Sand and Cement Screed?

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Correct way to lay sand and cement screed?

FreeBear

A finishing layer of screed is usually around 50mm thick - Never head of chicken wire being put in (a heavy reinforcing mesh, yes. But for industrial strength floors).

If you are concerned about the screed bonding with the concrete, give it a coat of SBR or acrylic primer first. Or even a bit of PVA at a push.

FreeBear

I would bring the insulation up to the finished floor height - That would give you a little bit of room for the slab to expand/contract. Concrete has a coefficient of expansion of 12-14µm per °C. So for a temperature increase of 5°C on a slab that is 3m wide, it will expand by (approximately) 0.2mm.

To limit screed from cracking, add some polyester fibres in to the mix - http://www.tdpltd.com/our-products/concrete-and-screed-fibres/ - The modern equivalent to horse hair or chicken wire with the added advantage that they won't rust.

FreeBear

genau wrote: »

so am I correct in assuming that even the "sand and cement screed slab" will expand and contract in the same way that concrete does and so needs this insulation/expanding material for the same reason?

A sand & cement screed will have a very similar rate of expansion. The reason for continuing the insulation up still remain.

FreeBear

Your plaster is going to be at least 12mm thick, perhaps as much as 20mm depending on the type & finish... With solid brick walls, I'd suggest looking at Pavatex and/or hemp plaster which will quite easily be 25mm or more thick. Add to that, the thickness of skirting boards, and you don't have to worry about tiles on top of the polystyrene.

Pavatex & hemp plasters will give you a bit of insulation on the walls and maintain breathability - bringing the polystyrene boards up to the top of the screed will go some way to avoiding cold bridging.