and thats with two diy people.

I would start on the small room as a practice. First put a mark on the wall at the finished height of screed. This is easily achieved with a spinning lazer level. A straight edge and spirit level was then used. When the screed is being put down it must pushed down hard or wacked down to get rid of as much air as possible. This will make it stronger. Fibres are also a good option especially if youre going for a 65mm screed. When its layed cover it with polythene and leave it for a week. If you dont, you run the risk of cracking or even curling. This is where the corners curl up and sound hollow when tapped.

Rule of thumb for drying 1day per mm of thickness. If after a week you can open the windows/doors and let a draft run over it it will dry a bit quicker. Dont be suprised if you still need to put self levelling latex down after the screed has dried.

You dont need a dpm under your insulation. If you use celotex you would need to tape the joints anyway to stop the screed dropping down the gaps and would give you a vapour barrier. If your BC is involved he may have a say in what can be used. There are also regs for thicknesess of ins. Depending on how your beam floor is uneven may effect the way the insulation is layed. Go for the cheapest ins especially as your not going for ufh.