QUESTION

I have had a large terrace laid (3000 sq.ft.) and a large percentage sound completely hollow as they are not adhered to the layer underneath. They were layed on Schlüter®-DITRA-DRAIN 4 matting using a layer of adhesive beween the tiles and the matting and, obviously the matting was glued to the screed beneath which was laid on a 1 in 70 slope as specified by Schlüter. If the tiles which sound completely hollow are to be replaced by the tiler then I have two questions:-
1 How will the tiles which must be replaced be identified compared to the ones which are just a bit hollow sounding - is there a machine or device to quantify the slightly defective from the seriously defective?
2 How will the integrity of the drainage system be maintained because I imagine removing the defective tiles will damage the defective matting and the screed on which it is laid. Will simply replacing the part of the matting beneath the tile be enough?
Thank you,

ANSWER

ANSWER -  First of all having tiles that sound hollow is not a defect in itself.  Normally when tiles are installed over a membrane of some sort; like Ditra it will cause hollow sounds because you are not bonded to a solid surface.

Although sometimes if the tile is making hollow sounds it can be a symptom of a problem.  Particularly if some tiles sound hollow and some doesn't or if a portion of a tile sounds hollow and the other portions of the same tile sound solid.  The only way to determine if it is a problem is if you actually have resultant damages from the condition.  Then you would need to remove some of the tiles under various conditions to determine what is causing the hollow sound and if it is a problem, then determine how to repair it.

the industry standards recommend a 1:50 slope (2%) for exterior floors and interior wet areas.

The Schlüter®-DITRA-DRAIN 4 matting is a perforated drainage mat that is installed over the ditra waterproof membrane.  So to make repairs you will have to see how much damage is done to the two ditra mats.  The bottom waterproof mat is the most critical one to make sure that if you have to repair it that it remains waterproof.  You will have to overlap the old membrane with the new membrane to make sure it remains waterproof.  Then apply the drainage mat on top of it.  You should check with Schluter to see if they have any special requirements or recommendations for making repairs.