reassurance/ ideas/advice screed floor

[simonsnorkel]

I have some concerns on a floor I am going to lay in the near future. Due to the practicalities of the job I think a fibre screed seems the better option. However I am scared it may crack.

The reasons I've opted for screed are
It has worked out within my budget to get the chosen substance delivered by a local firm than buy the materials individually and hire mixer.

Access to the room is down stairs and a short walk. As I have no who can help me, I feel concrete will start to become unworkable before I have got the 1 cubic meter into the room.

The floor cant go much higher than 130mm this includes the 50mm insulation sheets I have already laid across the floor. I will get a much better finish with screed.

My Concerns
The existing floor is quite uneven. I have estimated the screed will be 80mm at its deepest and a minimum of 50mm thick at the thinnest point. However I am anticipating, in practice this may become 40m. Although, this will only be in a ½ meter squared area. Will this weaken it too much?

The floor slopes in several directions. The insulation I have put down seems to rock slightly in places. I am very concerned this may encourage the screed to crack. Before I cover the insulation with another polythene membrane, would it help to cut along the top of the insulation – in order to try and get it to follow the general direction of the slope? (very tricky in places – sloping in 2 directions etc)

Once its down will I be able to drill into it and fix a stud wall in the same way as I would concrete.

Previous, but limited experience in employment down south has given me the confidence to do the job myself; although it was always working with someone far more knowledgeable than me. A very tight budget also means I have to do it myself and means I only get one shot at it. For this reason any advice is much needed and would be greatly appreciated.

thanks

 

[noseall]

you can not have unstable insulation sheets beneath the screed.

you will need to sand blind the existing floor, lay polythene then lay the insulation.

50mm is the recommended minimum though i would say 75mm if laying directly onto insulation.

 

[simonsnorkel]

Thanks for the advice. I am very scared the slab will crack

I had aready raised a low corner with some building sand i had left over after building a wall.

My next question would be - Is there a more suitable sand for blinding?

I have another concern - does the layer need to be compacted with a whacker plate? I used a board of wood laid on the sand in the corner and hit it with a lump hammer to try and compact it better. This seemed to work???

 

[chappers]

no real need to compact it but make sure you put enough down to completly take the flex out of the insulation, building sand is fine.
I wouldn't really recommend going below 75mm for an unbonded screed.You might get away with it in a low traffic area, but I reckon it will eventualy give up the ghost.
also depends on what you intend to cover the screed with if you are tiling then tiles will probably crack or come loose if the screed cracks, whereas if covered with a floating floor you probably won't notice so much if the screed cracks below it.

 

[simonsnorkel]

thanks again chappers

I have got more sand and will make a start on leveling under the boards that are wonky.

I am still considering splitting the area in 2 and laying the same slab in concrete.