Screed directly onto steel decking?

[dougalhouse]

Is there any reason why I wouldn’t want to screed directly onto steel decking instead of concreting worst.

It’ll be 100mm deep approximately 4m x 2m on an external steel framed balcony.

 

[conny]

If the steelwork is in direct sunlight for any length of time it may expand/contract throughout the day and cause the screed to crack, especially around the edges.
Not sure if there is some expansion compound you could put around it, like they do when casting large slabs of concrete.

 

[dougalhouse]

Thanks for the reply but wouldn’t that still be the case with concrete too? Ie cracking due to expansion or is screed less flexible?

 

[conny]

When concrete expands it basically 'stretches' and if mixed correctly does not crack. However, when they lay large slabs, (say, 12' x12'), they leave a gap by pouring into box surrounds. After the concrete has set they remove the boards forming the box and fill with an expandable filler. I would imagine that if the screed is mixed correctly this would act in the same way. I would think for the size you want to do you could put a 10 mm board around the edges then remove after it's set then in fill with maybe a clear/opaque silicon.