Mortar bed height

[Brian_West]

So I’ve just laid my dry mortar bed for a sloped shower. It slopes to one side towards a Kerdi line linear drain.

The mortar bed turned out well I think (first one ever). It’s good and solid and the slope is right.

However I thought I was erring on the side of caution by leaving the height of the bed about 4mm lower than necessary below the drain edge.

I’m faced with having to lay thinset (compressed height 5mm) then ditra heat, then another 5mm thinset (compressed) on top before finally tile on top.

So essentially looking at using a 10mm notched trowel to lay the thinset instead of a 6mm.

Have I created a problem here or am I worrying about nothing.

I have read threads about trying to raise the height of a mortar bed but it doesn’t seem to be recommended.

 

[Dumbo]

Ditra heat I believe is recommended 6mm trowel (check this ) . There are tech reasons for this as to much adhesive will fill the cavities in the back of the mat and stop it functioning properly as a decoupling membrane . Also a 10 mm trowel doesn't give you a 5mm bed depth as when using the trowel it is used a a 45 degree so 10 mm is the hypoteneuse of your triangle and you vertical height before compression is 7mm Therefore offering 3.5 mm after compression

 

[Brian_West]

So any recommendations on what to do.

Is laying a 4mm bed of tile adhesive and levelling it and allowing it to dry advisable.

 

[Dumbo]

So what is your height between bed and shower drain lip . Also how thick are your tiles .

 

[Brian_West]

Height difference is 18 mm

Ditra heat is 7.5mm thick. Measured with calipers just now.

Tiles are 10 mm thick but will be laid on top of the ditra heat and linear drain junction. It is one of those drains you tile over.

You can see in the image the Ditra heat should be level with the top of the foam block and then that strip of kerdi band goes over the junction and then tile on top.

 

[Dumbo]

I don't get your problem .
So i going to say differently.
What is the difference between what you have now and your desired finish as that is what counts

 

[Brian_West]

I'll try explaining a little differently. The distance from the mortar bed to the top edge of the foam block is 15mm. In this depth I have to fit the Ditra heat on a thinset bed so that it is level with the top of the foam block. Ditra heat is 7.5mm thick so that leaves a compressed thinset bed of about 7.5mm. Not good.

Schluter say use a 6mm trowel which would give a compressed bed of about 2-3mm. So if I use a 6mm trowel the Ditra heat will be too low by about 4-5mm. Then I would end up having to put a thick bed under the tile. Not good either.

Attached image shows where I have zoomed in on where the Ditra heat should butt against the foam block (hashed area).

 

[Dumbo]

In reality it would probably be lower . I would stick down some insulation board prior to fixing the ditra.
If I was you cut off a piece of ditra and stick it to something , I think you will find it will pretty much bed to nothing . Then see what thickness ins board you can use

 

[Unused Account 1]

Theres always trouble getting the heights correct

 

[Brian_West]

Thanks. That’s interesting on the stickiness of ditra. They make a big enough deal about checking for full contact on the underside during installation.

I’ve been considering medium bed mortar for underneath the ditra. Would allow a deeper bed but must check if any incompatibilities
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Theres always trouble getting the heights correct

The irony is that I hit the levels I aimed for pretty much spot on. I shouldn’t have worried about allowing for errors and it would have been near perfect.