Why sand and cement for shower trays?

[Al Getaroundtuit]

I have a Simpsons/Crosswater 1100 X 900, low profile, stone resin shower tray to fit.
My question is, why do the manufacturers insist on a sand cement bed when sand and cement tends to crack, shrink and not stick to wood very well?

My father in law is a bricky by trade and he seems to think it is strange that they insist on S/C, when it is not unknown to use flexible tile adhesives on wedi boards etc in wet rooms.

Would there be a greater risk of the S/C cracking and needing the warranty on the tray than the risk of not following the manufacturer's instructions, using a flexible adhesive that never gives way?

What would you use and have you ever had any call backs to either?

Thanks in advance for your comments and advice.

 

[Flintstone]

I don't fit trays generally but I've fitted one or two and I used an S2 adhesive.

 

[Dumbo]

I have fitted many trays with flexible adhesive and never had a comeback , but much the same as tiling you must get a good coverage on the tray as if it fails it will good the warranty .

 

[Qwerty]

Always an S1 tile adhesive for me

 

[Boggs]

Always S1/S2 adhesive here, always clean the back of the tray to remove the dust.

 

[Bopster]

Flex adhesive here to.
With s/c a tray would be easy to lift of if ever required. In adhesive if ever required to lift it maybe harder, much harder

 

[Ajax123]

It's not about sticking it down it's about giving it a solid level base.

 

[acaciaguy]

I once fitted a toilet with s1. I was doing a cloakroom and found the toilet was cemented in. Why have they done that i thought. Further investigation determined there was a gas pipe running under the bog. So s1 it was. I think the flex adhesive helps for the use of the toilet. I would use under tray

 

[Wayne Brown]

I always bed them down with a flexible powdered adhesive treat it as a big tile as stated above remove the dust from the back of the tray from manufacture. If it's to support the tray on legs maybe ? then a sand and cement screed is the norm. Kop