# Kerdi = unmodified, Ditra = modified - why?



## bob_cntrctr (Jan 30, 2008)

Gentlemen,

It's been a while. Hope everyone is well. 

Anyway - here's a question that has stuck in the back ofmy mind for some time:

Install instructions specify un-modified thinset between Kerdi and the tiles. Schluter's official explanation is that the impermeable Kerdi will not allow moisture to escape, causing polymer-modified mortars to not cure properly.

But Ditra instructions specify modified thinset between the Ditra and the tiles - and Ditra is no less impermeable than Kerdi.

Anyone know what's really going on?

Thanks.


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## kevjob (Aug 14, 2006)

Bob, modified for ditra when installing over wood sub-floor, un-modified is what schluter wants between tiles and ditra. I like kerabond.


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## angus242 (Oct 20, 2007)

bob_cntrctr said:


> Gentlemen,
> 
> It's been a while. Hope everyone is well.
> 
> ...


You've got it wrong. The ONLY time modified thinset is used is UNDER Ditra when installed over WOOD.

Always unmodified OVER Ditra. Unmodified UNDER Ditra over concrete.


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## bob_cntrctr (Jan 30, 2008)

You know, I just installed about a mile of Ditra a couple months ago, and I know I did exactly that - modified underneath 'cause it was going over 3/4" plywood, then unmodified on top. But the brain must be getting old, I remembered it backwards, and realized it made no sense that way....

Thanks for jogging the old memory.


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