Ask Andy “Floating Screed Over Insulation. What Is The Minimum Depth Allowed?”

I’m Andy Parkin Managing Director of the Multi-Award Winning Speed Screed. Thank you very much for your question, Alex.

Your question is:

“If I do a floating screed over Celotex insulation, what is the minimum depth allowed?”

Watch Andy’s video to find out his answer:

Having laid Celotex floor insulation, you are correct in thinking that there will be some movement in the screed. If you think about it, the screed is extremely thin. Whilst it has excellent compressive strength, its flexural strength will not be as strong because you have a very thin screed.

The minimum screed depth needs to be a bit thicker than if it was bonded directly to the substrate when deriving its strength from the concrete substrate. If it is going onto the insulation, the minimum realistically is 35mm, and it is going to be from the high spot.

Across the floor, you will probably have slight variations so that it might be 35mm at the high spot, but nominally it could be around 40mm as such.

You are looking at 35mm at the highspot and looking domestically at products like liquid screeds. Liquid calcium sulphate and the cementitious screeds will go down in domestic environments at 35mm. Hence, the flexural strength is strong enough. And if you are looking at specialist modified screeds — sand and cement screeds, the minimum depth would be 35 mm.

A traditional sand and cement screed needs 65 mm. You can see that it is not going to be suitable for your application. At 35mm, you need a specialist screed, and the one that comes to mind is a product called Retonal Xtreme.  It is very high strength and fast-drying product which can be laid at 35 mm.

Your minimum options are a semi-dry sand and cement product, and some liquid screeds can be laid at 35mm.

****watch out for the new Gyvlon Finio which can be laid at 30mm when the insulation is at least 140 kPa****

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