Screed info please

happyhero

Hi sorry to bring this subject up yet again. I have been on here about this job before but I was just wondering if someone could clear a few things up for me please.

Basically I have a downstairs extension which had a wall in it dividing it into two halves. It has parquet floor on hardboard sheets laid on the floor.
I have removed the wall and it has left a trench which I have removed anything loose from and this trench varies from about 3cm to nearly 5cm, most of it being about 4cm. The floor is out by about 1cm either side of this trench so I was going to peal away at the parquet until I can lose the difference in a gentle unnoticeable slope if neccasery.

I was going to screed the trench to just below the lowest floor level which would make the screed closer to 3cm. Then peel back the floor until I get something acceptable and then use some of the Ardex k15 leveller to level from the top of the screed to the face level of the parquet.

Thing is the more I read the more I get confused. Some suggest 3 to 1 mixture and some 4 to 1, can someone tell me which one is best and why please?

Also as this screed is quite thin for screed what should I do with it, ie what should I put in it, PVA (how much if so), expanded metal, anything else?

How thin can a screed be and get away with it?

Also by the wall the removed wall was joined to, I can see there is a membrane under this floor. In the trench you cannot see this, just concrete so I am assuming that this is a layer of concrete over the membrane. Do I have to do anything to this concrete to make my screed stick better to it, ie PVA it?

Any advice or recommendation appreciated.

plumb1_2

I would use granite chippings, 3-1 with cemment.
Just mix a little cement into a slurry and paint the sides of the hole before you infill, no need for PVA.

ormus

you can lay a screed from 10mm to 75mm.
but most screeders use a general guide of 40mm minimum, to minimise any problems and to get a good bond.

obviously the thinner you go the more likely to be a problem.
perhaps a very minimum of say 25mm thick?

happyhero

plumb1 wrote:

I would use granite chippings, 3-1 with cemment.
Just mix a little cement into a slurry and paint the sides of the hole before you infill, no need for PVA.

Cheers for the tip, I have never heard of this slurry thing is it better than PVA? How does it work? By slurry I assume you mean I should make a very watery cement and water mix and paint it on like PVA, and then I assume let it go off for about an hour before filling my trench?

One thing I for got to ask, bearing in mind it is a central heated house, should I cover the screed after to slow it drying too quick?

Also how long before it is strong enough to walk on?

ormus

i use a 3 to 1 mix.
make sure you mix it very nearly dry, ie nothing like concrete nor mortar.

24 hrs should be ok for light foot traffic, but 48 hrs is even better.
if you weigh 20 stone, give it a weekend eh!

plumb1_2

happyhero wrote:

Cheers for the tip, I have never heard of this slurry thing is it better than PVA? How does it work? By slurry I assume you mean I should make a very watery cement and water mix and paint it on like PVA, and then I assume let it go off for about an hour before filling my trench?

One thing I for got to ask, bearing in mind it is a central heated house, should I cover the screed after to slow it drying too quick?

Also how long before it is strong enough to walk on?

Yes, just mix a couple of trowels of cement with water the paint it on, it will give a good key for the granite mix,you dont wait until it has dried.
You only need the granite/cememt mix dampish dont have it sloppy, dont worry about the central heating and it will set within 24hrs.

ps- make sure the hole is dust free before you add the mix.

ritesh

If you do are going to have a thin layer of screed you can use an additive called RONAFIX when you make the screed mixture. This is similar to PVA but a lot stronger. When the screed has dried you will probably need a kangol to break it off!!

nelly_2

PVA is the best primer as it also stops the water being sucked out of the screed making it dry too fast as well as ensuring adhesion.

Sand and cement shouldnt really be less than 2 inch deep neither.

Use self leveling to fill it up most of them can be bulked up with sand then another pure coat added on top which can be troweld down to 1 mill thick.