Any other reason to blind type 1 other than to protect dpm?

[24sevenv24]

Hi All

I am in the middle of constructing a raft foundation for my extension. My DPM will sit on top of the slab below the insulation and screed so that I can tie it into the external leaf blockwork as my internal leaf will be timber. Probably a stupid question but is the only purpose of blinding type 1 with sand is to protect the dpm from being punctured? As my dpm will sit above the slab, do I need to blind the type 1 sub base still? Any advantages of doing so?

Cheers

Will

 

[George_engineer]

Sand is cheaper than concrete.

You will still want a polythene sheet to prevent water loss from the concrete but it doesn't really matter if it gets slightly damaged.